This document describes the detection of an unidentified emission line in the stacked X-ray spectrum of 73 galaxy clusters observed by XMM-Newton. The line was detected at an energy of 3.55-3.57 keV in independent analyses of the MOS and PN instruments. The line was also seen in Chandra observations of the Perseus cluster. Possible explanations discussed include an atomic transition in thermal plasma, or the decay of sterile neutrino dark matter particles. However, the origin is unclear and requires further observation.
Detection of an_unindentified_emission_line_in_the_stacked_x_ray_spectrum_of_...Sérgio Sacani
1. Researchers detected a previously unknown emission line in the stacked X-ray spectrum of 73 galaxy clusters observed by XMM-Newton. 2. The line was detected at an energy of 3.55-3.57 keV and was seen independently in subsamples of clusters. 3. The line was also detected in Chandra observations of the Perseus cluster but not in observations of the Virgo cluster. 4. The nature of this line is unclear - it could be a thermal line from an undetected element, or potentially the decay line of a hypothesized dark matter particle called a sterile neutrino. Further observations are needed to determine the origin of the line.
An unindetified line_in_xray_spectra_of_the_adromeda_galaxy_and_perseus_galax...Sérgio Sacani
This document summarizes an analysis of X-ray spectra from the Andromeda galaxy and Perseus galaxy cluster observed with the XMM-Newton X-ray observatory. The analysis identified a weak unidentified line at an energy of approximately 3.5 keV in the spectra of both objects. The line strength increases towards the centers of the objects and is stronger in Perseus than in Andromeda. The line properties are consistent with originating from the decay of dark matter particles, though an instrumental or astrophysical source cannot be ruled out based on individual objects. Future detections or non-detections in additional targets could help reveal the nature of this line.
Topology of charge density from pseudopotential density functional theory cal...Alexander Decker
nl
2(2l + 1)Rnl2 (r )
(6)
n,l
The document discusses the challenges of determining the topology of charge density from pseudopotential density functional theory calculations due to the absence of core electrons. Specifically, it notes that pseudopotential calculations lack critical points at nuclear positions defined by core electrons. To address this, the document examines methods to reconstruct the correct topology, such as adding an isolated atomic core density or using orthogonalized core orbitals. It also provides background on the quantum theory of atoms in molecules and defines key concepts like critical points, atomic basins, and charge density topology. Results are reported for several molecules to analyze
Structural, electronic, elastic, optical and thermodynamical properties of zi...Alexander Decker
nl
2(2l + 1)Rnl2 (r )
(6)
n,l
The document discusses the challenges of determining the topology of charge density from pseudopotential density functional theory calculations due to the absence of core electrons. Specifically, it notes that pseudopotential calculations lack critical points at nuclear positions where core electrons have been removed. To address this, the document examines methods to reconstruct the correct topology, such as adding back core densities or using orthogonalized densities. It also explores analyzing charge density topology using Bader's Quantum Theory of Atoms in Molecules and discusses applications to molecules like alanine.
The non gravitational_interactions_of_dark_matter_in_colliding_galaxy_clustersSérgio Sacani
1) The document analyzes 72 galaxy cluster collisions observed with Hubble and Chandra telescopes to test theories of non-gravitational dark matter interactions.
2) It detects the existence of dark matter at 7.6 sigma significance by comparing the positions of dark mass concentrations to stars and gas.
3) Combining measurements from all collisions, it constrains the dark matter self-interaction cross-section to be less than 0.47 cm^2/g (95% confidence level), ruling out some proposed dark matter models with stronger interactions.
In search of multipath interference using large moleculesGabriel O'Brien
This document summarizes an experiment that tested the quantum mechanical principle of superposition using large dye molecules. The experiment measured interference patterns when the molecules passed through single, double, and triple slits. It observed less than 1% deviation from the expected interference patterns based on quantum mechanics, providing evidence that the superposition principle applies even to massive particles like these large molecules. The experiment is one of the first to directly observe quantum interference using massive particles rather than light or single particles.
This document discusses the observation of ortho-Positronium formation in the Double Chooz neutrino experiment. Positronium is a bound state of an electron and positron that can form either para or ortho configurations. Ortho-Positronium has a longer lifetime that can be measured. The Double Chooz experiment observes an excess of events with longer time differences between electron-positron annihilation signals, indicating ortho-Positronium formation. By fitting the time difference distribution, the fraction of ortho-Positronium formed is measured to be 42% with a lifetime of 3.68 ns, consistent with previous dedicated measurements.
This document analyzes extensions of the Standard Model that naturally accommodate tiny neutrino masses through the exchange of heavy particles. It discusses how neutrino masses are generated at tree-level by seesaw mechanisms involving fermionic singlets/triplets or scalar triplets. Dimension-six operators are also explored, as they may allow observable low-energy effects even with suppressed dimension-five operators. Phenomenological consequences are then analyzed, with a focus on charged lepton flavor violating processes and constraints from rare decays.
Detection of an_unindentified_emission_line_in_the_stacked_x_ray_spectrum_of_...Sérgio Sacani
1. Researchers detected a previously unknown emission line in the stacked X-ray spectrum of 73 galaxy clusters observed by XMM-Newton. 2. The line was detected at an energy of 3.55-3.57 keV and was seen independently in subsamples of clusters. 3. The line was also detected in Chandra observations of the Perseus cluster but not in observations of the Virgo cluster. 4. The nature of this line is unclear - it could be a thermal line from an undetected element, or potentially the decay line of a hypothesized dark matter particle called a sterile neutrino. Further observations are needed to determine the origin of the line.
An unindetified line_in_xray_spectra_of_the_adromeda_galaxy_and_perseus_galax...Sérgio Sacani
This document summarizes an analysis of X-ray spectra from the Andromeda galaxy and Perseus galaxy cluster observed with the XMM-Newton X-ray observatory. The analysis identified a weak unidentified line at an energy of approximately 3.5 keV in the spectra of both objects. The line strength increases towards the centers of the objects and is stronger in Perseus than in Andromeda. The line properties are consistent with originating from the decay of dark matter particles, though an instrumental or astrophysical source cannot be ruled out based on individual objects. Future detections or non-detections in additional targets could help reveal the nature of this line.
Topology of charge density from pseudopotential density functional theory cal...Alexander Decker
nl
2(2l + 1)Rnl2 (r )
(6)
n,l
The document discusses the challenges of determining the topology of charge density from pseudopotential density functional theory calculations due to the absence of core electrons. Specifically, it notes that pseudopotential calculations lack critical points at nuclear positions defined by core electrons. To address this, the document examines methods to reconstruct the correct topology, such as adding an isolated atomic core density or using orthogonalized core orbitals. It also provides background on the quantum theory of atoms in molecules and defines key concepts like critical points, atomic basins, and charge density topology. Results are reported for several molecules to analyze
Structural, electronic, elastic, optical and thermodynamical properties of zi...Alexander Decker
nl
2(2l + 1)Rnl2 (r )
(6)
n,l
The document discusses the challenges of determining the topology of charge density from pseudopotential density functional theory calculations due to the absence of core electrons. Specifically, it notes that pseudopotential calculations lack critical points at nuclear positions where core electrons have been removed. To address this, the document examines methods to reconstruct the correct topology, such as adding back core densities or using orthogonalized densities. It also explores analyzing charge density topology using Bader's Quantum Theory of Atoms in Molecules and discusses applications to molecules like alanine.
The non gravitational_interactions_of_dark_matter_in_colliding_galaxy_clustersSérgio Sacani
1) The document analyzes 72 galaxy cluster collisions observed with Hubble and Chandra telescopes to test theories of non-gravitational dark matter interactions.
2) It detects the existence of dark matter at 7.6 sigma significance by comparing the positions of dark mass concentrations to stars and gas.
3) Combining measurements from all collisions, it constrains the dark matter self-interaction cross-section to be less than 0.47 cm^2/g (95% confidence level), ruling out some proposed dark matter models with stronger interactions.
In search of multipath interference using large moleculesGabriel O'Brien
This document summarizes an experiment that tested the quantum mechanical principle of superposition using large dye molecules. The experiment measured interference patterns when the molecules passed through single, double, and triple slits. It observed less than 1% deviation from the expected interference patterns based on quantum mechanics, providing evidence that the superposition principle applies even to massive particles like these large molecules. The experiment is one of the first to directly observe quantum interference using massive particles rather than light or single particles.
This document discusses the observation of ortho-Positronium formation in the Double Chooz neutrino experiment. Positronium is a bound state of an electron and positron that can form either para or ortho configurations. Ortho-Positronium has a longer lifetime that can be measured. The Double Chooz experiment observes an excess of events with longer time differences between electron-positron annihilation signals, indicating ortho-Positronium formation. By fitting the time difference distribution, the fraction of ortho-Positronium formed is measured to be 42% with a lifetime of 3.68 ns, consistent with previous dedicated measurements.
This document analyzes extensions of the Standard Model that naturally accommodate tiny neutrino masses through the exchange of heavy particles. It discusses how neutrino masses are generated at tree-level by seesaw mechanisms involving fermionic singlets/triplets or scalar triplets. Dimension-six operators are also explored, as they may allow observable low-energy effects even with suppressed dimension-five operators. Phenomenological consequences are then analyzed, with a focus on charged lepton flavor violating processes and constraints from rare decays.
This document describes an experiment to observe and record the surface plasmon resonance (SPR) curve for a thin metal film. Light from a laser is shone through a glass prism onto the metal film at varying angles of incidence. The intensity of the reflected light is recorded versus the angle to generate the SPR curve. Surface plasmons are quantum phenomena that can be excited at the metal-air interface by photons and decay back into photons. The SPR curve depends on the dielectric constant of the metal film and its thickness. Matching the wavevector of incident light to that of surface plasmons requires increasing the wavevector by passing light through a higher index material like glass before it reaches the metal film.
1) Researchers performed multi-microsecond simulations of a new fast-folding protein domain to study its folding transitions.
2) Analysis of the simulations identified a folded ensemble with time steps matching previous folding events.
3) The researchers analyzed various structural properties to characterize the folded state and folding transitions between folded and unfolded conformations.
Investigation of Steady-State Carrier Distribution in CNT Porins in Neuronal ...Kyle Poe
In this work, the carrier distribution of a carbon nanotube inserted into the spinal ganglion neuronal membrane is examined. After primary characterization based on previous work, the nanotube is approximated as a one-dimensional system, and the Poisson and Schrödinger equations are solved using an iterative finite-difference scheme. It was found that carriers aggregate near the center of the tube, with a negative carrier density of ⟨ρn⟩ = 7.89 × 10^13 cm−3 and positive carrier density of ⟨ρp⟩ = 3.85 × 10^13 cm−3. In future work, the erratic behavior of convergence will be investigated.
2014 NJP - Oscillatory solitons and time-resolved phase locking of two polari...Guilherme Tosi
This document summarizes time-resolved measurements of two polariton condensates formed in a semiconductor microcavity under nonresonant excitation. The measurements directly observe oscillatory behavior as dark or bright soliton-like waves form between the excitation spots. They also observe phase locking of the two initially independent condensates over time. These phenomena provide insights into the underlying dynamics of polariton-polariton interactions and propagation of polariton condensates.
1. The document proposes a new experiment involving entanglement and gravitational decoherence using a dual Mach-Zehnder interferometer setup.
2. In the proposed experiment, one interferometer would be placed in a gravitational field, while the other would not. This would result in non-unitary evolution and allow for nonlocal signaling between the two locations.
3. By moving his interferometer in and out of the gravitational field, one experimenter could encode and transmit binary messages to the other, who could decode the messages by observing changes in interference patterns resulting from the non-unitary evolution.
This document summarizes research on two-dimensional solid-state nutation NMR experiments for determining quadrupole parameters of half-integer quadrupolar nuclei. It presents:
1) A complete series of simulated nutation spectra for spins I = 3/2 to I = 5 calculated using density matrix formalism to serve as fingerprints for parameter determination.
2) Applications of the method to 27Al in spodumene and 45Sc in Sc2(SO4)3 to determine their quadrupole parameters by comparing experimental spectra to simulations.
3) Discussion of experimental aspects like resonance offset and magic angle spinning and how they affect the nutation spectra.
This document analyzes the nonlinear interaction of an elliptical laser beam with collisional plasma. It sets up and solves nonlinear differential equations for the semi-major and semi-minor axes of the elliptical beam as it propagates through the plasma. The analysis shows that increasing the plasma density or decreasing the absorption coefficient increases the extent of self-focusing of the laser beam. Higher plasma densities and lower absorption coefficients result in greater dominance of the nonlinear refractive term over the diffractive term, leading to stronger self-focusing.
Dynamics of Twointeracting Electronsinthree-Dimensional LatticeIOSR Journals
The physical property of strongly correlated electrons on a three-dimensional (3D) 3 x 3 x 3 cluster of the simple cubic lattice is here presented.In the work we developed the unit step Hamiltonian as a solution to the single band Hubbard Hamiltonian for the case of two electrons interaction in a finite three dimensional lattice. The approximation to the Hubbard Hamiltonian study is actually necessary because of the strong limitation and difficulty pose by the Hubbard Hamiltonian as we move away from finite - size lattices to larger N - dimensional lattices. Thus this work has provided a means of overcoming the finite - size lattice defects as we pass on to a higher dimension. We have shown in this study, that the repulsive Coulomb interaction which in part leads to the strong electronic correlations, would indicate that the two electron system prefer not to condense into s-wave superconducting singlet state (s = 0), at high positive values of the interaction strength. This study reveals that when the Coulomb interaction is zero, that is, for free electron system (non-interacting), thevariational parameters which describe the probability distribution of lattice electron system is the same. The spectra intensity for on-site electrons is zero for all values of the interaction strength
This document summarizes a research paper that studied the quantum criticality of a two-channel pseudogap Anderson model. The paper investigates the quantum phase transition from a two-channel Kondo phase to a local moment phase using the non-crossing approximation and numerical renormalization group approaches. It finds novel power-law scalings in the linear and non-linear conductance at the quantum critical point. The scaling behaviors are distinct between equilibrium and non-equilibrium conditions, providing insights into non-equilibrium quantum criticality.
This document summarizes a research article that develops a theoretical model to describe how decoherence effects rubidium vapor in an electromagnetically induced transparency (EIT) experiment. The model accounts for decoherence from both dephasing and population relaxation. It quantifies the impact of decoherence on various experimental measurements, including Faraday rotation, susceptibility, transmission, and coherence relationships. The model is in good agreement with previous experimental results. It also discusses how the model could be applied to other EIT-based experiments and how Faraday rotation could be used to detect single atoms.
This document provides an overview of Coherent X-ray Diffraction Imaging (CXDI) and its application to nanostructures. CXDI allows imaging of a sample without using lenses by measuring the diffraction pattern and reconstructing the image using iterative phase retrieval algorithms. The document discusses coherent scattering from finite size crystals, partially coherent illumination, and experimental examples of CXDI for studying crystalline structures at the nanoscale.
Dynamical dark energy in light of the latest observationsSérgio Sacani
A flat Friedmann–Robertson–Walker universe dominated by a cosmological constant (Λ) and cold dark matter (CDM) has been the working model preferred by cosmologists since the discovery of cosmic acceleration1,2. However, tensions of various degrees of significance are known to be present among existing datasets within the ΛCDM framework3–11. In particular, the Lyman-α forest measurement of the baryon acoustic oscillations (BAO) by the Baryon Oscillation Spectroscopic Survey3 prefers a smaller value of the matter density fraction ΩM than that preferred by cosmic microwave background (CMB). Also, the recently measured value of the Hubble constant, H0 = 73.24 ± 1.74 km s−1 Mpc−1 (ref. 12), is 3.4σ higher than the 66.93 ± 0.62 km s−1 Mpc−1 inferred from the Planck CMB data7. In this work, we investigate whether these tensions can be interpreted as evidence for a non-constant dynamical dark energy. Using the Kullback–Leibler divergence13 to quantify the tension between datasets, we find that the tensions are relieved by an evolving dark energy, with the dynamical dark energy model preferred at a 3.5σ significance level based on the improvement in the fit alone. While, at present, the Bayesian evidence for the dynamical dark energy is insufficient to favour it over ΛCDM, we show that, if the current best-fit dark energy happened to be the true model, it would be decisively detected by the upcoming Dark Energy Spectroscopic Instrument survey14.
This document proposes that dark energy may be an artifact of the free energy required to encode classical information about quantum systems into the environment. It applies the framework of quantum Darwinism to model how the positions of stars are encoded in the ambient photon field. Assuming Landauer's principle, encoding the positions of 10^25 stars with 10 km resolution requires a free energy equivalent to the observed dark energy density. Finer encodings would require much higher energies inconsistent with observations. Thus dark energy may represent the cost of encoding classical information rather than a property of empty space.
The document describes a project analyzing clustering of galaxies using the Shapley Galaxy dataset. The dataset contains information on 4215 galaxies. The author applies hierarchical clustering algorithms and Gaussian mixture modeling to determine the optimal number of clusters in the data. Single, complete, and average linkage clustering are applied but do not clearly show clustering structure. Gaussian mixture modeling with the EM algorithm and BIC criterion indicates the best fit model has 8 clusters. Scatter plots are presented for solutions with 1 to 8 clusters.
1. The document estimates the maximum intensity of dark matter glow based on Dr. Hewett's Time Symmetric Cosmology theory, which predicts dark matter is a boson resulting from Hawking evaporation of primordial black holes.
2. Using classical approximations, the author estimates the total photon intensity of the Andromeda galaxy's dark matter halo would be 4.67x1024, far greater than its visible light intensity of 3.19x109.
3. However, the author acknowledges the model's assumptions and approximations likely overestimate the intensity by many orders of magnitude, and more rigorous modeling is needed to test if dark matter glow could be detectable.
Artigo que descreve o trabalho feito com o Chandra nos aglomerados de galáxias de Perseus e Virgo sobre a descoberta de uma turbulência cósmica que impede a formação de novas estrelas.
Supersymmetry (SUSY) is a proposed symmetry between bosons and fermions that could help solve issues in the Standard Model such as the hierarchy problem. SUSY introduces new "quantum" dimensions beyond the usual 3 spatial and 1 time dimension. SUSY generators called Q transform fermions into bosons and vice versa. The SUSY algebra involves the generators Q satisfying anticommutation relations in addition to the usual commutation relations of generators like momentum P and angular momentum M. While experimental evidence for SUSY is still lacking, it is an attractive theoretical idea that may be discovered at energy scales below 1 TeV.
This document summarizes observations of the W49 giant molecular cloud (GMC) using the PMO 14m telescope and the Submillimeter Array (SMA). The PMO observations mapped the entire GMC in various molecular lines at scales up to 113 pc, while the SMA mosaic mapped the central star-forming region W49N at scales down to 0.5 pc. The observations are used to derive the mass structure of the GMC across all scales. The main findings are that the W49 GMC has a total gas mass of 1.1 million solar masses within 60 pc and 2x10^5 solar masses within 6 pc. The mass is distributed in a hierarchical network of filaments converging toward the central
The document summarizes findings from the Microwave Instrument on the Rosetta Orbiter (MIRO) regarding the subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko. Key points:
- MIRO detected water vapor emissions from the comet beginning in early June 2014 and measured the total water production rate, which varied from 0.3 kg/s to 1.2 kg/s between June and August.
- Water outgassing displayed periodic variations correlated with the comet's 12.4-hour rotation period and seemed to originate primarily from the comet's "neck" region.
- Subsurface temperatures measured by MIRO showed seasonal and diurnal variations, indicating radiation
This document describes an experiment to observe and record the surface plasmon resonance (SPR) curve for a thin metal film. Light from a laser is shone through a glass prism onto the metal film at varying angles of incidence. The intensity of the reflected light is recorded versus the angle to generate the SPR curve. Surface plasmons are quantum phenomena that can be excited at the metal-air interface by photons and decay back into photons. The SPR curve depends on the dielectric constant of the metal film and its thickness. Matching the wavevector of incident light to that of surface plasmons requires increasing the wavevector by passing light through a higher index material like glass before it reaches the metal film.
1) Researchers performed multi-microsecond simulations of a new fast-folding protein domain to study its folding transitions.
2) Analysis of the simulations identified a folded ensemble with time steps matching previous folding events.
3) The researchers analyzed various structural properties to characterize the folded state and folding transitions between folded and unfolded conformations.
Investigation of Steady-State Carrier Distribution in CNT Porins in Neuronal ...Kyle Poe
In this work, the carrier distribution of a carbon nanotube inserted into the spinal ganglion neuronal membrane is examined. After primary characterization based on previous work, the nanotube is approximated as a one-dimensional system, and the Poisson and Schrödinger equations are solved using an iterative finite-difference scheme. It was found that carriers aggregate near the center of the tube, with a negative carrier density of ⟨ρn⟩ = 7.89 × 10^13 cm−3 and positive carrier density of ⟨ρp⟩ = 3.85 × 10^13 cm−3. In future work, the erratic behavior of convergence will be investigated.
2014 NJP - Oscillatory solitons and time-resolved phase locking of two polari...Guilherme Tosi
This document summarizes time-resolved measurements of two polariton condensates formed in a semiconductor microcavity under nonresonant excitation. The measurements directly observe oscillatory behavior as dark or bright soliton-like waves form between the excitation spots. They also observe phase locking of the two initially independent condensates over time. These phenomena provide insights into the underlying dynamics of polariton-polariton interactions and propagation of polariton condensates.
1. The document proposes a new experiment involving entanglement and gravitational decoherence using a dual Mach-Zehnder interferometer setup.
2. In the proposed experiment, one interferometer would be placed in a gravitational field, while the other would not. This would result in non-unitary evolution and allow for nonlocal signaling between the two locations.
3. By moving his interferometer in and out of the gravitational field, one experimenter could encode and transmit binary messages to the other, who could decode the messages by observing changes in interference patterns resulting from the non-unitary evolution.
This document summarizes research on two-dimensional solid-state nutation NMR experiments for determining quadrupole parameters of half-integer quadrupolar nuclei. It presents:
1) A complete series of simulated nutation spectra for spins I = 3/2 to I = 5 calculated using density matrix formalism to serve as fingerprints for parameter determination.
2) Applications of the method to 27Al in spodumene and 45Sc in Sc2(SO4)3 to determine their quadrupole parameters by comparing experimental spectra to simulations.
3) Discussion of experimental aspects like resonance offset and magic angle spinning and how they affect the nutation spectra.
This document analyzes the nonlinear interaction of an elliptical laser beam with collisional plasma. It sets up and solves nonlinear differential equations for the semi-major and semi-minor axes of the elliptical beam as it propagates through the plasma. The analysis shows that increasing the plasma density or decreasing the absorption coefficient increases the extent of self-focusing of the laser beam. Higher plasma densities and lower absorption coefficients result in greater dominance of the nonlinear refractive term over the diffractive term, leading to stronger self-focusing.
Dynamics of Twointeracting Electronsinthree-Dimensional LatticeIOSR Journals
The physical property of strongly correlated electrons on a three-dimensional (3D) 3 x 3 x 3 cluster of the simple cubic lattice is here presented.In the work we developed the unit step Hamiltonian as a solution to the single band Hubbard Hamiltonian for the case of two electrons interaction in a finite three dimensional lattice. The approximation to the Hubbard Hamiltonian study is actually necessary because of the strong limitation and difficulty pose by the Hubbard Hamiltonian as we move away from finite - size lattices to larger N - dimensional lattices. Thus this work has provided a means of overcoming the finite - size lattice defects as we pass on to a higher dimension. We have shown in this study, that the repulsive Coulomb interaction which in part leads to the strong electronic correlations, would indicate that the two electron system prefer not to condense into s-wave superconducting singlet state (s = 0), at high positive values of the interaction strength. This study reveals that when the Coulomb interaction is zero, that is, for free electron system (non-interacting), thevariational parameters which describe the probability distribution of lattice electron system is the same. The spectra intensity for on-site electrons is zero for all values of the interaction strength
This document summarizes a research paper that studied the quantum criticality of a two-channel pseudogap Anderson model. The paper investigates the quantum phase transition from a two-channel Kondo phase to a local moment phase using the non-crossing approximation and numerical renormalization group approaches. It finds novel power-law scalings in the linear and non-linear conductance at the quantum critical point. The scaling behaviors are distinct between equilibrium and non-equilibrium conditions, providing insights into non-equilibrium quantum criticality.
This document summarizes a research article that develops a theoretical model to describe how decoherence effects rubidium vapor in an electromagnetically induced transparency (EIT) experiment. The model accounts for decoherence from both dephasing and population relaxation. It quantifies the impact of decoherence on various experimental measurements, including Faraday rotation, susceptibility, transmission, and coherence relationships. The model is in good agreement with previous experimental results. It also discusses how the model could be applied to other EIT-based experiments and how Faraday rotation could be used to detect single atoms.
This document provides an overview of Coherent X-ray Diffraction Imaging (CXDI) and its application to nanostructures. CXDI allows imaging of a sample without using lenses by measuring the diffraction pattern and reconstructing the image using iterative phase retrieval algorithms. The document discusses coherent scattering from finite size crystals, partially coherent illumination, and experimental examples of CXDI for studying crystalline structures at the nanoscale.
Dynamical dark energy in light of the latest observationsSérgio Sacani
A flat Friedmann–Robertson–Walker universe dominated by a cosmological constant (Λ) and cold dark matter (CDM) has been the working model preferred by cosmologists since the discovery of cosmic acceleration1,2. However, tensions of various degrees of significance are known to be present among existing datasets within the ΛCDM framework3–11. In particular, the Lyman-α forest measurement of the baryon acoustic oscillations (BAO) by the Baryon Oscillation Spectroscopic Survey3 prefers a smaller value of the matter density fraction ΩM than that preferred by cosmic microwave background (CMB). Also, the recently measured value of the Hubble constant, H0 = 73.24 ± 1.74 km s−1 Mpc−1 (ref. 12), is 3.4σ higher than the 66.93 ± 0.62 km s−1 Mpc−1 inferred from the Planck CMB data7. In this work, we investigate whether these tensions can be interpreted as evidence for a non-constant dynamical dark energy. Using the Kullback–Leibler divergence13 to quantify the tension between datasets, we find that the tensions are relieved by an evolving dark energy, with the dynamical dark energy model preferred at a 3.5σ significance level based on the improvement in the fit alone. While, at present, the Bayesian evidence for the dynamical dark energy is insufficient to favour it over ΛCDM, we show that, if the current best-fit dark energy happened to be the true model, it would be decisively detected by the upcoming Dark Energy Spectroscopic Instrument survey14.
This document proposes that dark energy may be an artifact of the free energy required to encode classical information about quantum systems into the environment. It applies the framework of quantum Darwinism to model how the positions of stars are encoded in the ambient photon field. Assuming Landauer's principle, encoding the positions of 10^25 stars with 10 km resolution requires a free energy equivalent to the observed dark energy density. Finer encodings would require much higher energies inconsistent with observations. Thus dark energy may represent the cost of encoding classical information rather than a property of empty space.
The document describes a project analyzing clustering of galaxies using the Shapley Galaxy dataset. The dataset contains information on 4215 galaxies. The author applies hierarchical clustering algorithms and Gaussian mixture modeling to determine the optimal number of clusters in the data. Single, complete, and average linkage clustering are applied but do not clearly show clustering structure. Gaussian mixture modeling with the EM algorithm and BIC criterion indicates the best fit model has 8 clusters. Scatter plots are presented for solutions with 1 to 8 clusters.
1. The document estimates the maximum intensity of dark matter glow based on Dr. Hewett's Time Symmetric Cosmology theory, which predicts dark matter is a boson resulting from Hawking evaporation of primordial black holes.
2. Using classical approximations, the author estimates the total photon intensity of the Andromeda galaxy's dark matter halo would be 4.67x1024, far greater than its visible light intensity of 3.19x109.
3. However, the author acknowledges the model's assumptions and approximations likely overestimate the intensity by many orders of magnitude, and more rigorous modeling is needed to test if dark matter glow could be detectable.
Artigo que descreve o trabalho feito com o Chandra nos aglomerados de galáxias de Perseus e Virgo sobre a descoberta de uma turbulência cósmica que impede a formação de novas estrelas.
Supersymmetry (SUSY) is a proposed symmetry between bosons and fermions that could help solve issues in the Standard Model such as the hierarchy problem. SUSY introduces new "quantum" dimensions beyond the usual 3 spatial and 1 time dimension. SUSY generators called Q transform fermions into bosons and vice versa. The SUSY algebra involves the generators Q satisfying anticommutation relations in addition to the usual commutation relations of generators like momentum P and angular momentum M. While experimental evidence for SUSY is still lacking, it is an attractive theoretical idea that may be discovered at energy scales below 1 TeV.
This document summarizes observations of the W49 giant molecular cloud (GMC) using the PMO 14m telescope and the Submillimeter Array (SMA). The PMO observations mapped the entire GMC in various molecular lines at scales up to 113 pc, while the SMA mosaic mapped the central star-forming region W49N at scales down to 0.5 pc. The observations are used to derive the mass structure of the GMC across all scales. The main findings are that the W49 GMC has a total gas mass of 1.1 million solar masses within 60 pc and 2x10^5 solar masses within 6 pc. The mass is distributed in a hierarchical network of filaments converging toward the central
The document summarizes findings from the Microwave Instrument on the Rosetta Orbiter (MIRO) regarding the subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko. Key points:
- MIRO detected water vapor emissions from the comet beginning in early June 2014 and measured the total water production rate, which varied from 0.3 kg/s to 1.2 kg/s between June and August.
- Water outgassing displayed periodic variations correlated with the comet's 12.4-hour rotation period and seemed to originate primarily from the comet's "neck" region.
- Subsurface temperatures measured by MIRO showed seasonal and diurnal variations, indicating radiation
- The document derives the second order Friedmann equations from the quantum corrected Raychaudhuri equation (QRE), which includes quantum corrections terms.
- One correction term can be interpreted as dark energy/cosmological constant with the observed density value, providing an explanation for the coincidence problem.
- The other correction term can be interpreted as a radiation term in the early universe that prevents the formation of a big bang singularity and predicts an infinite age for the universe by avoiding a divergence in the Hubble parameter or its derivative at any finite time in the past.
The herschel view_of_massive_star_formation_in_dense_and_cold_filament_w48Sérgio Sacani
The Herschel Space Observatory observed the IRDC filament G035.39–00.33 in the W48 molecular cloud complex. The observations revealed 28 compact dense cores, 13 of which have masses greater than 20 solar masses. These massive dense cores are excellent candidates to form intermediate- to high-mass stars. Most of the massive dense cores are located within the G035.39–00.33 filament and contain infrared-quiet high-mass protostars. The large number of protostars suggests a "mini-burst" of star formation is occurring within the filament, with an efficiency of about 15% and a formation rate of around 40 solar masses per year per square kiloparsec. Some extended Si
An ultraluminous quasar_with_a_twelve_billion_solar_mass_black_hole_at_redshi...Sérgio Sacani
1) Researchers discovered an ultraluminous quasar, SDSS J010013.021280225.8, at a redshift of 6.30, making it the most distant and luminous quasar known.
2) Spectral analysis estimates the black hole at its center has a mass of 1.2 billion solar masses, consistent with an Eddington-limited accretion rate.
3) The quasar has an ionized proximity zone estimated to be 26 million light years across, significantly larger than found for other high-redshift quasars, suggesting its high luminosity.
Detectcion of noble_gas_molecular_ion_arh_in_the_crab_nebulaSérgio Sacani
Scientists detected emission lines from the ionized argon hydride (36ArH+) molecule in spectra of the Crab Nebula obtained with the Herschel Space Observatory. The detection of 36ArH+ confirms that argon originated from explosive nucleosynthesis during the core-collapse supernova that created the Crab Nebula. The likely excitation mechanism is electron collisions in partially ionized regions with electron densities of a few hundred per cubic centimeter. This is the first detection of a noble gas molecule in space.
The most luminous_galaxies_discovered_by_wiseSérgio Sacani
This document presents a sample of 20 extremely luminous galaxies discovered by the Wide-field Infrared Survey Explorer (WISE). Five of these galaxies have infrared luminosities exceeding 1014 solar luminosities, the highest infrared luminosity threshold yet observed. They were selected using criteria requiring weak or no detection in the first two WISE bands but strong detections in the third and fourth bands. Spectral energy distribution modeling suggests their high luminosities are powered by obscured active galactic nuclei with hot dust temperatures around 450 Kelvin. The existence of such luminous galaxies at redshifts above 3 provides constraints on the early growth of supermassive black holes through rapid accretion.
This document summarizes a study on Mercury's shrinking surface over geologic time. The study finds that Mercury has contracted by at least 5-7 km in radius over the past 4 billion years due to cooling and the solidification of its large iron core. Global imaging by the MESSENGER spacecraft revealed numerous compressional features across Mercury's entire surface, including lobate scarps and wrinkle ridges, indicating substantial surface area loss from contraction. This degree of contraction is consistent with thermal evolution models and suggests Mercury has shrunk more than previously estimated based on earlier Mariner 10 imaging of only 45% of the surface.
Assymetries in core_collapse_supernovae_from_maps_of_radioactive_ti_in_cassio...Sérgio Sacani
The document summarizes findings from observations of Cassiopeia A using the Nuclear Spectroscopic Telescope Array (NuSTAR).
1) NuSTAR detected two clear emission lines from the decay of radioactive titanium-44, confirming previous measurements of titanium-44 yield with high significance. The spatial distribution of titanium-44 emission shows it is clumpy and extended along the jet axis seen in X-ray images, with knots off the jet axis.
2) There is no correlation between the distribution of titanium-44 and iron detected by Chandra X-ray Observatory. This suggests much of the iron-rich ejecta has not been shock-heated and is "invisible", constraining models of the remnant.
A vlt flames_survey_for_massive_binaries_in_westerlund_1Sérgio Sacani
1) The authors conducted a radial velocity survey of stars in the young massive cluster Westerlund 1 to search for a potential pre-supernova companion to the magnetar CXO J1647-10.2-455216 located within the cluster.
2) They identified a candidate star, Wd1-5, that has anomalous velocities compared to other stars in the cluster, suggesting it was impacted by the supernova that created the magnetar.
3) Analysis of Wd1-5 found evidence of chemical enrichment that is difficult to explain by single star evolution, but could be explained if Wd1-5 was once part of a close binary system where it accreted material from
Extensive Noachian fluvial systems in Arabia Terra: Implications for early Ma...Sérgio Sacani
Valley networks are some of the strongest lines of evidence for
extensive fluvial activity on early (Noachian; >3.7 Ga) Mars. However,
their purported absence on certain ancient terrains, such as
Arabia Terra, is at variance with patterns of precipitation as predicted
by “warm and wet” climate models. This disagreement has contributed
to the development of an alternative “icy highlands” scenario,
whereby valley networks were formed by the melting of highland ice
sheets. Here, we show through regional mapping that Arabia Terra
shows evidence for extensive networks of sinuous ridges. We interpret
these ridge features as inverted fluvial channels that formed in
the Noachian, before being subject to burial and exhumation. The
inverted channels developed on extensive aggrading flood plains. As
the inverted channels are both sourced in, and traverse across, Arabia
Terra, their formation is inconsistent with discrete, localized sources
of water, such as meltwater from highland ice sheets. Our results are
instead more consistent with an early Mars that supported widespread
precipitation and runoff.
Alma observations of_feed_and_feedback_in_nearby_seyfert_galaxiesSérgio Sacani
The ALMA observations of NGC 1433 reveal a nuclear gaseous spiral structure within the central kpc. This spiral winds up into a pseudo-ring at ~200 pc from the center. Near the nucleus, there is intense high-velocity CO emission up to 200 km/s that is interpreted as an outflow, involving 3.6 million solar masses of molecular gas and a flow rate of ~7 solar masses per year. The outflow could be driven by both the central star formation and AGN through its radio jets. Continuum emission at 0.87 mm is detected only at the very center and likely comes from thermal dust emission from the molecular torus expected in this Seyfert 2 galaxy.
This document discusses evidence that the Moon-forming impact occurred later than previously thought, at around 95 million years after the formation of the solar system. The study uses simulations of planetary formation to show a correlation between the timing of the last giant impact and the amount of mass later accreted by the planet. Comparing this to highly siderophile element abundances in Earth's mantle, which constrain the amount of late-accreted mass, the study determines the Moon-forming impact was most likely 95 million years after solar system formation. Earlier times of 40 million years or less are ruled out at a 99.9% confidence level. The simulations include both classical scenarios and scenarios where Jupiter and Saturn migrated inward early in the solar
A terrestrial planet_candidate_in_a_temperate_orbit_around_proxima_centauriSérgio Sacani
At a distance of 1.295 parsecs,1 the red-dwarf Proxima Centauri (α Centauri C, GL 551,
HIP 70890, or simply Proxima) is the Sun’s closest stellar neighbour and one of the best studied
low-mass stars. It has an effective temperature of only 3050 K, a luminosity of 0.1 per
cent solar, a measured radius of 0.14 R⊙
2 and a mass of about 12 per cent the mass of the
Sun. Although Proxima is considered a moderately active star, its rotation period is 83
days,3 and its quiescent activity levels and X-ray luminosity4 are comparable to the Sun’s. New
observations reveal the presence of a small planet orbiting Proxima with a minimum mass of
1.3 Earth masses and an orbital period of 11.2 days. Its orbital semi-major axis is 0.05 AU,
with an equilibrium temperature in the range where water could be liquid on its surface.5
The habitability of Proxima Centauri b - I. Irradiation, rotation and volatil...Sérgio Sacani
Proxima b is a planet with a minimum mass of 1.3 M⊕ orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass,
active star and the Sun’s closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b
and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet
and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time
evolution of the star’s spectrum, which is essential for modeling the flux received over Proxima b’s lifetime. We also show that Proxima
b’s obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet’s eccentricity and
level of triaxiality. Next we consider the evolution of Proxima b’s water inventory. We use our spectral energy distribution to compute
the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find
that Proxima b is likely to have lost less than an Earth ocean’s worth of hydrogen (EOH) before it reached the HZ 100–200 Myr after
its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We
conclude that Proxima b is a viable candidate habitable planet.
The 19 Feb. 2016 Outburst of Comet 67P/CG: An ESA Rosetta Multi-Instrument StudySérgio Sacani
On 19 Feb. 2016 nine Rosetta instruments serendipitously observed an outburst of gas and dust
from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras
and spectrometers ranging from UV over visible to microwave wavelengths, in-situ gas, dust and
plasma instruments, and one dust collector. At 9:40 a dust cloud developed at the edge of an image
in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature
of the outburst that signicantly exceeded the background. The enhancement ranged from 50% of
the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus.
Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest
enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3
and consequently the spacecraft potential changed from 16V to 20V during the outburst. A
clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15
minutes the Star Tracker camera detected fast particles ( 25 ms 1) while 100 m radius particles
were detected by the GIADA dust instrument 1 hour later at a speed of 6 ms 1. The slowest
were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst
originated just outside the FOV of the instruments, the source region and the magnitude of the
outburst could be determined.
Radial velocity monitoring has found the signature of a Msin i = 1:3 M planet located within the Habitable Zone of Proxima
Centauri, the Sun’s closest neighbor (Anglada-Escudé et al. 2016). Despite a hotter past and an active host star the planet Proxima b
could have retained enough volatiles to sustain surface habitability (Ribas et al. 2016). Here we use a 3D Global Climate Model (GCM)
to simulate Proxima b’s atmosphere and water cycle for its two likely rotation modes (the 1:1 and 3:2 spin-orbit resonances) while
varying the unconstrained surface water inventory and atmospheric greenhouse eect (represented here with a CO2-N2 atmosphere.)
We find that a broad range of atmospheric compositions can allow surface liquid water. On a tidally-locked planet with a surface water
inventory larger than 0.6 Earth ocean, liquid water is always present (assuming 1 bar of N2), at least in the substellar region. Liquid
water covers the whole planet for CO2 partial pressures & 1 bar. For smaller water inventories, water can be trapped on the night side,
forming either glaciers or lakes, depending on the amount of greenhouse gases. With a non-synchronous rotation, a minimum CO2
pressure of 10 mbar (assuming 1 bar of N2) is required to avoid falling into a completely frozen snowball state if water is abundant.
If the planet is dryer, 0.5 bar of CO2 would suce to prevent the trapping of any arbitrary small water inventory into polar ice
caps. More generally, any low-obliquity planet within the classical habitable zone of its star should be in one of the climate regimes
discussed here.
We use our GCM to produce reflection/emission spectra and phase curves for the dierent rotations and surface volatile inventories.
We find that atmospheric characterization will be possible by direct imaging with forthcoming large telescopes thanks to an angular
separation of 7=D at 1 m (with the E-ELT) and a contrast of 10 7. The magnitude of the planet will allow for high-resolution
spectroscopy and the search for molecular signatures, including H2O, O2, and CO2.
The observation of thermal phase curves, although challenging, can be attempted with JWST, thanks to a contrast of 210 5 at 10 m.
Proxima b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima b and
possibly determine whether this exoplanet’s surface is habitable.
SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE...Sérgio Sacani
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster,
using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these “ultradiffuse
galaxies” (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption
features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz=6280±120 km s−1 is within the 1σ velocity dispersion of the Coma cluster. The galaxy has an effective
radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ∼2re. We show that UDGs are located in a previously
sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter
than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent
galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are
needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same
luminosity or to those of galaxies with the same size.
Forming intracluster gas in a galaxy protocluster at a redshift of 2.16Sérgio Sacani
Galaxy clusters are the most massive gravitationally bound structures in the Universe, comprising thousands of galaxies and
pervaded by a diffuse, hot “intracluster medium” (ICM) that dominates the baryonic content of these systems. The formation
and evolution of the ICM across cosmic time1
is thought to be driven by the continuous accretion of matter from the large-scale
filamentary surroundings and dramatic merger events with other clusters or groups. Until now, however, direct observations of
the intracluster gas have been limited only to mature clusters in the latter three-quarters of the history of the Universe, and we
have been lacking a direct view of the hot, thermalized cluster atmosphere at the epoch when the first massive clusters formed.
Here we report the detection (about 6σ) of the thermal Sunyaev-Zeldovich (SZ) effect2
in the direction of a protocluster. In fact,
the SZ signal reveals the ICM thermal energy in a way that is insensitive to cosmological dimming, making it ideal for tracing
the thermal history of cosmic structures3
. This result indicates the presence of a nascent ICM within the Spiderweb protocluster
at redshift z = 2.156, around 10 billion years ago. The amplitude and morphology of the detected signal show that the SZ
effect from the protocluster is lower than expected from dynamical considerations and comparable with that of lower-redshift
group-scale systems, consistent with expectations for a dynamically active progenitor of a local galaxy cluster.
Solar abundance ratios of the iron-peak elements in the Perseus clusterSérgio Sacani
The metal abundance of the hot plasma that permeates galaxy
clusters represents the accumulation of heavy elements produced
by billions of supernovae1. Therefore, X-ray spectroscopy of the
intracluster medium provides an opportunity to investigate the
nature of supernova explosions integrated over cosmic time. In
particular, the abundance of the iron-peak elements (chromium,
manganese, iron and nickel) is key to understanding how the
progenitors of typical type Ia supernovae evolve and explode2–6.
Recent X-ray studies of the intracluster medium found that the
abundance ratios of these elements differ substantially from those
seen in the Sun7–11, suggesting differences between the nature of type
Ia supernovae in the clusters and in the Milky Way. However, because
the K-shell transition lines of chromium and manganese are weak
and those of iron and nickel are very close in photon energy, highresolution
spectroscopy is required for an accurate determination
of the abundances of these elements. Here we report observations
of the Perseus cluster, with statistically significant detections of the
resonance emission from chromium, manganese and nickel. Our
measurements, combined with the latest atomic models, reveal that
these elements have near-solar abundance ratios with respect to iron,
in contrast to previous claims. Comparison between our results and
modern nucleosynthesis calculations12–14 disfavours the hypothesis
that type Ia supernova progenitors are exclusively white dwarfs with
masses well below the Chandrasekhar limit (about 1.4 times the
mass of the Sun). The observed abundance pattern of the iron-peak
elements can be explained by taking into account a combination
of near- and sub-Chandrasekhar-mass type Ia supernova systems,
adding to the mounting evidence that both progenitor types make a
substantial contribution to cosmic chemical enrichment5,15,16.
The first X-ray look at SMSS J114447.77-430859.3: the most luminous quasar in...Sérgio Sacani
SMSS J114447.77-430859.3 (z = 0.83) has been identified in the SkyMapper Southern Survey as the most luminous quasar in
the last ∼ 9 Gyr . In this paper, we report on the eROSITA/Spectrum–Roentgen–Gamma (SRG) observations of the source from
the eROSITA All Sky Survey, along with presenting results from recent monitoring performed using Swift, XMM-Newton, and
NuSTAR. The source shows a clear variability by factors of ∼10 and ∼2.7 overtime-scales of a year and of a few days,respectively.
When fit with an absorbed power law plus high-energy cutoff, the X-ray spectra reveal a = 2.2 ± 0.2 and Ecut = 23+26
−5 keV
. Assuming Comptonization, we estimate a coronal optical depth and electron temperature of τ = 2.5 − 5.3 (5.2 − 8) and
kT = 8 − 18 (7.5 − 14) keV , respectively, for a slab (spherical) geometry. The broadband SED is successfully modelled by
assuming either a standard accretion disc illuminated by a central X-ray source, or a thin disc with a slim disc emissivity profile.
The former model results in a black hole mass estimate of the order of 1010 M , slightly higher than prior optical estimates;
meanwhile, the latter model suggests a lower mass. Both models suggest sub-Eddington accretion when assuming a spinning
black hole, and a compact (∼ 10 rg ) X-ray corona. The measured intrinsic column density and the Eddington ratio strongly
suggest the presence of an outflow driven by radiation pressure. This is also supported by variation of absorption by an order of
magnitude over the period of ∼ 900 d .
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
This document analyzes the gamma-ray signal from the central Milky Way that is consistent with emission from annihilating dark matter particles. The authors re-examine Fermi data using cuts on an event parameter to improve gamma-ray maps and more easily separate components. They find the GeV excess is robust and well-fit by a 36-51 GeV dark matter particle annihilating to bottom quarks with a cross section of 1-3×10−26 cm3/s. The signal extends over 10 degrees from the Galactic Center and is spherically symmetric, disfavoring explanations from millisecond pulsars or gas interactions.
Dissecting x ray_emitting_gas_around_the_center_of_our_galaxySérgio Sacani
1) The Chandra X-ray Observatory was used to observe the supermassive black hole at the center of the Milky Way, Sgr A*, for a total of 3 megaseconds.
2) The observations revealed extended X-ray emission around Sgr A* that aligns spatially with a surrounding disk of massive stars.
3) Spectral analysis ruled out low-mass stars as the origin of the X-ray emission and instead found evidence that the emission is from a radiatively inefficient accretion flow onto the black hole, with an outflow present.
Discovery of a radiation component from the Vela pulsar reaching 20 teraelect...Sérgio Sacani
Gamma-ray observations have established energetic isolated pulsars as
outstanding particle accelerators and antimatter factories. However, many
questions are still open regarding the acceleration and radiation processes
involved, as well as the locations where they occur. The radiation spectra of
all gamma-ray pulsars observed to date show strong cutofs or a break above
energies of a few gigaelectronvolts. Using the High Energy Stereoscopic
System’s Cherenkov telescopes, we discovered a radiation component from
the Vela pulsar which emerges beyond this generic cutof and extends up to
energies of at least 20 teraelectronvolts. This is an order of magnitude larger
than in the case of the Crab pulsar, the only other pulsar detected in the
teraelectronvolt energy range. Our results challenge the state-of-the-art
models for the high-energy emission of pulsars. Furthermore, they pave
the way for investigating other pulsars through their multiteraelectronvolt
emission, thereby imposing additional constraints on the acceleration and
emission processes in their extreme energy limit.
A density cusp of quiescent X-ray binaries in the central parsec of the GalaxySérgio Sacani
The existence of a ‘density cusp’1,2—a localized increase in
number—of stellar-mass black holes near a supermassive black
hole is a fundamental prediction of galactic stellar dynamics3
. The
best place to detect such a cusp is in the Galactic Centre, where
the nearest supermassive black hole, Sagittarius A*, resides. As
many as 20,000 black holes are predicted to settle into the central
parsec of the Galaxy as a result of dynamical friction3–5; however,
so far no density cusp of black holes has been detected. Low-mass
X-ray binary systems that contain a stellar-mass black hole are
natural tracers of isolated black holes. Here we report observations
of a dozen quiescent X-ray binaries in a density cusp within one
parsec of Sagittarius A*. The lower-energy emission spectra that
we observed in these binaries is distinct from the higher-energy
spectra associated with the population of accreting white dwarfs that
dominates the central eight parsecs of the Galaxy6
. The properties
of these X-ray binaries, in particular their spatial distribution and
luminosity function, suggest the existence of hundreds of binary
systems in the central parsec of the Galaxy and many more isolated
black holes. We cannot rule out a contribution to the observed
emission from a population (of up to about one-half the number of
X-ray binaries) of rotationally powered, millisecond pulsars. The
spatial distribution of the binary systems is a relic of their formation
history, either in the stellar disk around Sagittarius A* (ref. 7) or
through in-fall from globular clusters, and constrains the number
density of sources in the modelling of gravitational waves from
massive stellar remnants8,9
, such as neutron stars and black holes.
A 300 parsec-long jet-inflated bubble around a powerful microquasar in the ga...Sérgio Sacani
This document summarizes the discovery of a 300-parsec-long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793. Chandra X-ray observations revealed an aligned triple X-ray source within the optical nebula S26, consisting of a central core and two hot spots, interpreted as the core of the X-ray binary and where the jets interact with the ambient medium. Spectral analysis found the core has a hard power-law spectrum, while the hot spots have softer thermal plasma emission. The morphology and properties of S26 resemble those of a powerful FRII-type active galaxy, indicating it is powered by collimated jets with mechanical luminosity of ~10^40 er
This document reports on evidence for spatial variation in the fine structure constant α from observations of quasar absorption spectra. A sample of 153 measurements from the ESO Very Large Telescope (VLT) probing a different region of the universe suggests α was larger in the past, opposite to previous findings from the Keck telescope. Combining the two datasets reveals a significant spatial dipole in α, with the maximum variation in the direction of right ascension 17.3 hours, declination -61 degrees. Detailed analysis found no systematic effects that could mimic this result.
A rapidly spinning_supermassive_black_hole_at_the_centre_of_ngc1365Sérgio Sacani
1) XMM-Newton and NuSTAR observed the galaxy NGC 1365 simultaneously over 130 ks, detecting broadband X-ray emission from 3-79 keV.
2) The observations revealed variable absorption below 10 keV and prominent broad emission features between 5-7 keV and 10-30 keV, indicative of relativistic reflection from an accretion disk near a spinning supermassive black hole.
3) Time-resolved spectral analysis was able to disentangle the variable absorption component from the relativistic reflection component. Absorption-dominated models without reflection could be ruled out statistically and on physical grounds.
the paper focuses on the fabrication and characterization of heterostructures using transition metal dichalcogenide (TMDC) monolayers. The authors describe the process of mechanical exfoliation to obtain thin flakes of TMDC material, which are then placed on a viscoelastic polydimethylsiloxane film. These monolayers are subsequently stamped onto a silicon wafer covered with thermal oxide to create heterobilayers .
The paper also discusses the use of ultrafast optical-pump/terahertz-probe near-field microscopy to study these heterostructures. The authors explain that this technique allows them to investigate the electric near fields and scattered fields of the emitted waveforms, as well as the photo-induced polarizability .
The experimental setup involves a high-average-power, low-noise Yb:YAG thin-disc oscillator, which generates terahertz probe pulses through optical rectification of 200-fs-long pulses. These pulses are centered at a wavelength of 1,030 nm and are generated in a gallium phosphide crystal .
The paper likely includes additional details on the experimental procedures, data analysis, and results obtained from the terahertz near-field microscopy experiments. It may also discuss the potential applications and implications of the findings
Resolved imaging confirms a radiation belt around an ultracool dwarfSérgio Sacani
Radiation belts are present in all large-scale Solar System planetary
30 magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune1. These persistent
31 equatorial zones of relativistic particles up to tens of MeV in energy can extend farther
32 than 10 times the planet’s radius, emit gradually varying radio emissions2–4 and impact
33 the surface chemistry of close-in moons5. Recent observations demonstrate that very low
34 mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet35
like radio emissions such as periodically bursting aurorae6–8 from large-scale
36 magnetospheric currents9–11. They also exhibit slowly varying quiescent radio
37 emissions7,12,13 hypothesized to trace low-level coronal flaring14,15 despite departing from
38 empirical multi-wavelength flare relationships8,15. Here we present high resolution
39 imaging of the ultracool dwarf LSR J1835+3259 at 8.4 GHz demonstrating that its
40 quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetric
41 structure similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf
42 radii separate the two lobes, which are stably present in three observations spanning
43 more than one year. For plasma confined by the magnetic dipole of LSR J1835+3259, we
44 estimate 15 MeV electron energies consistent with Jupiter’s radiation belts4. Our results
45 confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16–
46 19 and support broader re-examination of rotating magnetic dipoles in producing non47
thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20, and
4
Wind from the_black_hole_accretion_disk_driving_a_molecular_outflow_in_an_act...Sérgio Sacani
Artigo descreve estudo inédito que mostra que os ventos gerados pelos buracos negros nos centros das galáxias pode acabar com o processo de formação de estrelas nas galáxias hospedeiras.
Effect of a_high_opacity_on_the_light_curves_of_radioactively_powered_transie...Sérgio Sacani
This document discusses how higher opacities from lanthanide elements in the ejecta of neutron star mergers could dramatically affect the predicted light curves of electromagnetic counterparts. The key points are:
1) Ab initio calculations show r-process element opacities are orders of magnitude higher than previously assumed iron opacities, particularly from lanthanide elements.
2) With these higher opacities, radiation transport models predict light curves that are longer (lasting about a week), dimmer, and redder, with emission peaked in the infrared rather than optical/ultraviolet.
3) A two-component light curve may result if there is both lanthanide-rich ejecta and a secondary
1) PSR J033711715 is a millisecond pulsar discovered to be in a hierarchical triple system with two white dwarf companions, making it the first known millisecond pulsar triple system.
2) Precise timing observations using multiple radio telescopes determined the masses of the pulsar (1.4378 solar masses), inner white dwarf companion (0.19751 solar masses), and outer white dwarf companion (0.4101 solar masses) to high precision.
3) The unexpectedly coplanar and nearly circular orbits of the system indicate an exotic evolutionary history and provide an opportunity to test theories of general relativity by studying the interactions between the bodies.
A population of red candidate massive galaxies ~600 Myr after the Big BangSérgio Sacani
Galaxies with stellar masses as high as ~ 1011 solar masses have been identified1–3 out to
33 redshifts z ~ 6, approximately one billion years after the Big Bang. It has been difficult to
34 find massive galaxies at even earlier times, as the Balmer break region, which is needed
35 for accurate mass estimates, is redshifted to wavelengths beyond 2.5 μm. Here we make
36 use of the 1-5 μm coverage of the JWST early release observations to search for
37 intrinsically red galaxies in the first ≈ 750 million years of cosmic history. In the survey
38 area, we find six candidate massive galaxies (stellar mass > 1010 solar masses) at 7.4 ≤ z ≤
39 9.1, 500–700 Myr after the Big Bang, including one galaxy with a possible stellar mass of
40 ~1011 solar masses. If verified with spectroscopy, the stellar mass density in massive
41 galaxies would be much higher than anticipated from previous studies based on rest42
frame ultraviolet-selected samples.
Magnetic fields and relativistic electrons fill entire galaxy clusterSérgio Sacani
- The authors analyzed deep LOFAR radio observations of the galaxy cluster Abell 2255, detecting radio synchrotron emission distributed over an unprecedented scale of at least 5 megaparsecs, well beyond the cluster outskirts.
- This pervasive radio emission indicates that shocks and turbulence efficiently transfer kinetic energy into relativistic particles and magnetic fields throughout the cluster, including the periphery.
- The strength of the emission requires magnetic field energy densities at least 100 times higher than expected from simple compression of primordial fields, suggesting efficient dynamo action even in the cluster outskirts.
Probing the jet_base_of_blazar_pks1830211_from_the_chromatic_variability_of_i...Sérgio Sacani
This document summarizes ALMA observations of the blazar PKS 1830-211 taken over multiple epochs in 2012. The blazar is lensed by a foreground galaxy, producing two resolved images (NE and SW) separated by 1". The observations were taken at frequencies corresponding to 350-1050 GHz in the blazar rest frame. Analysis of the flux ratio between the two images over time and frequency revealed a remarkable frequency-dependent behavior, implying a "chromatic structure" in the blazar jet. This is interpreted as evidence for a "core-shift effect" caused by plasmon ejection very near the base of the jet. The observations provide a unique probe of activity in the region where plasma acceleration occurs in blazar
A filament of dark matter between two clusters of galaxiesCarlos Bella
1) The document reports the detection of a dark matter filament connecting two galaxy clusters, Abell 222 and Abell 223, using weak gravitational lensing.
2) Parametric modeling finds that a filament component provides a significantly better fit than models with just three galaxy clusters, and the filament contributes a mass comparable to an additional galaxy cluster.
3) Combining the lensing detection with X-ray observations places an upper limit of 0.09 on the hot gas fraction in the filament.
Similar to Detection of an_unidentified_emission_line_in_the_stacked_xray_spectrum_of_galaxy_clusters (20)
Compositions of iron-meteorite parent bodies constrainthe structure of the pr...Sérgio Sacani
Magmatic iron-meteorite parent bodies are the earliest planetesimals in the Solar System,and they preserve information about conditions and planet-forming processes in thesolar nebula. In this study, we include comprehensive elemental compositions andfractional-crystallization modeling for iron meteorites from the cores of five differenti-ated asteroids from the inner Solar System. Together with previous results of metalliccores from the outer Solar System, we conclude that asteroidal cores from the outerSolar System have smaller sizes, elevated siderophile-element abundances, and simplercrystallization processes than those from the inner Solar System. These differences arerelated to the formation locations of the parent asteroids because the solar protoplane-tary disk varied in redox conditions, elemental distributions, and dynamics at differentheliocentric distances. Using highly siderophile-element data from iron meteorites, wereconstruct the distribution of calcium-aluminum-rich inclusions (CAIs) across theprotoplanetary disk within the first million years of Solar-System history. CAIs, the firstsolids to condense in the Solar System, formed close to the Sun. They were, however,concentrated within the outer disk and depleted within the inner disk. Future modelsof the structure and evolution of the protoplanetary disk should account for this dis-tribution pattern of CAIs.
Signatures of wave erosion in Titan’s coastsSérgio Sacani
The shorelines of Titan’s hydrocarbon seas trace flooded erosional landforms such as river valleys; however, it isunclear whether coastal erosion has subsequently altered these shorelines. Spacecraft observations and theo-retical models suggest that wind may cause waves to form on Titan’s seas, potentially driving coastal erosion,but the observational evidence of waves is indirect, and the processes affecting shoreline evolution on Titanremain unknown. No widely accepted framework exists for using shoreline morphology to quantitatively dis-cern coastal erosion mechanisms, even on Earth, where the dominant mechanisms are known. We combinelandscape evolution models with measurements of shoreline shape on Earth to characterize how differentcoastal erosion mechanisms affect shoreline morphology. Applying this framework to Titan, we find that theshorelines of Titan’s seas are most consistent with flooded landscapes that subsequently have been eroded bywaves, rather than a uniform erosional process or no coastal erosion, particularly if wave growth saturates atfetch lengths of tens of kilometers.
SDSS1335+0728: The awakening of a ∼ 106M⊙ black hole⋆Sérgio Sacani
Context. The early-type galaxy SDSS J133519.91+072807.4 (hereafter SDSS1335+0728), which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). This variability behaviour, coupled with the host-galaxy properties, suggests that SDSS1335+0728 hosts a ∼ 106M⊙ black hole (BH) that is currently in the process of ‘turning on’. Aims. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728. Methods. We used archival photometry (from WISE, 2MASS, SDSS, GALEX, eROSITA) and spectroscopic data (from SDSS and LAMOST) to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We analysed the variability of SDSS1335+0728 in the X-ray/UV/optical/mid-infrared range, modelled its spectral energy distribution prior to and after December 2019, and studied the evolution of its UV/optical spectra. Results. From our multi-wavelength photometric analysis, we find that: (a) since 2021, the UV flux (from Swift/UVOT observations) is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1−W2 WISE colour has become redder; and (c) since February 2024, the source has begun showing X-ray emission. From our spectroscopic follow-up, we see that (i) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (ii) broad emission lines are not detected; and (iii) the [OIII] line increased its flux ∼ 3.6 years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region. Conclusions. We conclude that the variations observed in SDSS1335+0728 could be either explained by a ∼ 106M⊙ AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGNobserved in the process of activating. If the latter were found to be the case, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour. Key words. galaxies: active– accretion, accretion discs– galaxies: individual: SDSS J133519.91+072807.4
Discovery of An Apparent Red, High-Velocity Type Ia Supernova at 𝐳 = 2.9 wi...Sérgio Sacani
We present the JWST discovery of SN 2023adsy, a transient object located in a host galaxy JADES-GS
+
53.13485
−
27.82088
with a host spectroscopic redshift of
2.903
±
0.007
. The transient was identified in deep James Webb Space Telescope (JWST)/NIRCam imaging from the JWST Advanced Deep Extragalactic Survey (JADES) program. Photometric and spectroscopic followup with NIRCam and NIRSpec, respectively, confirm the redshift and yield UV-NIR light-curve, NIR color, and spectroscopic information all consistent with a Type Ia classification. Despite its classification as a likely SN Ia, SN 2023adsy is both fairly red (
�
(
�
−
�
)
∼
0.9
) despite a host galaxy with low-extinction and has a high Ca II velocity (
19
,
000
±
2
,
000
km/s) compared to the general population of SNe Ia. While these characteristics are consistent with some Ca-rich SNe Ia, particularly SN 2016hnk, SN 2023adsy is intrinsically brighter than the low-
�
Ca-rich population. Although such an object is too red for any low-
�
cosmological sample, we apply a fiducial standardization approach to SN 2023adsy and find that the SN 2023adsy luminosity distance measurement is in excellent agreement (
≲
1
�
) with
Λ
CDM. Therefore unlike low-
�
Ca-rich SNe Ia, SN 2023adsy is standardizable and gives no indication that SN Ia standardized luminosities change significantly with redshift. A larger sample of distant SNe Ia is required to determine if SN Ia population characteristics at high-
�
truly diverge from their low-
�
counterparts, and to confirm that standardized luminosities nevertheless remain constant with redshift.
Evidence of Jet Activity from the Secondary Black Hole in the OJ 287 Binary S...Sérgio Sacani
Wereport the study of a huge optical intraday flare on 2021 November 12 at 2 a.m. UT in the blazar OJ287. In the binary black hole model, it is associated with an impact of the secondary black hole on the accretion disk of the primary. Our multifrequency observing campaign was set up to search for such a signature of the impact based on a prediction made 8 yr earlier. The first I-band results of the flare have already been reported by Kishore et al. (2024). Here we combine these data with our monitoring in the R-band. There is a big change in the R–I spectral index by 1.0 ±0.1 between the normal background and the flare, suggesting a new component of radiation. The polarization variation during the rise of the flare suggests the same. The limits on the source size place it most reasonably in the jet of the secondary BH. We then ask why we have not seen this phenomenon before. We show that OJ287 was never before observed with sufficient sensitivity on the night when the flare should have happened according to the binary model. We also study the probability that this flare is just an oversized example of intraday variability using the Krakow data set of intense monitoring between 2015 and 2023. We find that the occurrence of a flare of this size and rapidity is unlikely. In machine-readable Tables 1 and 2, we give the full orbit-linked historical light curve of OJ287 as well as the dense monitoring sample of Krakow.
Candidate young stellar objects in the S-cluster: Kinematic analysis of a sub...Sérgio Sacani
Context. The observation of several L-band emission sources in the S cluster has led to a rich discussion of their nature. However, a definitive answer to the classification of the dusty objects requires an explanation for the detection of compact Doppler-shifted Brγ emission. The ionized hydrogen in combination with the observation of mid-infrared L-band continuum emission suggests that most of these sources are embedded in a dusty envelope. These embedded sources are part of the S-cluster, and their relationship to the S-stars is still under debate. To date, the question of the origin of these two populations has been vague, although all explanations favor migration processes for the individual cluster members. Aims. This work revisits the S-cluster and its dusty members orbiting the supermassive black hole SgrA* on bound Keplerian orbits from a kinematic perspective. The aim is to explore the Keplerian parameters for patterns that might imply a nonrandom distribution of the sample. Additionally, various analytical aspects are considered to address the nature of the dusty sources. Methods. Based on the photometric analysis, we estimated the individual H−K and K−L colors for the source sample and compared the results to known cluster members. The classification revealed a noticeable contrast between the S-stars and the dusty sources. To fit the flux-density distribution, we utilized the radiative transfer code HYPERION and implemented a young stellar object Class I model. We obtained the position angle from the Keplerian fit results; additionally, we analyzed the distribution of the inclinations and the longitudes of the ascending node. Results. The colors of the dusty sources suggest a stellar nature consistent with the spectral energy distribution in the near and midinfrared domains. Furthermore, the evaporation timescales of dusty and gaseous clumps in the vicinity of SgrA* are much shorter ( 2yr) than the epochs covered by the observations (≈15yr). In addition to the strong evidence for the stellar classification of the D-sources, we also find a clear disk-like pattern following the arrangements of S-stars proposed in the literature. Furthermore, we find a global intrinsic inclination for all dusty sources of 60 ± 20◦, implying a common formation process. Conclusions. The pattern of the dusty sources manifested in the distribution of the position angles, inclinations, and longitudes of the ascending node strongly suggests two different scenarios: the main-sequence stars and the dusty stellar S-cluster sources share a common formation history or migrated with a similar formation channel in the vicinity of SgrA*. Alternatively, the gravitational influence of SgrA* in combination with a massive perturber, such as a putative intermediate mass black hole in the IRS 13 cluster, forces the dusty objects and S-stars to follow a particular orbital arrangement. Key words. stars: black holes– stars: formation– Galaxy: center– galaxies: star formation
JAMES WEBB STUDY THE MASSIVE BLACK HOLE SEEDSSérgio Sacani
The pathway(s) to seeding the massive black holes (MBHs) that exist at the heart of galaxies in the present and distant Universe remains an unsolved problem. Here we categorise, describe and quantitatively discuss the formation pathways of both light and heavy seeds. We emphasise that the most recent computational models suggest that rather than a bimodal-like mass spectrum between light and heavy seeds with light at one end and heavy at the other that instead a continuum exists. Light seeds being more ubiquitous and the heavier seeds becoming less and less abundant due the rarer environmental conditions required for their formation. We therefore examine the different mechanisms that give rise to different seed mass spectrums. We show how and why the mechanisms that produce the heaviest seeds are also among the rarest events in the Universe and are hence extremely unlikely to be the seeds for the vast majority of the MBH population. We quantify, within the limits of the current large uncertainties in the seeding processes, the expected number densities of the seed mass spectrum. We argue that light seeds must be at least 103 to 105 times more numerous than heavy seeds to explain the MBH population as a whole. Based on our current understanding of the seed population this makes heavy seeds (Mseed > 103 M⊙) a significantly more likely pathway given that heavy seeds have an abundance pattern than is close to and likely in excess of 10−4 compared to light seeds. Finally, we examine the current state-of-the-art in numerical calculations and recent observations and plot a path forward for near-future advances in both domains.
Anti-Universe And Emergent Gravity and the Dark UniverseSérgio Sacani
Recent theoretical progress indicates that spacetime and gravity emerge together from the entanglement structure of an underlying microscopic theory. These ideas are best understood in Anti-de Sitter space, where they rely on the area law for entanglement entropy. The extension to de Sitter space requires taking into account the entropy and temperature associated with the cosmological horizon. Using insights from string theory, black hole physics and quantum information theory we argue that the positive dark energy leads to a thermal volume law contribution to the entropy that overtakes the area law precisely at the cosmological horizon. Due to the competition between area and volume law entanglement the microscopic de Sitter states do not thermalise at sub-Hubble scales: they exhibit memory effects in the form of an entropy displacement caused by matter. The emergent laws of gravity contain an additional ‘dark’ gravitational force describing the ‘elastic’ response due to the entropy displacement. We derive an estimate of the strength of this extra force in terms of the baryonic mass, Newton’s constant and the Hubble acceleration scale a0 = cH0, and provide evidence for the fact that this additional ‘dark gravity force’ explains the observed phenomena in galaxies and clusters currently attributed to dark matter.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Elevate Your Nonprofit's Online Presence_ A Guide to Effective SEO Strategies...TechSoup
Whether you're new to SEO or looking to refine your existing strategies, this webinar will provide you with actionable insights and practical tips to elevate your nonprofit's online presence.
Andreas Schleicher presents PISA 2022 Volume III - Creative Thinking - 18 Jun...EduSkills OECD
Andreas Schleicher, Director of Education and Skills at the OECD presents at the launch of PISA 2022 Volume III - Creative Minds, Creative Schools on 18 June 2024.
Philippine Edukasyong Pantahanan at Pangkabuhayan (EPP) CurriculumMJDuyan
(𝐓𝐋𝐄 𝟏𝟎𝟎) (𝐋𝐞𝐬𝐬𝐨𝐧 𝟏)-𝐏𝐫𝐞𝐥𝐢𝐦𝐬
𝐃𝐢𝐬𝐜𝐮𝐬𝐬 𝐭𝐡𝐞 𝐄𝐏𝐏 𝐂𝐮𝐫𝐫𝐢𝐜𝐮𝐥𝐮𝐦 𝐢𝐧 𝐭𝐡𝐞 𝐏𝐡𝐢𝐥𝐢𝐩𝐩𝐢𝐧𝐞𝐬:
- Understand the goals and objectives of the Edukasyong Pantahanan at Pangkabuhayan (EPP) curriculum, recognizing its importance in fostering practical life skills and values among students. Students will also be able to identify the key components and subjects covered, such as agriculture, home economics, industrial arts, and information and communication technology.
𝐄𝐱𝐩𝐥𝐚𝐢𝐧 𝐭𝐡𝐞 𝐍𝐚𝐭𝐮𝐫𝐞 𝐚𝐧𝐝 𝐒𝐜𝐨𝐩𝐞 𝐨𝐟 𝐚𝐧 𝐄𝐧𝐭𝐫𝐞𝐩𝐫𝐞𝐧𝐞𝐮𝐫:
-Define entrepreneurship, distinguishing it from general business activities by emphasizing its focus on innovation, risk-taking, and value creation. Students will describe the characteristics and traits of successful entrepreneurs, including their roles and responsibilities, and discuss the broader economic and social impacts of entrepreneurial activities on both local and global scales.
A Free 200-Page eBook ~ Brain and Mind Exercise.pptxOH TEIK BIN
(A Free eBook comprising 3 Sets of Presentation of a selection of Puzzles, Brain Teasers and Thinking Problems to exercise both the mind and the Right and Left Brain. To help keep the mind and brain fit and healthy. Good for both the young and old alike.
Answers are given for all the puzzles and problems.)
With Metta,
Bro. Oh Teik Bin 🙏🤓🤔🥰
THE SACRIFICE HOW PRO-PALESTINE PROTESTS STUDENTS ARE SACRIFICING TO CHANGE T...indexPub
The recent surge in pro-Palestine student activism has prompted significant responses from universities, ranging from negotiations and divestment commitments to increased transparency about investments in companies supporting the war on Gaza. This activism has led to the cessation of student encampments but also highlighted the substantial sacrifices made by students, including academic disruptions and personal risks. The primary drivers of these protests are poor university administration, lack of transparency, and inadequate communication between officials and students. This study examines the profound emotional, psychological, and professional impacts on students engaged in pro-Palestine protests, focusing on Generation Z's (Gen-Z) activism dynamics. This paper explores the significant sacrifices made by these students and even the professors supporting the pro-Palestine movement, with a focus on recent global movements. Through an in-depth analysis of printed and electronic media, the study examines the impacts of these sacrifices on the academic and personal lives of those involved. The paper highlights examples from various universities, demonstrating student activism's long-term and short-term effects, including disciplinary actions, social backlash, and career implications. The researchers also explore the broader implications of student sacrifices. The findings reveal that these sacrifices are driven by a profound commitment to justice and human rights, and are influenced by the increasing availability of information, peer interactions, and personal convictions. The study also discusses the broader implications of this activism, comparing it to historical precedents and assessing its potential to influence policy and public opinion. The emotional and psychological toll on student activists is significant, but their sense of purpose and community support mitigates some of these challenges. However, the researchers call for acknowledging the broader Impact of these sacrifices on the future global movement of FreePalestine.
Temple of Asclepius in Thrace. Excavation resultsKrassimira Luka
The temple and the sanctuary around were dedicated to Asklepios Zmidrenus. This name has been known since 1875 when an inscription dedicated to him was discovered in Rome. The inscription is dated in 227 AD and was left by soldiers originating from the city of Philippopolis (modern Plovdiv).
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This document provides an overview of wound healing, its functions, stages, mechanisms, factors affecting it, and complications.
A wound is a break in the integrity of the skin or tissues, which may be associated with disruption of the structure and function.
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There are 4 phases of wound healing: hemostasis, inflammation, proliferation, and remodeling. This document also describes the mechanism of wound healing. Factors that affect healing include infection, uncontrolled diabetes, poor nutrition, age, anemia, the presence of foreign bodies, etc.
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Detection of an_unidentified_emission_line_in_the_stacked_xray_spectrum_of_galaxy_clusters
1. Submitted to ApJ, 2014 February 10
Preprint typeset using LATEX style emulateapj v. 04/17/13
DETECTION OF AN UNIDENTIFIED EMISSION LINE IN THE STACKED X-RAY SPECTRUM OF GALAXY
CLUSTERS
Esra Bulbul1,2
, Maxim Markevitch2
, Adam Foster1
, Randall K. Smith1
Michael Loewenstein2
, and
Scott W. Randall1
1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138.
2 NASA Goddard Space Flight Center, Greenbelt, MD, USA.
Submitted to ApJ, 2014 February 10
ABSTRACT
We detect a weak unidentified emission line at E = (3.55 − 3.57) ± 0.03 keV in a stacked XMM
spectrum of 73 galaxy clusters spanning a redshift range 0.01 − 0.35. MOS and PN observations
independently show the presence of the line at consistent energies. When the full sample is divided
into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is seen at
> 3σ statistical significance in all three independent MOS spectra and the PN “all others” spectrum.
The line is also detected at the same energy in the Chandra ACIS-S and ACIS-I spectra of the Perseus
cluster, with a flux consistent with XMM-Newton (however, it is not seen in the ACIS-I spectrum of
Virgo). The line is present even if we allow maximum freedom for all the known thermal emission
lines. However, it is very weak (with an equivalent width in the full sample of only ∼ 1 eV) and located
within 50–110 eV of several known faint lines; the detection is at the limit of the current instrument
capabilities and subject to significant modeling uncertainties. On the origin of this line, we argue that
there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is
the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark
matter is in sterile neutrinos with ms = 2E = 7.1 keV, our detection in the full sample corresponds to
a neutrino decay mixing angle sin2
(2θ) ≈ 7 × 10−11
, below the previous upper limits. However, based
on the cluster masses and distances, the line in Perseus is much brighter than expected in this model,
significantly deviating from other subsamples. This appears to be because of an anomalously bright
line at E = 3.62 keV in Perseus, which could be an Arxvii dielectronic recombination line, although
its emissivity would have to be 30 times the expected value and physically difficult to understand. In
principle, such an anomaly might explain our line detection in other subsamples as well, though it
would stretch the line energy uncertainties. Another alternative is the above anomaly in the Ar line
combined with the nearby 3.51 keV K line also exceeding expectation by factor 10–20. Confirmation
with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new
line.
1. INTRODUCTION
Galaxy clusters are the largest aggregations of hot in-
tergalactic gas and dark matter. The gas is enriched
with heavy elements (Mitchell et al. (1976); Serlemitsos
et al. (1977) and later works) that escape from galaxies
and accumulate in the intracluster/intergalactic medium
(ICM) over billions of years of galactic and stellar evo-
lution. The presence of various heavy ions is seen from
their emission lines in the cluster X-ray spectra. Data
from large effective area telescopes with spectroscopic ca-
pabilities, such as ASCA, Chandra, XMM-Newton and
Suzaku, uncovered the presence of many elements in the
ICM, including O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni
(for a review see, e.g., B¨ohringer & Werner 2010). Re-
cently, weak emission lines of low-abundance Cr and Mn
were discovered (Werner et al. 2006; Tamura et al. 2009).
Relative abundances of various elements contain valuable
information on the rate of supernovae of different types in
galaxies (e.g., Loewenstein 2013) and illuminate the en-
richment history of the ICM (e.g., Bulbul et al. 2012b).
Line ratios of various ions can also provide diagnostics
of the physical properties of the ICM, uncover the pres-
ence of multi-temperature gas, nonequilibrium ionization
ebulbul@cfa.harvard.edu
states and nonthermal emission processes such as charge
exchange (Paerels & Kahn 2003).
As for dark matter, 80 years from its discovery by
(Zwicky 1933, 1937), its nature is still unknown (though
now we do know for sure it exists — from X-ray and
gravitational-lensing observations of the Bullet Cluster,
Clowe et al. (2006), and we know accurately its cosmo-
logical abundance, e.g., Hinshaw et al. (2013)). Among
the various plausible dark matter candidates, one that
has motivated our present work is the hypothetical ster-
ile neutrino that is included in some extensions to the
standard model of particle physics (Dodelson & Widrow
(1994) and later works; for recent reviews see, e.g.,
Abazajian et al. (2007); Boyarsky et al. (2009)). Ster-
ile neutrinos should decay spontaneously with the rate
Γγ(ms, θ) = 1.38 × 10−29
s−1 sin2
2θ
10−7
ms
1 keV
5
,
(1)
where the particle mass ms and the “mixing angle” θ
are unknown but tied to each other in any particular
neutrino production model (Pal & Wolfenstein 1982).
The decay of sterile neutrino should produce a photon of
E = ms/2 and an active neutrino. The mass of the ster-
ile neutrino may lie in the keV range, which would place
arXiv:1402.2301v1[astro-ph.CO]10Feb2014
2. 2
its decay line in the range accessible to current X-ray
telescopes. So far, searches in various types of massive
systems resulted only in upper limits (e.g., Boyarsky et
al. 2012; Abazajian et al. 2012).
Current X-ray archives of XMM-Newton, Chandra and
Suzaku contain vast collections of galaxy cluster obser-
vations. Mining these databases can result in significant
improvement in sensitivity to faint spectral features com-
pared to individual cluster observations, with respect to
both the statistical and (in a less obvious way) systematic
or instrumental uncertainties. In this paper, we under-
take a fishing expedition that combines the spectra of
many bright clusters from the XMM-Newton archive in
order to search for any kind of faint, unidentified X-ray
emission lines — be it thermal lines from previously un-
detected rare elements in the ICM or the elusive sterile
neutrino decay line.
To improve the sensitivity to weak spectral lines and
reduce systematic effects, we stack the X-ray spectra
from clusters at different redshifts in their the rest frame,
rescaling the photon energies to z = 0. After blue-
shifting each cluster spectrum to z = 0, any background
lines or instrumental response artifacts are smeared out
(since they occur in the detector frame), but a weak
intrinsic line would be amplified and may become de-
tectable in the combined spectrum. In this paper, we use
this method to detect a previously unknown, extremely
faint emission line at E ≈ 3.55−3.57 keV. It was detected
in the stacked XMM-Newton X-ray spectra of 73 bright
galaxy clusters in the redshift range 0.01 < z < 0.35, and
independently in several subsamples.
This paper is organized as follows. §2 describes the
XMM-Newton data processing, background modeling,
and spectra stacking methods. We also determine the
best-fit flux and energy of the detected spectral feature
using the XMM-Newton MOS and PN instruments. In
§3, we confirm the existence of this spectral line in the
Chandra ACIS-I and ACIS-S spectra of the Perseus clus-
ter, and obtain an upper limit from the ACIS-I observa-
tions of the Virgo cluster. In §4, we discuss the possi-
ble interpretations of this new emission line. All errors
quoted throughout the paper correspond to 68% (while
in parentheses, 90%) single-parameter confidence inter-
vals; upper limits are at 90% confidence, unless otherwise
stated.
2. CLUSTER SAMPLE SELECTION AND DATA ANALYSIS
2.1. Sample Selection
We searched the XMM-Newton archive for galaxy clus-
ter observations that yielded large numbers of X-ray
counts. We first selected clusters below a redshift of 0.4;
higher-redshift clusters are too faint to contribute signifi-
cantly into the stacked spectrum. We then calculated the
total X-ray counts expected from these XMM-Newton
observations using the ROSAT count rates reported in
eBCS (Ebeling et al. 2000), NORAS (B¨ohringer et al.
2000), REFLEX (B¨ohringer et al. 2004), XBACs (Ebel-
ing et al. 1996), and MACS catalogues (Ebeling et al.
2001) and XMM-Newton exposures. To prevent nearby
clusters from dominating the stacked spectrum, we used
different cluster count limits for different redshift ranges.
We chose clusters with a minimum of 105
counts per
cluster for clusters with z < 0.1, and 104
counts per
0
1
2
3
4
5
6
7
8
9
10
0.05 0.10 0.15 0.20 0.25 0.30 0.35
z
NNumberofClusters
Redshift
Figure 1. Redshift histogram of the total of 73 galaxy clusters in
the sample, selected from the XMM-Newton archive.
cluster for clusters with redshifts 0.1 < z < 0.4 to have a
wide enough range for the redshift-smearing effect. Off-
set pointings were excluded from the sample. In the end,
a sample of 73 clusters were selected. Included in Table 1
are the XMM-Newton observation identification (ObsID)
numbers, total MOS and PN clean exposure times, count
rates, and our best-fit redshifts (see §2.2). The redshift
histogram of the sample is given in Figure 1. The count
rates reported in Table 1 have been used only for sample
selection.
2.2. XMM-Newton Data Analysis
The EPIC data processing and background model-
ing were carried out with the XMM-Newton Extended
Source Analysis Software (XMM-ESAS) (Kuntz & Snow-
den 2008; Snowden et al. 2008). We reduced MOS and
PN data with the XMM-Newton Science Analysis Sys-
tem (SAS) version 12. Our XMM-Newton analysis is
described fully in Bulbul et al. (2012a,b); here we give
relevant details.
The light curve filtering was applied to eliminate pe-
riods of elevated background. Cleaned events files were
created using the good time interval file (GTI) produced
by this process. The net exposure time after filtering the
event files for good time intervals is given in Table 1.
Images were created in the 0.4−7.0 keV band for MOS
and PN observations and used for detecting point sources
with the CIAO tool wavdetect. The images were exam-
ined carefully for any missed point sources, as well as for
individual MOS CCDs operating in an anomalous state.
The CCDs in an anomalous state and all point sources
were excluded from further analysis.
Subtle errors in the detector energy gain may cause a
fraction of a percent shifts of the location of the emis-
sion lines in different X-ray observations of the same
cluster. In addition, a redshift measured from the op-
tical observations of a cluster may differ from an X-ray
redshift of the gas. To be able to stack spectra in the
same frame, we determined the best-fit X-ray redshift for
each XMM-Newton observation using the bright Fe lines.
These redshifts (Table 1), which correct for both of the
above-mentioned effects, were then used to scale the in-
dividual spectra in the source frame of each observation,
as will be described in §2.3. Our selected observations
provide adequate statistics to fit an X-ray redshift for
each spectrum.
5. 5
TABLE 1 – continued from previous page
Cluster RA DEC ObsID XMM-Newton XMM-Newton Count Rate Best-fit
MOS Exp PN Exp redshift
(ks) (ks) (cnts s−1 )
0401170101 0.295
1E 0657-558 06: 58: 31.1 -55: 56:49 0112980201 43.7 17.4 4.1 0.296
MS 2137.3-2353 21: 40: 15.28 -23.0: 39: 43.5 0008830101 21.4 6.3 0.2 0.313
0673830201 0.313
MACS J2229.7-2755 22: 29: 45.13 -27.0: 55: 33.7 0651240201 58.3 14.0 0.2 0.320
MACS J1532.8+3021 15: 32: 48.98 30.0: 21: 14.8 0039340101 21.5 8.0 0.2 0.350
0651240101 0.345
AS1063 22: 48: 46.69 -44.0: 30: 48.9 0504630101 21.6 18.0 0.4 0.354
For most clusters, the spectra were extracted within
the overdensity radius R500. The overdensity radii
were calculated using the Vikhlinin et al. (2009) mass-
temperature scaling relation for each cluster. Due to
the large solid angle of nearby clusters, e.g., Coma,
Perseus and Centaurus, their spectra were extracted
within the full field of view (FOV). Redistribution matrix
files (RMFs) and ancillary response files (ARFs) were
created with the SAS tools rmfgen and arfgen, respec-
tively.
Although we stack the cluster spectra in this work
(and end up using only the 2–10 keV band for the line
search), it is still important to accurately subtract the
background from each individual observation. For each
extracted spectrum, we model a superposition of five
main background components: quiescent particle back-
ground, soft X-ray background emission (including solar
wind charge exchange, Galactic halo, local hot bubble,
and unresolved extragalactic sources), as well as residual
contamination from soft protons. We use the ROSAT
All−Sky Survey (RASS) background spectrum to model
the soft X-ray background using the background tool at
the High Energy Astrophysics Science Archive Research
Center (HEASARC) Web site. The RASS spectrum was
extracted from an annulus from 1◦
to 2◦
surrounding the
cluster center, with the assumption that this spectrum
reasonably represents the soft X-ray background in the
direction of the cluster.
We simultaneously modeled the soft X-ray emission
from the local hot bubble (LHB) or heliosphere with a
cool unabsorbed single temperature thermal component
(E ∼ 0.1 keV), while the Galactic hotter halo and inter-
galactic medium were modeled with an absorbed ther-
mal component (E ∼ 0.2 keV). The energies of the apec
model were restricted but allowed to vary with free nor-
malizations. The abundances were set to 1A . We model
the contamination due to unresolved point sources using
an absorbed power law component with a spectral index
of α 1.46 and normalization of 8.88 × 10−7
photons
keV−1
cm−2
s−1
at ∼1 keV (Kuntz & Snowden 2008).
Soft-proton flares are largely removed by the light curve
filtering. However after the filtering some soft-proton
residuals may remain in the data and were modeled by
including an extra power law model component and diag-
onal response matrices provided in the SAS distribution
in the final spectral analysis (Snowden et al. 2008).
The EPIC-MOS quiescent particle background (QPB)
spectra have two bright instrumental fluorescent lines:
the Al-K (1.49 keV) and the Si-K (1.74 keV) lines. The
PN QPB spectra have fluorescent lines of Al-K (1.49
keV), Ni-K (7.48 keV), Cu-K (8.05, 8.91 keV), and Zn-K
(8.64, 9.57 keV). Since small variations in the gain and
the line strengths between the source and background
spectra can lead to residuals in the spectral fitting (Kuntz
& Snowden 2008) and XMM-ESAS software does not in-
clude these instrumental lines in the QPB spectra, we
modeled these instrumental lines spectrally by adding
Gaussian models to our spectral fits to determine the
best-fit energies, widths, and normalizations. The total
background was constructed by adding the models for
the Al-K, Si-K, Ni-K, Cu-K, and Zn-K lines with the
best-fit energies, widths, and normalizations to the QPB
produced in the XMM-ESAS analysis for all pointings.
These total QBP spectra were directly subtracted from
the summed observation to obtain source spectra.
The fitting of the source spectra was done with the
spectral fitting package XSPEC 12.8.0 (Arnaud 1996).
The 0.3−10 keV energy interval was used for MOS spec-
tra, whereas the 0.4 − 10.0 keV band was used for the PN
fits. To determine the best-fit cluster redshifts for each
observation (given in Table 1), the cluster spectra were fit
with a standard absorbed multi-temperature collisional
equilibrium plasma model (apec) (Smith et al. 2001) and
AtomDB v2.0.2 (Foster et al. 2012). We did not observe
any differences beyond a fraction of a percent in terms
of the detector gain variations.
2.3. Spectra Stacking Methods
The best way of distinguishing a real spectral feature in
a class of distant objects from instrumental artifacts and
the X-ray background features is to detect that feature in
multiple objects at different redshifts in their rest frame,
in which case the line coming from an object will stay
at the same energy, unlike the detector artifacts. To
accomplish this, we stacked the spectra of our selected
73 clusters, blue-shifting them to the source frame using
the best-fit X-ray redshift of each observation determined
above.
Technically, the energies of the source and background
X-ray events were rescaled to the source frame using
the best-fit redshifts. The scaled event files were then
used to extract the source and particle background spec-
tra within r = R500 or the full FOV of MOS, and the
same extraction region was used for PN observations for
nearby clusters that fill the FOV. Counts from each indi-
vidual spectrum were co-added into a single stacked spec-
6. 6
trum using the FTOOL mathpha to produce the stacked
source and the particle background spectra. At the end
of the stacking process, we obtained spectra with ∼ 6
Ms of good cluster exposure with MOS 1 and MOS 2
(that were co-added) and ∼ 2 Ms with PN for the full
XMM-Newton sample.
Energy (keV)
Flux(countss
-1
keV
-1
)
Ti Cr
Fe XXV
at 6.7 keV
PN Bkg
MOS Bkg
Al-K
Si-K
Fe Ni
Cu-K
Zn-K
Figure 2. XMM-Newton MOS and PN background subtracted
source spectra and particle background spectra for the Perseus clus-
ter. The spectra were obtained by co-adding the observations of
the cluster in the cluster’s rest frame. In the co-added scaled spec-
tra, the Fe xxv line is located at its rest energy, ∼ 6.7 keV. Energy
of background and instrumental lines are blue-shifted according to
the cluster’s redshift.
The RMF and ARF to be used with the stacked spec-
trum were constructed by averaging the responses for
individual observations with proper weighting. The in-
dividual RMFs and ARFs were first remapped to the
source frame using the best-fit redshifts. The weighing
factors for stacking RMFs and ARFs were calculated us-
ing the total counts in the energy band we will use for
our line search (2–10 keV). These factors (ωcnt) are given
in Table 4. The weighted and normalized ARFs and
RMFs were stacked using the FTOOLS addarf and ad-
drmf. These X-ray count-weighted response files were
used to model the continuum and the known plasma
emission lines; we will also try a different weighting of
responses for the possibly non-thermal new line, as will
be described below.
For a check, each background-subtracted, blue-shifted,
single-cluster spectrum was fit with an apec model us-
ing the corresponding scaled ARF and RMF to verify
that the best-fit redshifts were consistent with zero. For
illustration, the co-added MOS and PN source and back-
ground spectra of the Perseus cluster in its source frame
are shown in Figure 2. We note that the Fe xxv line is lo-
cated at its rest energy ∼ 6.7 keV, while the background
and instrumental lines are blue-shifted.
The stacked MOS and PN source and background spec-
tra of the clusters in the sample are shown in Figure 3.
The background spectra show the smearing effect on the
background lines, e.g., Al-K (1.48 keV), Si-K (1.75 keV),
Cr (5.4 keV), Mn (5.8 keV), Fe-K (6.4 keV), Cu-K (8.05
keV, 8.91 keV), Zn-K (8.64 keV, 9.61 keV) and Au (9.1
keV). They are much less prominent in the stacked spec-
trum compared with the single-source spectrum shown in
Figure 2. Similarly, any residuals from inaccurate back-
ground subtraction are smeared. We will see other ad-
vantages of this smearing below.
3. ANALYZING THE STACKED XMM-NEWTON SPECTRA
We will limit our line search to the 2 − 10 keV en-
ergy band. After looking at the stacked spectra, we con-
cluded that the band below 2 keV is hopelessly crowded
with lines, such as the strong Ne x (1.21 keV), Fe xxiv
(1.55 keV), Mg xii (1.74 keV), and Si xii (1.86 keV)
features, making the detection of any weak emission fea-
tures between them difficult, given the ∼ 100 eV energy
resolution of XMM-Newton and other CCD detectors.
To search for any unidentified spectral lines in the
stacked spectra, we need to model the known lines and
the continuum emission to a very good precision. We do
not necessarily need to obtain a physically meaningful
model (which would be a mixture of all the thermal com-
ponents in all the clusters), but one that allows enough
freedom to account for all known lines and the possible
errors in their theoretical emissivities. To this end, we fit
the background-subtracted stacked source spectra with a
line-free multi-temperature apec model to represent the
continuum emission with high accuracy, and then add
individual lines. We start with four continuum compo-
nents to represent the multi-temperature nature of the
stacked spectra. The line-free apec model accounts for
the continuum due to thermal bremsstrahlung, radiative
recombination, and two-photon emissions. The best-fit
temperature and normalization parameters of line-free
apec models are shown in Table 2. The best-fit temper-
atures in the table do not have physical meaning, since
they are obtained by fitting the stacked blue-shifted spec-
tra. (We note that the continuum of a redshifted thermal
model can be well represented by a continuum with a dif-
ferent redshift and a different temperature.) The abun-
dance was set to 0.3 in order to include the recombination
edges in the fitting process. The abundance parameter
does not affect the line modeling, since the line-free apec
model does not include lines.
In order to account for the known plasma emission lines
in a model-independent way, for each known line in the
2.0 − 10.0 keV band, were added a Gaussian line to the
model. Initially we have added Gaussian models for the
known strong emission lines from the AtomDB database1
with emissivities > 5 × 10−19
photons cm3
s−1
for the
lowest temperature given in Table 2. The strong emis-
sion lines (which can be resolved with a CCD detector)
included in our model at their rest energies are: Al xiii
(2.05 keV), Si xiv (2.01 keV and 2.51 keV), Si xii (2.18
keV, 2.29 keV, and 2.34 keV), S xiv (2.62 keV), S xv
(complex at 2.45 keV, 2.88 keV), Ar xvii (triplet at 3.12
keV, 3.62 keV, 3.68 keV), K xviii (3.47 keV and 3.51
keV), K xix (3.71 keV), Ca xix (complex at 3.86 keV,
3.90 keV, 4.58 keV), Ar xviii (3.31 keV, 3.93 keV), Ca xx
(4.10 keV), Cr xxiii (5.69 keV), Fe xxiv (complex at 6.62
keV), Fe xxv (complex at 6.70 keV, 8.29 keV, 7.81 keV,
7.88 keV), Fe xxvi (6.95 keV, 8.3 keV, and 8.70 keV),
and Ni xxvii (7.79 keV). Initially, a total of 28 Gaussian
model components were included in the 2–10 keV energy
band. Individual Gaussian components were then re-
moved if they were not required to accurately model the
spectra (to improve convergence of the fit). The widths
of Gaussians were left free, but restricted to the range 0
1 http://www.atomdb.org/Webguide/webguide.php
7. 7
1 10
Energy (keV)
0.01
0.1
1
10
Flux(countss
-1
keV
-1
)
PN Background
MOS Background
PN
MOS
2 4 6 8
Fe XXV
(6.7 keV)
Fe XXVI
(6.97 keV)
Cu K
(8.05, 8.91 keV)
Zn K
(8.64, 9.57 keV)
Cr (5.4 keV)
Mn (5.8 keV)
Al K
(1.49 keV)
Si K
(1.75 keV)
Fe-K (6.4 keV)
5 6 7
Energy (keV)
0.1
1
Flux(countss
-1
keV
-1
)
Perseus MOS Background
Perseus PN Background
Stacked PN Background
Stacked MOS Background
Cr Mn Fe-K
Figure 3. Left Panel: Stacked XMM-Newton MOS and PN background-subtracted source spectra and particle background spectra of
the full sample. The spectrum of each observation was scaled to the rest frame prior to stacking. The total filtered exposure time was 6
Ms for MOS and 2 Ms for PN. The background MOS (in blue) and PN (in green) spectra show the effect of smearing of instrumental lines,
such as Cr, Mn, Fe and Ni, as well as Al-K and Si-K fluorescent lines. The effect is due to the stacking of background spectra which are
scaled by different cluster redshifts. Right Panel: Close-up view of 5.0 − 8.0 keV band of the background XMM-Newton MOS and PN
spectra of the Perseus cluster compared to the stacked XMM-Newton MOS and PN background spectra. The background lines are less
prominent in the stacked background spectra than in the single source background spectra.
< ∆E/E < 10−2
. The energies of the Gaussian compo-
nents were allowed to vary by up to 5 eV to account for
residual uncertainties in the gain and in the energies in
the atomic database. This way, we were able to model
the continuum emission and strong known emission lines
accurately, leaving a clean residual spectrum to search
for any unidentified lines.
We also fit a power-law model in the full band to repre-
sent the residual soft proton background contamination
(see §2.2), and used these power law indices and normal-
izations for further narrower band fits (see §3.1). The
spectral counts in each energy bin were sufficiently high
to allow the use of the Gaussian statistics in this analysis
(Protassov et al. 2002).
3.1. Stacked Spectra of the Full Cluster Sample
After the stacking process we obtained a total 8.5× 106
source counts in the 6 Ms MOS spectra, while the 2 Ms
PN stacked spectra has a total source counts of 5.1× 106
.
The line-free apec model with Gaussian lines produces
an acceptable fit to the stacked MOS and PN spectra
with χ2
s of 564.8 for 566 dof (MOS) and 510.5 for 564
dof (PN). After modeling all the known thermal plasma
lines in the stacked spectrum, we examined the residuals
in each 1 keV band carefully. We found one significant
unidentified residual emission feature at E ≈ 3.55 − 3.57
keV, which is not associated with any plasma emission
lines in the band. Near this line, there are four tabulated
weak thermal emission lines of K xviii (1s1
2s1
→ 1s2
)
at a rest energy of 3.47 keV, K xviii (1s1
2p1
→ 1s2
) at
3.51 keV, a dielectronic recombination line of Ar xvii at
3.62 keV, Ar xvii (1s1
3p1
→ 1s2
) at 3.68 keV, and K
xix (2p1
→ 1s1
) at 3.72 keV.
In order to separate the excess emission feature from
these weak contaminating K and Ar lines, we make con-
servative estimates of their flux using AtomDB. Ideally,
line flux measurements would be based on other lines
of the same ions; however, there are no other strong K
xviii, K xix lines in the spectrum. Therefore, we use
the lines from relatively clean part of the band, namely,
the S xvi (2p1
→ 1s1
), Ca xix (1s1
2p1
→ 1s2
), and
Ca xx (2p1
→ 1s1
) lines at 2.63 keV, 3.90 keV and 4.11
keV, respectively, to estimate the flux of the 3.47 keV,
3.51 keV, 3.68 keV and 3.72 keV lines. The best-fit flux
measurements of these S xvi, Ca xix, and Ca xx lines
are given in Table 2.
We assume the relative abundances of S, Ca, Ar, and K
are proportional to their abundances in the solar photo-
sphere (Anders & Grevesse 1989). While this may not be
exactly true, it gives a reasonable starting point (we will
relax this assumption below). Then, using AtomDB, we
calculated the relative emissivity of the K xviii, K xix,
and Ar xvii lines compared to the the S xvi, Ca xix, and
Ca xx lines based on the equilibrium collisional plasma
conditions at the various temperatures of our line-free
apec components. In practice, the emissivities of K xviii,
K xix, and Ar xvii lines are stronger at the lowest tem-
peratures of each model, so the other components can be
ignored. The curves in Figure 4 represent the emissivities
of K and Ar lines as a function of plasma temperature
for the normalizations of the lowest temperature compo-
nents measured in our spectra.
Having obtained the relative theoretical emissivity of
the lines from AtomDB, we estimated the flux as
Γl = Γr
i
Normi εl (Te)/εr (Te), (2)
where subscripts l and r represent the lines of interest
(K xviii and Ar xvii) and reference lines (S xvi, Ca
xix, and Ca xx) respectively, Γ is the flux in the line,
ε(Te) is the calculated emissivity from AtomDB at the
electron temperature Te, and the sum over i represents
the different temperature components listed in Table 2
with their normalizations Normi. We use 0.1 and 3 times
of the maximum values of these fluxes as lower and up-
per bounds for the normalizations of the Gaussian lines
in the XSPEC fitting. The lower limits of 0.1 is set to
avoid the lines vanishing and posing problems for the
minimization routine. The factor 3 represents a conser-
vative allowance for variation of the relative elemental
8. 8
Table 2
Best-fit Temperature and Normalizations of line-free apec Model in 2 − 10 keV fit to the Stacked MOS and PN spectra for various
samples. The temperature (kTi) normalization (Ni) are in the units of keV and (10−2 cm−5), and the line fluxes of the S xvi, Ca xix, Ca
xx are in the units of 10−5 photons cm−2 s−1 at the rest energies 2.63 keV, 3.90 keV, 4.11 keV.
Full Coma Excluding
Sample + Centaurus Nearby Perseus
+ Ophiuchus Clusters
Parameters MOS PN MOS PN MOS PN MOS PN
kT1 5.9 ± 0.1 7.3 ± 0.2 3.9 ± 0.1 2.5 ± 0.2 3.5 ± 0.2 2.0 ± 0.3 3.6 ± 0.6 2.17 ± 0.9
N1 2.2 ± 0.1 1.1 ± 0.1 6.5 ± 0.1 5.4 ± 0.1 0.6 ± 0.1 0.3 ± 0.1 15.7 ± 7.8 10.2 ± 6.9
kT2 6.1 ± 0.1 2.3 ± 0.3 6.8 ± 0.1 6.5 ± 0.2 6.8 ± 0.1 9.4 ± 0.2 7.6 ± 0.7 6.25 ± 0.8
N2 1.8 ± 0.1 0.6 ± 0.1 8.9 ± 0.1 6.1 ± 0.1 0.8 ± 0.1 0.1 ± 0.1 44.0 ± 6.8 50.2 ± 14.1
kT3 7.3 ± 0.2 18.7 ± 0.2 10.7 ± 0.2 15.4 ± 0.6 10.3 ± 0.3 4.4 ± 0.7 − −
N3 1.6 ± 0.1 0.4 ± 0.1 8.9 ± 0.1 7.2 ± 0.2 0.7 ± 0.1 0.1 ± 0.02 − −
kT4 10.9 ± 0.5 6.9 ± 0.1 7.4 ± 0.2 4.0 ± 0.2 6.9 ± 0.2 − − −
N4 0.9 ± 0.1 1.0 ± 0.1 6.9 ± 0.1 4.6 ± 0.2 0.6 ± 0.1 − − −
Flux of S xvi 7.9 ± 0.1 3.9 ± 0.1 39.1 ± 6.6 13.1 ± 0.9 2.9 ± 0.1 2.8 ± 0.1 49.1 ± 7.3 55.5 ± 4.9
Flux of Ca xix 2.4 ± 0.1 0.9 ± 0.2 13.5 ± 4.8 4.6 ± 0.6 0.7 ± 0.1 0.6 ± 0.1 25.6 ± 1.5 11.9 ± 2.9
Flux of Ca xx 1.7 ± 0.1 0.4 ± 0.2 8.5 ± 0.5 1.8 ± 0.6 0.5 ± 0.1 0.4 ± 0.1 14.7 ± 1.2 11.1 ± 7.3
Table 3
Estimated maximum fluxes of K xviii at the rest energies 3.47 keV, 3.51 keV, Ar xvii at the rest energies 3.68 keV, and K xix at the rest
energy 3.71 keV lines obtained from AtomDB in the units of photons cm−2 s−1. Estimates were performed based on best-fit fluxes
obtained from the fluxes of S xvi, Ca xix, and Ca xx lines in the line-free apec model. The fits were allowed to go a factor 3 above these
estimates. The maximum flux for the Ar xvii DR at 3.62 keV line was initially set to 1% of the Ar xvii line at 3.12 keV in the spectral fits.
Sample Inst. Flux Flux Flux Flux Flux
K xviii K xviii Ar xvii Ar xvii K xix
(3.47 keV) (3.51 keV) (3.62 keV) (3.68 keV) (3.71 keV)
( 10−7 ) ( 10−7 ) ( 10−7 ) ( 10−6 ) ( 10−6 )
MOS 1.3 ± 0.7 3.5 ± 1.8 0.12 1.0 ± 0.5 1.2 ± 0.6
Full
Sample PN 0.9 ± 0.4 1.8 ± 0.9 0.14 0.7 ± 0.3 0.3 ± 0.1
Coma + MOS 2.7 ± 2.1 8.2 ± 6.3 7.0 2.5 ± 1.9 5.2 ± 4.1
Centaurus +
Ophiuchus PN 3.3 ± 2.3 6.8 ± 4.7 1.4 2.5 ± 1.8 0.8 ± 0.6
Perseus MOS 18.5 ± 9.9 45.7 ± 24.4 6.4 15.1 ± 8.1 11.6 ± 6.2
PN 13.8 ± 6.8 36.0 ± 17.8 1.99 10.8 ± 5.4 9.15 ± 4.5
All MOS 0.5 ± 0.2 1.3 ± 0.5 0.10 0.4 ± 0.1 0.29 ± 0.1
Other
Clusters PN 1.3 ± 0.5 2.6 ± 0.9 0.90 1.1 ± 0.4 1.2 ± 0.4
abundances between the S and Ca (the measured lines
on which the predictions are based) on one hand and K
and Ar on the other. (This factor 3 is not included in
Table 3.)
Since our detected emission line is only 50 eV away
from the Ar xvii dielectronic recombination (DR) at the
rest energy 3.62 keV, we calculated the emissivity of the
Ar xvii DR line in a conservative way, using AtomDB
9. 9
1 5 10
Plasma Temperature (keV)
LineFlux(photonscm
-2
s
-1
)
Ar XVII (3.68 keV)
K XVIII (3.51 keV)
MOS Detection
K XVIII (3.47 keV)
K XIX (3.71 keV)
10
10
10
10
- 9
- 8
- 6
- 5
10
- 7
107
108
10-20
10-19
10-18
10-17
Emissivity(photonscms)
Log (Temperature) (K)
Ar XVII 3.12 keV
n=2à 1‘triplet’ lines
Ar XVII 3.62 keV DR lines
Based on
AtomDB v2.0.2
T ~ 2 keV
Figure 4. Left Panel: Estimated line fluxes of the K xviii at the rest energies 3.47 keV, 3.51 keV, the Ar xvii at the rest energies 3.68
keV, and the K xix at the rest energy 3.71 keV as a function of plasma temperature.The line fluxes are calculated based on the observed
fluxes of S xvi, Ca xix, and Ca xx from the stacked XMM-Newton MOS observations of the full sample. The flux detection and 90% errors
on the flux of the unknown spectral feature measured from the stacked MOS observations of the full sample is shown with the red shaded
area. Right Panel: A comparison of emissivities of the Ar xvii triplet lines at 3.12 keV and Ar xvii DR line at 3.62 keV. The figure
shows that the flux ratio of the Ar xvii at 3.12 keV to the Ar xvii DR line at 3.62 keV could at most be 1% at the lowest temperature we
observe in our fits (T∼ 2 keV indicated with the dashed line). This fraction was used as an upper limit to the flux of the Ar xvii DR line
in our spectral fits and given in Table 3 for each sample.
v2.0.2. The He-like Argon ‘triplet’ including four lines
(known either as w, x, y, z or R, I1, I2, and F) was
summed, since the components cannot be distinguished
at the CCD resolution. The two Ar xvii DR lines at 3.62
keV, known in AtomDB as 10077 → 2 and 10078 → 3,
and which are the result of a He-like Ar ion recombining
to Li-like Ar and emitting a photon at 3.62 keV, were
similarly extracted and summed. The right panel of the
Figure 4 shows the comparison of the emissivity of Ar
xvii DR and He-like Argon triplet at E ≈ 3.12 keV. To
model the flux of the Ar xvii DR line in our spectral fits
in a conservative way, we set the lower and upper limits of
the flux to be 0.001 and 0.01 times the flux of the He-like
Ar. The upper limit corresponds to the highest flux that
Ar xvii DR can have for the ICM plasma temperatures
that we see in our spectra (this will be further discussed
in §3.4). The lower limit has been set to avoid problems
with the fitting procedure.
Once the lower and upper limits on flux estimates of
K xviii, Kxix, and Ar xvii lines are set, we performed
the fit in a narrower band 3 − 6 keV energy band (to
avoid strong S and Si lines below 3 keV and Fe lines
above 6 keV). This band is sufficiently wide to measure
the continuum accurately (to better than 1%). The weak
residual emission line at E ≈ 3.57 keV was detected in
the fits. The excess emission after the Gaussian K and Ar
lines were included in the model at their maximum fluxes
(as described above) in MOS and PN spectra is shown
in Figure 5. We have then added a Gaussian model to
fit the remaining residuals, leaving its flux and energy to
vary. The fit was improved by ∆χ2
of 22.8 for MOS and
∆χ2
of 13.9 for PN for an additional two degrees of free-
dom (energy and normalization). The best-fit energy of
the added Gaussian line is 3.57 ± 0.02 (0.03) keV in the
stacked MOS and 3.51 ± 0.03 (0.04) keV in the stacked
PN observations. The line energies from MOS and PN
are in significant tension, 2.8σ apart (Fig. 8). However,
given the systematic uncertainties of the fitting proce-
dure, we consider it acceptable; this tension disappears
once another level of complexity is introduced in model-
ing (see §3.5 below). The width of the new line is unre-
solved and broadened only by the instrumental response.
This is the only significant unidentified feature we have
detected in the 2–10 keV band of MOS and PN spectra.
To measure the flux of this line, we have to use a
statistically proper response file, which will depend on
the physical interpretation of the line. If the line were
coming from the thermal plasma, then the same spec-
tral responses that were used for the thermal components
are appropriate. However, there are no known thermal
plasma lines at this energy, so we explore a possible in-
terpretation of the detected line as a decay signature of
the sterile neutrino (see §1). In this interpretation, the
spectral fitting procedure has to be slightly modified. In
particular, when co-adding the instrumental responses
used for the DM line component, the individual cluster
responses should be weighted by the factor ωdm propor-
tional to the estimated dark matter photon flux from
each cluster (as opposed to the X-ray flux used for the
response averaging so far). These response files will be
solely used to measure the flux of the detected 3.57 keV
line; for the rest of the components, clearly originating in
the ICM, the X-ray flux weighting is correct. The dark-
matter response weighting was done using the following
approach.
The surface brightness of the DM decay signal
is proportional to the DM column density SDM =
l.o.s.
ρDM (r)dr. The observed photon flux from the DM
decay into a solid angle ΩF OV is given by
FDM =
MF OV
DM
4πD2
L
Γγ
ms
(1 + z) photons cm−2
s−1
. (3)
where Γγ and ms are the decay rate and mass of the
sterile neutrino (see eq. 1 and Pal & Wolfenstein (1982)),
MF OV
DM is the projected DM mass within the spectral ex-
traction region (Rext, which is either R500 or RF OV ),
and DL is the luminosity distance. The expected contri-
10. 10
0.6
0.7
0.8
Flux(cntss
-1
keV
-1
)
-0.02
-0.01
0
0.01
0.02
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
300
305
310
315
Eff.Area(cm
2
)
3.57 ± 0.02 (0.03)
XMM-MOS
Full Sample
6 Ms
1
1.5
Flux(cntss
-1
keV
-1
)
-0.02
0
0.02
0.04
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
980
1000
1020
Eff.Area(cm
2
)
3.51 ± 0.03 (0.05)
XMM-PN
Full Sample
2 Ms
-0.04
0
0.04
0.08
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
280
285
Eff.Area(cm
2
)
XMM-MOS
Centaurus +
Coma +
Ophiuchus
525.3 ks
-0.2
-0.1
0
0.1
0.2
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
630
640
650
Eff.Area(cm
2
)
XMM-PN
Centaurus +
Coma +
Ophiuchus
168 ks
-0.002
0
0.002
0.006
0.008
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
290
295
300
305
310
315
Eff.Area(cm
2
)
XMM-MOS
Rest of the
Sample
(69 Clusters)
4.9 Ms
-0.02
0
0.02
0.04
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
1220
1240
1260
Eff.Area(cm
2
)
XMM-PN
Rest of the
Sample
(69 Clusters)
1.8 Ms
Figure 5. Top panels: 3−4 keV band of the stacked MOS (left panel) and stacked PN (right panel) spectra of the samples. The figures
show the energy band where the new spectral feature is detected. The Gaussian lines with maximum values of the flux normalizations of K
xviii and Ar xvii estimated using AtomDB were included in the models. The red lines in the top panels (shown only for the full sample)
show the model and the excess emission. The blue lines show the total model after another Gaussian line is added, representing the new
line. Middle panels shows the residuals before (red) and after (blue) the Gaussian line is added. The bottom panels show the effective area
curves (the corresponding ARF). Redshift smearing greatly reduces variations of the effective area in the high-z sample.
bution of each cluster i to the total DM line flux in the
stacked spectrum is
ωi,dm =
Mproj
i,DM (< Rext)(1 + zi)
4πD2
i,L
ei
etot
. (4)
where zi is the redshift of ith cluster, and ei and etot are
the exposure time of ith cluster and the total exposure
time of the sample.
The dark matter mass within the extraction radius is
11. 11
estimated as
MDM (Rext) = Mtot(Rext) − Mgas(Rext) − M∗(Rext),
(5)
where Mtot(Rext), Mgas(Rext), and M∗(Rext) are the to-
tal mass, gas mass, and stellar mass in the extraction
radius Rext, respectively. The observed Vikhlinin et al.
(2009) temperature−mass scaling relation was used to
infer total masses for the intra-cluster gas temperatures
measured from the XMM-Newton observations. The gas
mass is determined following the method described in
Bulbul et al. (2010). The contribution of stars to the
total baryon budget is modest at large radii but more
important in the cluster centers because of the presence
of cD galaxies. At large radii (≥ R500), M∗ is 10%−15%
of the gas mass (Lin & Mohr 2004; Vikhlinin et al. 2006).
Stellar masses of each cluster were determined using the
stellar mass − total mass scaling relation (Gonzalez et
al. 2013). The calculated dark matter masses were cor-
rected using this factor. The projected dark matter
masses within Rext were then determined by project-
ing Navarro-Frenk-White (NFW) profiles (Bartelmann
1996; Golse & Kneib 2002; Loewenstein et al. 2009).
We used a concentration parameter c500 = 3 from the
Vikhlinin et al. (2006) c − M500 scaling relation and the
median total mass within R500 of the full sample, which
is ∼ 6 × 1014
M . The projected dark matter mass
within each spectral extraction radius is given in Table
4.
Weights for the responses to be included in the stacked-
spectrum response were calculated as follows. The num-
ber of dark matter decay photons in each cluster spec-
trum is
Si = α ωi,dm etot Ai, (6)
where Ai is the ancillary response (the instrument effec-
tive area) at photon energy E/(1+zi), and α is the ratio
of the decay rate of sterile neutrinos to the sterile neu-
trino mass ms (here we denote α ≡ Γγ/ms). The total
number of dark matter photons in the stacked line is
Sline =
i=73
i=0
Si
= α ωtot etot Aω,
(7)
where the weighted ARF Aω is a function of the total
weight ωtot,
Aω =
i
ωi
ωtot
Ai, (8)
and
ωtot =
i
ωi. (9)
The weighted responses Aω were used to model our
new line, while X-ray count-weighted response files were
used to model the other known emission lines and the
continuum components.
For MOS, the flux in the 3.57 keV line was 4.0+0.8
−0.8
(+1.8
−1.2) × 10−6
photons cm−2
s−1
, where the errors are
68% (90%). For PN, at the best-fit energy of 3.51 keV,
the line flux is 3.9+0.6
−1.0 (+1.0
−1.6) × 10−6
photons cm−2
s−1
.
If we fix the line energy from the MOS fit, for PN we
obtain the flux 2.5+0.6
−0.7 (+1.0
−1.1) × 10−6
photons cm−2
s−1
.
We note that the line energy detected in the stacked
PN observations of the full sample is consistent with the
K xviii line at 3.515 keV. However, the measured flux
from this line is a factor 20 above the flux of the Kxviii
line estimated from the AtomDB. In addition, the de-
tected energy in the stacked MOS observations of the
full sample is 3.5σ away from the K xviii line. This will
be further discussed later.
Since this is a blind search, we have examined ∼ 70 in-
dependent energy resolution elements in our search band.
Taking this into account, our 4 − 5σ detections corre-
spond to the probability of falsely detecting a line at
an unknown energy of 0.004% for MOS and 0.5% for
PN. However, the line is found at a consistent energy (or
at least in the same independent resolution element) in
these two completely independent samples coming from
different instruments. The statistical chance of such a
false detection at the same energy is negligibly low. We
caution that these are just the rough estimate of the
statistical probabilities; systematic uncertainties are also
important (§6).
We also fit the same MOS and PN spectra using
the X-ray count-weighted responses, to check if the de-
tection is dependent on the response weighting. For
MOS, the flux of the detected line was 4.1+1.0
−0.9 (+1.8
−1.56)
× 10−6
photons cm−2
s−1
; the fit was improved by ∆χ2
of 21.8 for 2 degrees of freedom. For PN, the line flux
was 3.9+1.3
−1.0 (+2.1
−2.0) × 10−6
photons cm−2
s−1
, while the
fit was improved by ∆χ2
of 13.8 for 2 degrees of free-
dom. This shows that the detection is robust and the
flux independent of the response scaling.
We will discuss the possible physical interpretations of
this emission line in §5. Here we will push forward with
one possible interpretation of the detected line, sterile
neutrino decay, because we need to describe the calcu-
lation of certain quantities that will be used below for
cross-checks and comparison of the subsamples of our
full sample.
For a DM particle decaying radiatively with Eγ =
ms/2, the detected flux from a clump of matter of a
known mass can be converted into the decay rate. The
energy of the detected line corresponds to a sterile neu-
trino particle mass of ms = 7.1 ± 0.07 keV, assuming
that the dark matter is solely composed of sterile neutri-
nos. The relation between the flux and mass implies a
mixing angle of
sin2
(2θ) =
FDM
12.76 cm−2 s−1
1014
M
MFOV
DM
DL
100 Mpc
2
1
1 + z
1 keV
ms
4 (10)
where FDM is the observed DM flux.
12. 12
Table 4 Columns (1) and (2) show the estimated projected dark matter masses in the spectral extraction radii Mproj
DM (Rext) and the
extraction radii Rext in Mpc, Column (3) is the projected dark matter masses per distance squared, and column (4) shows the ratio of
the exposure time to the total exposure stacked for each cluster, column (5) is the weighting factors (ωdm) calculated based on the
predicted dark matter flux used in the stacking of ARFs and RMFs of each cluster in the sample. These stacked ARFs and RMFs were
then used to determine the flux of the detected line, and column (6) shows the weighting factors (ωcnt) calculated based on the total
counts in the fitting band. The response files which were stacked using these factors were utilized to model plasma emission lines.
(1) (2) (3) (4) (5) (6)
Cluster Mproj
DM (Rext) Rext Mproj
DM /D2 Exp/Exptot ωdm ωcnt
(1014 M ) (Mpc) (1010 M / Mpc2)
Centaurus 0.63 0.17 2.41 0.044 0.138 0.074
A1060 0.58 0.19 1.82 0.010 0.024 0.009
A262 0.52 0.24 1.24 0.015 0.024 0.011
Perseus 1.49 0.25 2.89 0.048 0.180 0.39
AWM7 0.86 0.24 1.82 0.045 0.106 0.061
Coma 2.72 0.33 2.78 0.026 0.094 0.06 2
A3581 1.32 0.27 1.35 0.028 0.050 0.013
Ophiuchus 4.14 0.41 3.05 0.009 0.036 0.032
A4038 1.31 0.39 0.91 0.008 0.010 0.007
A496 2.29 0.47 1.24 0.038 0.061 0.044
A2063 1.92 0.51 0.88 0.008 0.009 0.0057
A2147 2.06 0.51 0.96 0.002 0.003 0.0016
A3571 3.94 0.57 1.42 0.007 0.0136 0.012
A3558 3.40 0.67 0.82 0.012 0.013 0.01
A4059 2.75 0.68 0.07 0.010 0.0008 0.007
Triangulum Australis 7.58 0.72 1.66 0.002 0.005 0.003
Hydra A 2.68 0.77 0.51 0.025 0.016 0.023
A754 15.65 0.77 2.93 0.004 0.015 0.0032
A2319 6.93 0.77 1.31 0.003 0.0004 0.033
Cygnus A 3.81 0.77 0.72 0.005 0.005 0.004
AS1101 1.95 0.69 0.34 0.025 0.011 0.0136
A3112 4.80 0.97 0.49 0.054 0.034 0.0337
A2597 3.61 0.91 0.29 0.004 0.002 0.002
A478 8.30 1.18 0.61 0.019 0.014 0.017
PKS0745−19 10.03 1.37 0.52 0.005 0.003 0.003
A2811 5.29 1.08 0.15 0.007 0.001 0.0018
A2034 8.07 1.29 0.35 0.005 0.002 0.002
RXC J0616.8−4748 3.97 0.95 0.16 0.0069 0.001 0.0007
RXC J0145.0−5300 6.11 1.14 0.45 0.011 0.003 0.0025
RXC J1044.5−0704 3.05 0.84 0.09 0.007 0.0009 0.0014
A1068 4.44 0.99 0.12 0.005 0.0009 0.0012
RXC J2218.6−3853 6.68 1.18 0.20 0.005 0.001 0.0013
RXC J0605.8−3518 4.91 1.14 0.20 0.005 0.001 0.0013
A1413 9.09 1.36 0.24 0.053 0.016 0.018
A2204 8.86 1.34 0.21 0.019 0.005 0.010
A3888 8.57 1.32 0.06 0.013 0.001 0.004
RXC J0958.3−1103 6.58 1.17 0.15 0.002 0.0005 0.0006
A545 10.79 1.46 0.25 0.002 0.0005 0.0004
RXC J2014.8-2430 6.18 1.14 0.13 0.006 0.001 0.002
RX J1720.1+2638 6.64 1.17 0.13 0.016 0.003 0.004
RXC J0645.4-5413 8.55 1.31 0.16 0.005 0.001 0.001
A1201 5.78 1.10 0.11 0.015 0.002 0.0017
A1914 13.93 1.62 0.26 0.006 0.002 0.002
A2345 7.65 1.24 0.14 0.015 0.003 0.002
A2218 7.48 1.23 0.13 0.012 0.002 0.002
A2254 7.47 1.23 0.13 0.017 0.003 0.002
A665 9.50 1.37 0.16 0.006 0.001 0.0015
A1689 12.55 1.55 0.20 0.010 0.002 0.004
A383 3.48 0.87 0.05 0.008 0.0005 0.009
A520 22.2 7.75 0.82 0.009 0.001 0.001
A2163 67.9 26.33 0.34 0.003 0.001 0.001
A209 8.82 1.32 0.11 0.005 0.0007 0.0007
A963 6.81 1.17 0.07 0.006 0.0006 0.001
RXC J1504.1-0248 8.87 15.6 0.09 0.011 0.003 0.004
Continued on next page
13. 13
TABLE 4 – continued from previous page
Cluster Mproj
DM (Rext) Rext Mproj
DM /D2 Exp/Exptot ωdm ωcnt
(1014 M ) (Mpc) (1010 M / Mpc2)
MS 0735.6+7421 3.89 0.91 0.04 0.014 0.0008 0.001
A773 9.34 1.35 0.11 0.004 0.0005 0.0004
AS0592 13.27 1.57 0.14 0.008 0.002 0.0017
A2390 12.07 1.66 0.13 0.003 0.0005 0.0008
A2667 9.66 1.30 0.10 0.006 0.0007 0.0011
A267 4.83 0.99 0.05 0.002 0.0001 0.0005
RXC J2129.6+0005 3.06 0.81 0.03 0.0097 0.0004 0.001
RXC J1314.4-2515 8.61 1.29 0.07 0.010 0.0009 0.004
A1835 12.15 1.18 0.10 0.037 0.005 0.009
A1758 4.54 1.17 0.03 0.009 0.0004 0.0008
A1763 10.47 1.41 0.11 0.004 0.0005 0.0005
A689 22.51 1.95 0.15 0.05 0.0001 0.0001
ZW 3146 5.52 1.23 0.04 0.059 0.003 0.010
A781 5.57 1.04 0.03 0.018 0.0007 0.001
Bullet 15.2 1.45 0.09 0.006 0.0007 0.001
MS 2137.3-2353 4.31 0.93 0.02 0.003 0.0001 0.0002
MACS J2229.7-2755 3.51 0.85 0.02 0.009 0.0001 0.0006
MACS J1532.8+3021 4.85 0.97 0.02 0.003 0.0007 0.0003
AS1063 16.80 1.68 0.07 0.004 0.0004 0.0008
Using the ωdm and the projected dark matter masses
given in Table 4, we find that the weighted projected
dark matter mass per distance squared is 1.82 × 1010
M /Mpc2
for the full sample observed with XMM-
Newton MOS. Using equation 3, one can calculate the
mixing angle for the full MOS cluster sample to be
sin2
(2θ) = 6.8+1.4
−1.4 (+2.0
−3.0) × 10−11
. The PN observa-
tions of the full sample give a mixing angle measurement
of sin2
(2θ) = 6.7+1.7
−1.0 (+2.7
−1.7) × 10−11
for a weighted mass
per distance squared of 1.80 × 1010
M /Mpc2
. These are
given in Table 5. The PN and MOS full-sample measure-
ments are consistent with each other and the constraints
placed by previous studies, e.g. the unresolved cosmic
X-ray background (CXB) in the Chandra Deep Fields
(Abazajian et al. 2007) and the XMM-Newton blank-sky
background spectrum (Boyarsky et al. 2006), Chandra
observation of the Bullet cluster (Boyarsky et al. 2008),
Chandra observations of M31 (Watson et al. 2012; Hori-
uchi et al. 2013), and XMM-Newton observations of M31,
the Willman 1, and Fornax dSph (Boyarsky et al. 2010).
For the PN flux for the line fixed at the best-fit MOS
energy, the corresponding mixing angle is sin2
(2θ) =
4.3+1.2
−1.0 (+1.8
−1.7) × 10−11
. This measurement is consistent
with that obtained from the stacked MOS observations
at a 1σ level. Since the most confident measurements
are provided by the highest signal-to-noise stacked MOS
observations of the full sample, we will use the flux at
energy 3.57 keV when comparing the mixing angle mea-
surements for the sterile neutrino interpretation of this
line.
3.2. Excluding bright nearby clusters from the sample
We now divide the full cluster sample into three inde-
pendent subsamples, in order to check that our line does
not originate from any single object. The full stacked
spectra examined in §3.1 have a significant contribu-
tion of photons from several nearby bright clusters, e.g.
Perseus, Coma, Centaurus, and Ophiuchus. In order to
determine whether the line detection is dominated by
these bright sources, we excluded them from the sample
and stacked the MOS and PN spectra of the remaining 69
fainter galaxy clusters. We have performed the stacking
process following the same approach described in §2.3.
A total of 4.9 Ms of good stacked MOS and 1.7 Ms good
stacked PN exposure were obtained for this sub-sample.
The weighted mean redshift was 0.06. The stacked MOS
and PN spectra contain 34% (2.95× 106
source counts)
and 55% (2.79× 106
source counts) of the total source
counts of the full cluster sample.
We fit the stacked spectra using the line-free apec
model and additional Gaussian models as described in
§3.1 in the 3−6 keV band. The best-fit temperatures,
normalizations of the line-free apec model, and the fluxes
of S xvi, Ca xix, and Ca xx lines are given in Table 2.
We then carefully examined the spectra for any uniden-
tified emission features in the 3.4 − 3.7 keV energy inter-
val. Similarly, we determined the maximum fluxes of the
K xviii, K xix, and Ar xvii lines based on the plasma
temperatures and fluxes of hydrogen-like S xvi, helium-
like Ca xix, and hydrogen-like Ca xx lines at 2.63 keV,
3.90 keV, and 4.11 keV, measured from the spectral fits,
and AtomDB as described in §3.1. As before, the lower
and upper limits of the fluxes of K xviii, K xix, and
Ar xvii lines were set to 0.1 to 3 times of the maximum
predicted fluxes. The Ar xvii DR line flux at 3.62 keV
was allowed to vary between 10−3
to 10−2
of the Ar xvii
triplet line at 3.12 keV.
We obtained an acceptable fit to the stacked MOS
spectrum of these 69 clusters. The total χ2
was 557
for 573 degrees of freedom. Adding in an extra Gaus-
sian model to the MOS spectrum at 3.57 keV improved
the fit by ∆χ2
of 16.5 for one additional degree of free-
dom. We found that the best-fit flux was 2.1 +0.4
−0.5 (+0.8
−0.8)
× 10−6
photons cm−2
s−1
. This flux corresponds to a
mixing angle of sin2
(2θ) = 6.0 +1.1
−1.4 (+2.3
−2.3) ×10−11
, consis-
tent with the mixing angle estimates obtained from the
14. 14
Table 5
Columns (2) and (3) are the measured rest energy and flux of the unidentified line in the units of photons cm−2 s−1 at the 68% (90%)
confidence level. The energy’s with asterisks are frozen to the indicated values; column (4) and (5) show the χ2 before the line is added to
the total model and change in the χ2 when an additional Gaussian component is added to the fit; column (6) is the weighted ratio of
mass to distance squared of the samples, and column (7) shows the mixing angle limits measured in each sample. Reported constraining
limits are 90% confidence upper limits.
(1) (2) (3) (4) (5) (6) (7)
Sample Inst. Energy Flux χ2 ∆χ2 Mproj
DM /D2 sin2(2θ)
(keV) (10−6 phts cm−2 s−1) (dof) (∆ dof) (1010 M /Mpc2) ( 10−11 )
MOS 3.57 ± 0.02 (0.03) 4.0 +0.8
−0.8 (+1.8
−1.2) 564.8 22.8 1.82 6.8 +1.4
−1.4 (+2.0
−3.0)
(566) (2)
Full XMM
Sample PN 3.51 ± 0.03 (0.05) 3.9 +0.6
−1.0 (+1.0
−1.6) 510.5 13.9 1.80 6.7 +1.7
−1.0 (+2.7
−1.7)
(564) (2)
PN 3.57 2.5 +0.6
−0.7 (+1.0
−1.1) 510.5 11.2 1.80 4.3 +1.2
−1.0 (+1.8
−1.7)
(564) (1)
MOS 3.57 15.9 +3.4
−3.8 (+6.7
−5.5) 562.3 17.1 2.68 18.2 +4.4
−3.9 (+12.6
−11.5)
Coma + (569) (1)
Centaurus + XMM
Ophiuchus PN 3.57 < 9.5 377.8 − − < 10.9
(387)
MOS 3.57 21.4 +7.0
−6.3 (+11.2
−10.5) 596.1 12.8 2.82 23.3 +7.6
−6.9 (+12.2
−11.5)
Perseus (574) (1)
(without XMM
the core) PN 3.57 < 16.1 539.1 − − < 17.6
(553)
MOS 3.57 52.0 +24.1
−15.2 (+37.0
−21.3) 613.8 15.7 2.89 55.3 +25.5
−15.9 (+39.3
−22.6)
Perseus (574) (1)
(with XMM
the core) PN 3.57 < 17.7 539.4 − − < 18.8
(554)
MOS 3.57 2.1 +0.4
−0.5 (+0.8
−0.8) 547.2 16.5 1.08 6.0 +1.1
−1.4 (+2.3
−2.3)
All (573) (1)
Other XMM
Clusters PN 3.57 2.0 +0.3
−0.5 (+0.5
−0.8) 741.9 15.8 1.15 5.4 +0.8
−1.3 (+1.3
−2.1)
(751) (1)
ACIS-S 3.56 ± 0.02 (0.03) 10.2 +3.7
−3.5 (+4.8
−4.7) 201 11.8 0.72 40.1 +14.5
−13.7 (+18.9
−18.2)
(197) (2)
Perseus Chandra
ACIS-I 3.56 18.6 +7.8
−8.0 (+1.2
−1.6) 152.6 6.2 1.86 28.3 +11.8
−12.1 (+18.2
−24.3)
(151) (1)
Virgo Chandra ACIS-I 3.56 < 9.1 189.1 − 2.41 < 10.5
(155)
full sample.
The overall fit to the stacked PN spectrum for these 69
clusters was acceptable with a total χ2
of 741.9 for 751
degrees of freedom. Adding an extra Gaussian line at
3.57 keV improved the fit by ∆χ2
of 15.8 for an additional
degree of freedom. The PN spectrum yields the best-fit
flux detection of 2.0 +0.3
−0.5 (+0.5
−0.8) ×10−6
photons cm−2
s−1
.
The mixing angle obtained from the stacked PN observa-
tions sin2
(2θ) = 5.4 +0.8
−1.3 (+1.3
−2.1) × 10−11
is also consistent
with the estimates from the full sample. Bottom panels
in Figure 5 show the residuals before and after a Gaus-
sian line is added at 3.57 keV to MOS and PN spectral
15. 15
5
6
7
Flux(cntss
-1
keV
-1
)
-0.2
-0.1
0
0.1
0.2
0.3
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
300
305
310
315
Eff.Area(cm
2
)
XMM - MOS
Perseus
(with core)
317 ks
10
12
14
16
Flux(cntss
-1
keV
-1
)
-0.8
-0.4
0
0.4
0.8
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
690
700
710
720
Eff.Area(cm
2
)
XMM - PN
Perseus
(with core)
38 ks
Figure 6. 3−4 keV band of the stacked MOS (left panel) and stacked PN (right panel) spectra of the Perseus cluster. The figures show
the energy band, where a new spectral feature at 3.57 keV is detected. The Gaussian lines with peak values of the flux normalizations of
K xviii and Ar xvii estimated using AtomDB were included in the models. The red lines in the top panels show the model and the excess
emission in both spectra. The blue lines show the total model after a Gaussian line is added, indicating that the unidentified spectral line
can be modeled with a Gaussian.
fits.
3.3. Stacked Spectra of the Nearby Bright Clusters;
Centaurus + Coma + Ophiuchus
We now check the MOS and PN spectra of the three
dominant nearby clusters, Coma, Ophiuchus, and Cen-
taurus. A total of 525.3 ks of good stacked MOS and
168 ks good stacked PN exposure times were obtained
for this sub-sample. The total source counts obtained in
the MOS and PN spectra were 3.2 × 106
and 2.1 × 106
,
respectively.
We performed the fits as above. The best determina-
tions for the continuum temperature and normalizations
and the fluxes of the S xvi, Ca xix, and Ca xx are given
in Table 2. We detected an excess emission feature in
the same band, i.e. 3.4 − 3.7 keV as in the stacked MOS
spectra. To determine the flux of the emission line at 3.57
keV, we estimated the maximum fluxes of the K xviii, K
xix, and Ar xvii lines using the AtomDB and the mea-
sured fluxes of S xvi, Ca xix, and Ca xx as described
in §3.1. Using the 0.1 and 3 times these fluxes as lower
and upper limits, we found that the unidentified line has
a flux of 15.9+3.4
−3.8 (+6.7
−5.5) × 10−6
photons cm−2
s−1
in the
stacked MOS observations. Adding this Gaussian to the
model improves the fit by ∆χ2
of 17.1 for an additional
degree of freedom for the stacked MOS spectrum.
We then allowed the energy of the additional Gaus-
sian model to vary to test whether the energy measured
from two different samples are the same. The best-fit
energy obtained from the stacked MOS observations of
Coma, Centaurus, and Ophiuchus clusters was 3.56 ±
0.02 (0.03), with a flux of 1.6+0.52
−0.44 (+0.81
−0.70) × 10−5
pho-
tons cm−2
s−1
. This measurement is consistent with the
energy measured in the MOS observations of the full sam-
ple. The sterile neutrino mixing angle that corresponds
to this flux is sin2
(2θ) = 18.2+4.4
−3.9 (+12.6
−11.5) × 10−11
, con-
sistent at 2σ with the full-sample value.
The fits to the stacked PN observations did not need an
additional Gaussian line, and resulted in a non-detection.
This could be due to the low count statistics of the
stacked PN observations (168 ks clean time). A 90%
upper limit on the flux of this line at 3.57 keV is 9.5
× 10−6
photons cm−2
s−1
from this spectrum; the upper
limit on the mixing angle from this flux limit is consistent
with the full-sample and MOS detections.
3.4. Perseus
Initially, we extracted the spectrum of the Perseus clus-
ter using the entire MOS and PN field-of-view. We have
co-added the XMM-Newton MOS and PN observations
of the Perseus cluster in the cluster’s frame. The total
exposure time in the stacked MOS spectrum was 317 ks
with a total of 7×106
source counts in the 2 − 10 keV
band and 38 ks total exposure with 2×106
source counts
in the stacked PN observations.
Following the same approach we used for modeling the
full cluster sample, we first fit the MOS and PN observa-
tions with the line-free apec model and additional Gaus-
sian models. Count-weighted responses were used to fit
the plasma emission lines and the continuum emission.
Probing the 3−4 keV band the MOS observations re-
vealed residuals around 3.57 keV, at the same energy
band where we detected line emission in the previous
samples. The left panel of Figure 6 shows the detection
in the co-added MOS observations of the Perseus cluster.
Using the limits on the K and Ar lines (Table 3) as above
and adding a Gaussian model to the MOS spectrum at
the fixed energy of 3.57 keV improved the fit by ∆χ2
of
15.7. The best-fit flux at 3.57 keV was 5.2+2.41
−1.52 (+3.70
−2.13)
× 10−5
photons cm−2
s−1
.
This flux corresponds to a mixing angle of sin2
(2θ) =
5.5+2.6
−1.6 (+3.9
−2.3) ×10−10
. This angle is not only an outlier in
our measurements from the other samples, it is also not
consistent with the upper limits on the mixing angle at
this value of ms from the previous studies (e.g., Horiuchi
et al. 2013).
We were unable to detect the line in the short (38 ks
clean time) PN observation of Perseus and placed a 90%
upper limit on the flux of the line of 17.7 photons cm−2
s−1
, which corresponds to an upper limit of sin2
(2θ) <
16. 16
1.9 × 10−10
, consistent with the MOS detection. Figure
6 shows both XMM-Newton Perseus spectra.
4.5
5.5
6.5
Flux(cntss
-1
keV
-1
)
-0.2
-0.1
0
0.1
0.2
Residuals
3 3.2 3.4 3.6 3.8 4
Energy (keV)
295
300
305
Eff.Area(cm
2
)
XMM - MOS
Perseus
(core cut)
317 ks
Figure 7. 3−4 keV band of the core-excised stacked MOS spec-
trum of the Perseus cluster. The figures show the energy band,
where a new spectral feature at 3.57 keV is detected. The Gaus-
sian lines with peak values of the flux normalizations of K xviii
and Ar xvii estimated using AtomDB were included in the mod-
els. The red lines in the top panels show the model and the excess
emission in both spectra. The blue lines show the total model after
a Gaussian line is added, indicating that the unidentified spectral
line can be modeled with a Gaussian.
Since this is a single-cluster spectrum, we first check
whether the Perseus signal is not an artifact of our blue-
shifting procedure. For this we fit the original, redshifted
MOS spectrum with a line-free apec model. We obtained
a best-fit χ2
463 for 385 dof. Adding a Gaussian line at
3.57 keV (rest energy) improved the fit by ∆χ2
of 16 for
an additional degree of freedom. The best-fit flux was 5.3
± 1.2 (2.0) × 10−5
photons cm−2
s−1
, is in agreement
with the flux obtained from the blue-shifted spectrum.
We conclude that our detection is independent of shifting
the spectrum.
Not ready to abandon the sterile neutrino explanation
based on the line flux incorrectly scaling with cluster
mass that we see for Perseus, we tried to investigate
possible astrophysical reasons behind the excess of the
line flux in Perseus. First, we investigated the depen-
dence of the energy and flux of this unidentified line on
the AtomDB predicted fluxes of nearby lines, i.e., the
K xviii line at 3.51 keV and the Ar xvii DR line at
3.62 keV. Allowing the energy of the Gaussian compo-
nent to vary produced a best-fit for an energy of 3.56
+0.01
−0.02 (+0.02
−0.03) keV, with a flux of 6.0+1.8
−1.4 (+2.4
−1.7) × 10−5
photons cm−2
s−1
(χ2
of 598.1 for 572 dof). The best-fit
energy is consistent with the energy measured from the
MOS observations of the full sample. However, the fluxes
of the nearby K xviii line at 3.51 keV and the Ar xvii
DR at 3.62 keV line were at their allowed upper limits
predicted from the AtomDB. Relaxing the upper limits
has shifted the line energy higher, to 3.59 +0.01
−0.03 (+0.02
−0.04)
keV with a flux of 5.5+1.7
−0.8 (+3.7
−1.5) × 10−5
photons cm−2
s−1
gave a slightly better fit (χ2
of 594.5 for 572 dof). We
note that the line energy of this extra line gets close to
the Ar xvii DR line at 3.62 keV. So we removed the extra
Gaussian line and re-fit the Perseus spectrum removing
the upper limits on the Ar xvii DR line. We obtained
only a slightly worse fit than the previous case, with a χ2
of 598.8 (574 dof). The measured flux of the Ar xvii DR
line at 3.62 keV in this case was 4.8+0.7
−0.8 (+1.3
−1.4) × 10−5
photons cm−2
s−1
, which is a factor of 30 above the pre-
dicted maximum flux of the Ar xvii DR line based on
the measured flux of the Ar xvii line at ∼3.12 keV and
AtomDB line rates. The predicted maximum flux of the
Ar xvii DR line for the Perseus spectrum was 1.6 × 10−6
photons cm−2
s−1
(< 0.01 times the flux of the Ar xvii
triplet at ∼3.12 keV).
This test showed that the line detected in the Perseus
cluster could also be interpreted as an abnormally bright
Ar xvii DR line. We note that, however, that obtaining
such a bright DR line relative to the He-like triplet at
3.12 keV is problematic. The emissivity of the satellite
line peaks at kT=1.8 keV, and declines sharply at lower
temperatures, in addition to the change in the ionization
balance which reduces the Ar+17
content of the plasma.
The emissivity ratio for the DR/3.12 keV has its max-
imum value of 0.04 at kT=0.7 keV, but the emissivity
of both lines is weak here, so any hotter component will
dominate and lead to a lower ratio being observed.
To avoid cool gas in the Perseus core contaminating
the flux of the nearby Ar and K lines, we also tried ex-
cising the central 1 region of the cluster and performed
the fit on the core-excised co-added MOS spectrum. We
found that adding an extra Gaussian line at 3.57 keV has
improved the fit by ∆χ2
of 12.8 for an additional degree
of freedom with a best-fit flux of 2.1 +0.7
−0.6 (+1.2
−1.1) × 10−5
photons cm−2
s−1
(see Figure 7). Excising the inner-
most 1 reduced the flux of the detected line by a factor
of two, indicating that the most of the flux of this emis-
sion originates from the cool core. The mixing angle that
corresponds to the line flux from the core-excised Perseus
spectrum is consistent within 1 − 2σ with those for the
bright clusters (Centaurus+Coma+Ophiuchus) and the
full sample, respectively (Table 5).
3.5. Refitting full sample with anomalous 3.62 keV line
With the knowledge that the 3.62 keV line can be
anomalously high (at least in Perseus), we should now
try to re-fit the stacked MOS spectrum of the full sample
to see if the line in the full sample is affected by the 3.62
keV excess from Perseus, which is part of the full sam-
ple. We set the flux of the 3.62 keV line to the Perseus
contribution of the Ar xvii DR line to the full-sample
spectrum (2.3 × 10−6
photons cm−2
s−1
), assuming all
the new line flux in Perseus originates from the abnor-
mally bright DR line. We note that this flux was already
a factor of 30 above the predicted upper limits by the
AtomDB. Adding an extra Gaussian component, repre-
senting the new line, to a model with the anomalous 3.62
keV line, still improves the fit by ∆χ2
of 6.52 for 2 de-
grees of freedom. The best-fit energy and flux were 3.55
± 0.03 (0.05) and 2.23+1.6
−0.9 (+2.2
−1.5) × 10−5
photons cm−2
s−1
, respectively. The new line is still required with 2.5σ
in the full sample; however, the energy of this line gets
lower and its confidence interval wider. The line energy
comes into agreement with the energy detected in PN
full sample (see Figure 8 left panel). If we completely
free the normalization of the 3.62 keV line in the full-
17. 17
3.5 3.55 3.6
Line Enegy (keV)
0
5
10
15
Free with Nominal Upper Limit
Set to Perseus Contribution
∆χ2
Ar XVII DR Line Flux
2.65σ
3 σ
3.62 keV
XMM - MOS
Full Sample
1σ
3.51 keV
3.48 3.5 3.52 3.54 3.56
Line Energy (keV)
2
4
6
8
3.55 keV
3.57keV
∆χ
2
XMM - PN
Full Sample
1σ
2.78σ
2.25σ
Figure 8. Left Panel: The change in the goodness-of-fit statistics as a function of the detected line energy at 3.55-3.57 keV obtained
from the stacked MOS observations of the full sample. The red continuous line show the confidence of the line energy when the flux of the
Ar xvii DR line at 3.62 keV were left free to vary within the AtomDB predicted boundaries. In this case, the detected line is 2.65σ from
the Ar xvii DR line at 3.62 keV line. The blue dashed line shows the confidence curve of the line energy when the flux of the Ar xvii DR
line at 3.62 keV was fixed at the maximum DR contribution from the Perseus cluster. In this case the line energy is consistent with the
Pn detection. Right Panel: The change in the goodness-of-fit statistics as a function of the line energy obtained from the stacked PN
observations of the full sample. The line energy is 2.2σ and 2.7σ away from the MOS detections.
sample MOS spectrum, it comes lower than the Perseus
contribution that we considered above.
4. CHANDRA OBSERVATIONS OF PERSEUS AND VIRGO
Due to the potential significance of the discovery of
an emission line due to the decay of sterile neutrinos in
clusters, it is necessary to confirm it with another instru-
ment. Pending a full stacking analysis of the Chandra
and Suzaku cluster archives (which is a current work in
progress), we analyze two Chandra observations of the
Perseus and Virgo clusters, which have over 1 Ms and
500 ks of total Chandra exposure, respectively. A sum-
mary of the Chandra observations used in this work to
confirm the detection is given in Table 6.
4.1. Chandra Data Analysis
The Chandra ACIS-I and ACIS-S data were processed
following Vikhlinin et al. (2005), using CIAO 4.5 and
CALDB 4.5.7. Each event list was filtered for high back-
ground periods. After this filtering the total good times
were 487 ks and 883 ks for the ACIS-I and ACIS-S ob-
servations of the Perseus cluster core, respectively. We
have extracted the ACIS-S spectra from the full S3 chip
excluding the 1 region surrounding the cluster centroid
and one of the observations (ID: 4950) with a background
flare. The ACIS-I spectrum was extracted using a cir-
cular region covering the full ACIS-I FOV. The filtered
ACIS-I good time for the Virgo cluster core was 481 ks.
Analysis steps include image creation, point source de-
tection with wavdetect and their removal.
Background corrections were made using the blank sky
background fields, including the “period-E” background
files. For each target event file, a corresponding back-
ground event file was generated and normalized by the
ratio of counts in the 9.0−12.0 keV energy range (Hickox
& Markevitch 2006). Because we are interested in the
high energy part of the spectrum, modeling of the soft
sky CXB is not relevant.
Each spectrum was fitted using a standard multi-
temperature apec model as described in §2.3 to determine
the best-fit X-ray redshift of each observation, shown in
Table 6. Each event file was then blue-shifted to the
cluster’s source frame using these best-fit redshifts. The
source and background spectra in the source’s frame were
obtained by generating spectra using the scaled event en-
ergy values in the event files. The ARFs and RMFs were
remapped based on the estimates of the best-fit redshifts.
The RMFs and ARFs were weighted by only the exposure
time of each observation. The scaled source and back-
ground spectra were co-added using the FTOOL math-
pha, whereas ARFs and RMFs were merged using the
FTOOLS addarf and addrmf tools, respectively.
4.2. Chandra Detection of the Emission Line in Perseus
Following the same method as described in §3.1, the
continuum emission was fit using the line-free apec model
with additional Gaussian models to represent the strong
emission lines. The best-fit temperature from the 2.0 −
6.0 keV band and normalizations of the line-free apec
model, fluxes and equivalent widths of S xvi, Ca xix,
and Ca xx lines at 2.63 keV, 3.90 keV and 4.11 keV, are
given in Table 7. We have searched especially the 3.0
− 4.0 keV interval where the 3.57 keV line emission was
detected in the stacked XMM-Newton observations. The
measured fluxes of S xvi, Ca xix, and Ca xx lines from
the ACIS-I and ACIS-S spectra with the AtomDB fluxes
yielded the maximum predicted fluxes of K xviii lines
at 3.47 keV and 3.51 keV, Ar xvii line at 3.68 keV, and
K xviii line at 3.71 keV as described in detail in §3.1.
The triplet emission line at Ar xvii 3.12 keV was used to
determine the maximum allowed flux of the Ar xvii DR
line at 3.62 keV at any plasma temperature as described
above. The predicted fluxes of these lines are given in
Table 8. Using 0.1 and 3 times of the upper bound of
these estimates as lower and upper limits for K xviii,
Ar xvii, and 10−3
to 10−2
times of the flux of the Ar
xvii triplet for the lower and upper bounds for the Ar
18. 18
Table 6
Summary of Chandra observations of the Perseus and Virgo clusters used in this work. The columns list (1) primary detector array used;
(2) observation number (3) & (4) right ascension and declination of the pointing (J2000); (6) good exposure time in ks after filtering.
(1) (2) (3) (4) (5)
Cluster Detector Obs ID RA DEC Exposure (ks) Redshift
Perseus ACIS-I 11713 03 19 31.8 +41 37 49.0 113.0 0.017
ACIS-I 11714 03 19 42.6 +41 34 07.0 92.3 0.017
ACIS-I 11715 03 19 44.2 +41 25 18.0 73.6 0.019
ACIS-I 11716 03 19 44.2 +41 25 18.0 39.4 0.017
ACIS-I 12025 03 19 31.8 +41 37 49.0 17.6 0.017
ACIS-I 12033 03 19 31.8 +41 37 49.0 18.6 0.018
ACIS-I 12036 03 19 31.8 +41 37 49.0 47.7 0.018
ACIS-I 12037 03 19 44.2 +41 25 18.0 85.0 0.018
Perseus ACIS-S 4289 03 19 47.6 +41 30 37.0 95.4 0.018
ACIS-S 3209 03 19 47.6 +41 30 37.0 95.7 0.018
ACIS-S 4946 03 19 48.2 +41 30 42.2 23.6 0.018
ACIS-S 6139 03 19 48.2 +41 30 42.2 56.4 0.018
ACIS-S 4947 03 19 48.2 +41 30 42.2 29.7 0.018
ACIS-S 6145 03 19 48.2 +41 30 42.2 85.0 0.018
ACIS-S 4948 03 19 48.2 +41 30 42.2 118.6 0.018
ACIS-S 4949 03 19 48.2 +41 30 42.2 29.4 0.018
ACIS-S 6146 03 19 48.2 +41 30 42.2 47.1 0.018
ACIS-S 4951 03 19 48.2 +41 30 42.2 96.1 0.018
ACIS-S 4952 03 19 48.2 +41 30 42.2 164.2 0.018
ACIS-S 4953 03 19 48.2 +41 30 42.2 30.1 0.018
Virgo ACIS-I 5826 12 30 49.5 +12 23 28.0 127.5 0.0040
ACIS-I 5827 12 30 49.5 +12 23 28.0 157.6 0.0038
ACIS-I 5828 12 30 49.5 +12 23 28.0 33.2 0.0036
ACIS-I 6186 12 30 49.5 +12 23 28.0 50.8 0.0040
ACIS-I 7210 12 30 49.5 +12 23 28.0 31.1 0.0033
ACIS-I 7211 12 30 49.5 +12 23 28.0 15.5 0.0038
ACIS-I 7212 12 30 49.5 +12 23 28.0 65.3 0.0036
Table 7
Best-fit Temperature and Normalizations of line-free apec Model Fit to the Co-added Chandra Observations of the Perseus and Virgo
clusters. Fluxes of the S xvi, Ca xix, Ca xx at the rest energies 2.63 keV, 3.90 keV, 4.11 keV are given.
Perseus Virgo
Model Paramaters ACIS-I ACIS-S ACIS-I
kT1 (keV) 4.58 ± 0.07 2.77 ± 0.18 1.18 ± 0.07
N1 (10−1 cm−5) 1.20 ± 0.01 3.21 ± 0.35 2.22 ± 0.10
kT2 (keV) 5.34 ± 0.02 4.79 ± 0.18 5.08 ± 0.37
N2 (10−1 cm−5) 3.83 ± 0.13 2.31 ± 0.24 1.02 ± 0.23
Flux of S xvi (10−4 pht cm−2 s−1) 3.94 ± 0.15 3.72 ± 0.85 6.18 ± 0.13
Flux of Ca xix (10−4 pht cm−2 s−1) 1.29 ± 0.08 1.05 ± 0.08 1.22 ± 0.88
Flux of Ca xx (10−4 pht cm−2 s−1) 1.10 ± 0.05 1.07 ± 0.05 0.35 ± 0.05
xvii DR line, we determined the best-fit flux of the weak
residual around 3.57 keV.
An additional Gaussian model improves the fit by ∆χ2
of 11.8 for an additional 2 degrees of freedom. The line
was unresolved and consistent with broadening by the
instrument response in the Perseus cluster spectra. The
19. 19
4
4.5
5
5.5
6
Flux(cntss
-1
keV
-1
)Residuals
3.2 3.4 3.6 3.8 4
Energy (keV)
320
330
340
350
Eff.Area(cm
2
)
0.02
-0.02
-0.06
0.06
0
3.56 keV Chandra ACIS-I
Perseus
487 ks
3
3.5
4
4.5
5
5.5
Flux(cntss
-1
keV
-1
)
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
Residuals
3 3.2 3.4 3.6 3.8
Energy (keV)
400
405
410
415
420
425
Eff.Area(cm2
)
Chandra
ACIS-S
Perseus
883 ks
3.56 ± 0.02 (0.03) keV
Figure 9. Close up 3.1 − 4.1 keV energy interval of the co-added spectrum obtained from Chandra ACIS-I and ACIS-S observations
of the Perseus cluster. The continuum emission was fit with 2T line-free apec model, while emission lines were modeled with additional
Gaussian components. The K xviii (3.51 keV), Ar xvii (3.62 keV) and Ar xvii (3.68 keV) lines are also included in the total model shown
in red line on the top panel. Blue line show the total model after a Gaussian component is to the total model, indicating that the weak
residual can be modeled with a Gaussian. The bottom panels show the weak residual before and after the Gaussian is added to the total
model.
Table 8
Best-fit Temperature and Normalizations of line-free apec Model Fit to the Co-added Chandra Spectra of Perseus and Virgo clusters. (1)
and (2) are the estimated fluxes of K xviii at the rest energy 3.51 keV and Ar xvii at the rest energy 3.68 keV lines obtained from
AtomDB in the units of photons cm−2 s−1; (3) and (4) are the estimated energy in keV and flux of the unidentified emission line in the
units of photons cm−2 s−1; (5) is the measured equivalent width of the spectral feature, indicates the ratio of line flux to continuum flux
in the units of keV.
Cluster Inst. Flux Flux Flux Flux Flux
K xviii K xviii Ar xvii Ar xvii K xix
(3.47 keV) (3.51 keV) (3.62 keV) (3.68 keV) (3.71 keV)
(10−6) (10−6) (10−7) (10−5) (10−6)
ACIS-I 3.4 ± 2.7 3.1 ± 2.4 4.3 ± 3.5 0.8 ± 0.4 2.6 ± 2.1
Perseus
ACIS-S 4.5 ± 2.3 4.15 ± 2.2 5.8 ± 2.8 1.3 ± 1.0 3.4 ± 2.7
Virgo ACIS-I 2.0 ± 0.5 3.6 ± 1.0 38.2 ± 10.6 1.7 ± 0.5 1.8 ± 0.6
Perseus ACIS-S spectra yields a best-fit energy of 3.56 ±
0.02 (0.03) keV for an additional Gaussian model, given
in Table 5. The flux of the detected signal is 1.02 +3.7
−3.5
(+4.8
−4.7) × 10−5
photons cm−2
s−1
. This detection corre-
sponds to a false detection probability of 0.5% in the
co-added ACIS-S spectrum. Figure 9 right panel shows
the signal in the Chandra ACIS-S observations of the
Perseus cluster before and after the Gaussian model is
added to the fit.
To further demonstrate that the detected flux is in-
dependent of the spectral modeling, we fit the ACIS-S
spectrum of the Perseus cluster with a two-temperature
vapec model with abundances of trace elements set to
that of Fe. We obtained an acceptable fit in the 3 − 6
keV energy band with the χ2
of 182.1 for 147 degrees of
freedom. An additional Gaussian model at 3.56 keV (rest
energy) improved the fit by ∆χ2
of 16 for an extra degree
of freedom. The best-fit flux of the line is 1.09 ± 0.26
(0.42) × 10−5
photons cm2
s−1
, which is consistent with
the flux measured in the line-free apec model fit with
additional Gaussian models. This test shows that the
detection is robust and independent of the method used
in the spectrum modeling. The Perseus co-added spec-
trum fit with a two-temperature vapec model is shown in
Figure 10.
We then performed the same search in the co-added
ACIS-I spectrum of the Perseus cluster. Fitting the 2.5-
6 keV band of the ACIS-I spectrum with a line-free apec
model with additional Gaussian lines as described above
produced a good fit overall with a total χ2
of 158.7 for
152 dof. Adding a Gaussian line at 3.56 keV, the en-
ergy where the line was detected in the co-added ACIS-
S spectrum of the Perseus cluster, improved the fit by
20. 20
3
3.5
4
4.5
5
5.5
Flux(cntss
-1
keV
-1
)
3 3.2 3.4 3.6 3.8
Energy (keV)
-0.04
-0.02
0
0.02
0.04
0.06
Residuals
Perseus ACIS-S
vapec Fit
Figure 10. The 3 − 4 keV energy interval for the co-added spec-
trum obtained from Chandra ACIS-S observations of the Perseus
cluster. The continuum emission was fit with two vapec models.
This detection demonstrates that the detected line is not a fitting
artifact.
∆χ2
of 6.2 for one additional degree of freedom. The
flux of the detected signal was 1.9 +8.0
−7.8 (+1.2
−1.6) × 10−5
photons cm−2
s−1
in the co-added ACIS-I spectrum. Fig-
ure 9 left panel shows the ACIS-I spectrum of the Perseus
cluster before and after an additional Gaussian model is
added to the total model, to demonstrate the detection
of the line.
The mixing angle sin2
(2θ) estimate from the co-added
Chandra ACIS-S observations of the Perseus cluster is 4.0
+1.5
−1.4 (+1.8
−1.8) × 10−10
is consistent with the angle obtained
from the co-added ACIS-I and XMM-Newton MOS ob-
servations of the Perseus cluster at the 1σ level. Since
ACIS-S chip covers the central 4 region of the Perseus
core, higher flux measured from ACIS-S observations also
indicates that this emission is concentrated in the core,
confirming the results from the XMM-Newton observa-
tions of the Perseus core.
4.3. Chandra Upper Limit on Line from Virgo
We have performed the same fitting strategy described
above to the co-added spectra of the Virgo cluster, e.g.
line-free apec model with additional Gaussian lines. We
used the lower and upper limits to the K and Ar line
in the 3.4 − 3.7 keV band based on the upper limits
estimated from the AtomDB (given in Table 8). The
overall fit was acceptable with the total χ2
of 82.5 for
62 dof. Unlike the Perseus cluster, the co-added Virgo
cluster did not show any residuals around 3.57 keV in
the fit with the line-free apec model. Adding a Gaussian
line did not significantly improve the fit. We were able
to place an upper limit of 9.1× 10−6
photons cm−2
s−1
at the 90% confidence level. This limit corresponds to
an upper limit on the mixing angle of sin2
(2θ) < 1.1 ×
10−10
.
We also fit the 2.5−4.0 keV band of the Virgo spectrum
using a two temperature standard vapec model. The fit
has a total χ2
obtained from the vapec model was 91.7 for
82 degrees of freedom. We overall obtained a better fit
with the standard vapec model than the fit with the line-
free apec model. The best-fit model also did not require
the addition of a line at 3.56 keV. The 90% upper limit
0
0.005
0.01
0.015
0.02
0.025
0.03
(phtscm
-2
s
-1
keV
-1
)
0
0.01
0.02
0.03
(phtscm
-2
s
-1
keV
-1
)
3 3.2 3.4 3.6 3.8 4
Energy (keV)
-0.04
-0.02
0
0.02
0.04
Residuals
S XVI Line-free APEC + Gaussians
S XVI S XVI
Ar XVII
Ca XIX
Ar XVII
Ar XVII Ar XVIII
S XV
VAPEC
Ar XVII
K XVIII
Figure 11. The 3 − 4 keV energy interval for the co-added spec-
trum obtained from Chandra ACIS-I observations of the Virgo clus-
ter at the redshifted frame. The continuum emission was fit with
line-free apec model with Gaussians components (upper pale) and
two-temperature vapec models (middle panel). The lower panel
shows the differences in the residuals for the two models. Red data
points show the residuals of the line-free apec model with Gaus-
sians components and black data points show the residuals of the
vapec model. The energy where the line is detected in the Chan-
dra ACIS observations of the Perseus cluster is indicated with an
arrow.
to the flux of this line is < 6.2 × 10−6
photons cm−2
s−1
.
The differences in the modeling approaches used in the
ACIS-I spectrum fits of the Virgo cluster (line-free apec
with Gaussians and vapec) are demonstrated in Figure
11. The factor of two difference in the upper limits on
the flux measurements indicates that the systematical
uncertainties in the flux measurements can be as large
as a factor of two depending on the modeling method
used in this analysis.
5. DISCUSSION
Stacking X-ray spectra of galaxy clusters in the source
frame enhances weak emission features while minimizing
the effects of instrumental and background features due
to the redshift smearing. Sanders & Fabian (2011) used
stacked XMM-Newton RGS observations of 62 clusters
to find, for example, the first evidence of Ovii in cluster
cores. (The RGS energy coverage is limited to E < 2
keV.) We stacked the XMM-Newton MOS (6 Ms) and
PN (2 Ms) spectra of 73 nearby (z < 0.35) well-exposed
galaxy clusters in their source frame and detected a weak
emission line at the rest energy of 3.57 ± 0.02 keV at
the 68% confidence level in XMM-Newton MOS obser-
vations. We have detected a similar emission feature
independently in the stacked PN observations of the
full sample, although the best-fit line energy was lower,
3.51 ± 0.03 keV. There is tension between these ener-
gies at a 2.8σ level, including only statistical errors; they
become consistent once we introduce another degree of
freedom in the model.
The best-fit fluxes of 4.0+0.8
−0.8 × 10−6
photons cm−2
s−1
photons cm−2
s−1
and 3.9+0.6
−1.0 × 10−6
photons cm−2
s−1
photons cm−2
s−1
obtained from the stacked MOS and
PN observations of the full sample are consistent with
each other. Even taking into account the fact that we
conducted a blind search in ∼ 70 independent energy
bins, the statistical probability of a false detection of
21. 21
such a 4−5σ line at the same energy in two independent
datasets is negligibly small.
We then divided the full sample into three subsamples
to test if the signal originates from one of the dominant
nearby clusters in the sample, i.e. the Perseus, Coma,
Centaurus, and Ophiuchus clusters. In the Centaurus +
Coma + Ophiuchus MOS spectrum, the line was with
the flux of 1.6+0.3
−0.4 × 10−5
photons cm−2
s−1
at 3.57 keV.
The lower signal-to-noise (128 kes total) PN spectrum
resulted in a no-detection, with a 90% upper limit of 9.5
× 10−6
photons cm−2
s−1
.
The stacked MOS and PN observations in the rest
frame of the fainter 69 clusters showed the emission line
at 3.57 keV with the best-fit flux of 2.1+0.4
−0.5 × 10−6
and
2.0+0.3
−0.5 × 10−6
photons cm−2
s−1
at the energy 3.57 keV.
The significant detection of the line originate from one
or a few dominant clusters, but is produced by all the
clusters in subsamples.
We investigated the detection in the XMM-Newton ob-
servations of the Perseus cluster in detail. The full-FOV
MOS spectrum of the Perseus cluster has best-fit 3.57
keV line at a flux of 5.2+2.4
−1.5 × 10−5
photons cm−2
s−1
,
with ∆χ2
of 15.7 for an additional degree of freedom. We
note that the flux of the detected line is dependent on
the predicted fluxes of the K xviii triplet feature at 3.51
keV and an Ar xvii DR line at 3.62 keV. In the spectral
fits of the Perseus cluster, the fluxes of these nearby lines
were at their allowed upper limits. Relaxing these upper
limits shifts the best fit line energy to 3.59 keV, suggest-
ing that the detected line could be the Ar xvii DR line
at 3.62 keV. To test this, we removed the Gaussian com-
ponent at 3.57 keV and found that the spectra could be
represented without an additional line. However, in this
case the implied flux of the Ar xvii DR line had to be
significantly increased from the AtomDB estimate that
it would be 1% of the strength of the Ar xvii triplet,
to 30%. Physically, it is difficult to create such a bright
Ar xvii DR line relative to the Ar xvii He-like triplet at
3.12 keV. The emissivity ratio for the Ar xvii DR line to
the Ar xvii triplet at 3.12 keV has its maximum value of
4% at kT=0.7 keV. Since the emissivity of both lines is
weak at this temperature, any hotter temperature com-
ponent will dominate the spectra, leading to a even lower
observed normalization ratio. However, in this case the
implied flux of the Ar xvii DR line had to be signifi-
cantly increased from the AtomDB estimate — by factor
30. This possibility is discussed below in §5.1.
To further investigate the origin of this excess emis-
sion, we excised the central 1 region of the Perseus core.
The best-fit flux of 2.1+7.0
−6.3 × 10−5
photons cm−2
s−1
at
3.57 keV decreased to its half of the flux measured when
the core was excluded. This decrease indicates that the
emission is highly concentrated in the immediate cool
core of the Perseus cluster.
In addition, we investigated the Chandra ACIS-S and
ACIS-I spectra of Perseus to confirm that the detected
signal is not an XMM-Newton detector feature. An in-
dependent search of 3.0 − 4.0 keV interval of the ACIS
spectra revealed a positive detection of the feature with
at a significance of 3.4σand 2.5σ. The measured best-
fit energies of 3.56 ± 0.02 keV in ACIS-S spectrum (1
core was excised) is consistent with the best-fit energy
obtained from the stacked XMM-Newton observations of
the full sample. The observed flux of the detected feature
is 1.0+3.7
−3.5 × 10−5
photons cm−2
s−1
. The same feature
was also observed in the co-added ACIS-I spectrum of
the Perseus cluster with the best-fit flux of and 1.8+7.8
−8.0
× 10−5
photons cm−2
s−1
, with a less significance (∆χ2
=
6.2 for 1 dof). However, the feature was not detected in
the Chandra ACIS-I observations of the Virgo cluster.
These observations allowed us to place an 90% upper
limit of 9.1 × 10−5
photons cm−2
s−1
.
5.1. Unknown plasma emission line?
One possible interpretation is that the detected line
is an unknown plasma emission line. The flux of the
line corresponds to a maximum emissivity of 3.3 × 10−18
photons cm3
s−1
, derived using the emission measure
appropriate for the lowest temperature (4.36 keV) com-
ponent as described in §3.1. For comparison, this is sim-
ilar to the maximum emissivity of the Ca xx Lyα line
at 4.1 keV. Given that the Ca xx line was previously
observed in individual galaxy cluster spectra, including
the Perseus cluster (e.g. Tamura et al. 2009), a line as
strong at ∼ 3.57 keV would have been observed had it
been expected. However, there is no likely candidate for
an atomic transition near 3.57 keV. The emission lines
of strong hydrogen- and helium-like ions are well known,
and none fall in this band. The only candidate is Cl xvi,
which has emission lines at 3.56 keV from n = 5 → 1
transitions, but would imply even stronger lines from
n = 3 → 1 at 3.27 keV and n = 4 → 1 at 3.44 keV
should be present. Emission lines from L-shell ions form
a far more complex pattern. However, the binding energy
of Li-like Zn (Z = 30) is only 2.782 keV, so the transi-
tion lines of all lighter elements or less ionized species
must be at lower energies than this. If this line is a
K-shell fluorescence transition, it must be from an ele-
ment whose neutral and Li-like K-shell fluorescent line
energies bound 3.57 keV. The only such atoms are argon
and potassium, but in this case the relevant Ar K-shell
fluorescence transition is simply another name for the
Ar xvii DR line discussed in detail above. The neutral
potassium Kα fluorescence line is at 3.313 keV while neu-
tral Kβ is at 3.59 keV, so there must be transitions at the
relevant energy. In this case, the best matches are the Kα
transitions of K XVI through K XIV ions, which occur at
∼ 3.57 keV (Palmeri et al. 2012). However, since at any
temperature above 1 keV potassium will have at most 2
bound electrons, any such line would have to be originat-
ing from an unknown source of photoionized potassium
in clusters. Thus this scenario is very unlikely, since the
compact sources, e.g. AGNs are not strong enough to
photoionize the low density ICM.
Although a complete analysis was not shown, adding
an Ar XVII DR line at 3.62 keV with unconstrained
flux into all of our spectra would significantly impact
both the fit results and detection level of a line at 3.57
keV. We have constrained this line to be at most 1% of
the strength of the unresolved Ar XVII triplet at 3.12
keV, but must consider the physical situation required
to maximize the 3.62 keV DR line. In thermal equilib-
rium, the maximum strength of this line is 4% of the Ar
XVII triplet, albeit at a temperature where the expected
emission is negligible. One might also consider an ex-
treme non-equilibrium situation with cold electrons that