X-ray Study of W28
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This is the ppt of my honours research.
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X-ray Study of W28
- 1. Supervisor: Dr. Jasmina Lazendic-Galloway presented by Alex Li
- 2. Target: SNR W28 (G6.4-0.1) The Northeastern Shell (XMM-Newton data) The Central Region (Chandra data) Aims: Detect powerlaw component (non-thermal X- ray radiation) from W28’s NE shell for broadband model Study plasma properties of SNR W28
- 3. Ejected stellar material (ejecta) from a supernova, mixed with interstellar medium (ISM) and compressed by forward shock, forms a SNR. What is supernova remnant (SNR)?
- 4. SNR Evolution Free Expansion Adiabatic or Sedov phase Radiative phase Mejecta >> Mswept-up Ejecta expands without deceleration Outer ISM shell by forward shock Mejecta ~ Mswept-up Significant deceleration Ejecta merges into forward shock Energy loss by adiabatic expansion Efficient particle acceleration by strong forward shock Tejecta < 106K Efficient cooling by radiation Well-mixing between ejecta and ISM No efficient particle acceleration due to slow forward shock Disappearance when velocity drops to the typical value of ISM ejecta outer shell of ISM by forward shock
- 5. Types of SNR Shell-type (e.g. Cassiopeia A) • non-thermal X-ray emitting shells Crab-like (e.g. Crab Nebula) • non-thermal X-ray or radio radiation from a pulsar at the centre • faint shells or no shell at all Composite-type • if appear as both shell-like and Crab-like Mixed-morphology (e.g. W44) • radio shell emission • thermal X-ray center-filled emission • uniform temperature profile not common in shell-type SNRs Ejecta Cassiopeia A Crab Nebula Radio shell Radio image Thermal X-ray center-filled emission X-ray image W44 Neutron star (a little spot)Non-thermal X- ray emitting shell by forward shock Pulsar
- 6. SNR W28 • Mixed-morphology SNR • 35000-15000 year old • 50’x45’ angular size • ~ 2kpc distant away • Possibly evolved into its radiative phase Previous X-ray observation with ROSAT and ASCA (2002): Northeastern : single thermal component (0.6 keV ), ISM abundance, Center: two thermal components (1.8 keV and 0.6 keV) Radio Image of W28 ROSAT X-ray Image of W28
- 7. Why Study SNR W28? • γ-ray emission near the northeastern shell of W28 detected by H.E.S.S in 2008 and Fermi-LAT in 2010. • γ-ray emission: Inverse Compton scattering (electrons) Non-thermal bremsstrahlung (electrons) π0 decay (protons) Detection of non-thermal X-ray • helps determine if γ-ray emission by electron or proton acceleration. Radio Image of W28 ROSAT X-ray Image of W28
- 8. Download raw data from data archive HEASARC and check them, e.g. version, mode, etc. Create a new event file Check light curve Filter the event file Select region and extract the region’s spectrum from the event file Background subtraction Spectral analysis and fitting: XSPEC ver.12.5.1 Need reprocessing for event file? Flaring exist? YES YES NO NO Basic Procedure of Preparing Spectra
- 9. The CCD image of W28’s NE shell from XMM (10 regions) The CCD image of W28 from Chandra (8 regions) Parameters concerned in spectral fitting: 1. Temperature 2. Abundances of elementsROSAT X-ray Image of W28
- 10. Northeastern ShellO Mg Si S MOS-CCD components pn-CCD component The CCD image of W28’s NE shell from XMM
- 11. O Mg Si S The CCD image of W28’s NE shell from XMM pn-CCD component MOS-CCD components Northeastern Shell
- 12. Results for Northeastern Shell All regions’ spectra are best fitted by two thermal components, different to single thermal component observed by ROSAT and ASCA. Low temperature components correspond to interstellar medium (ISM) and high temperature components correspond to SNR ejecta. Power-law model was also used for fitting spectra, but photon indices Γ > 4.0 for all the regions, except REG G that Γ = 3.8.
- 13. Enhanced abundances of O were detected in all regions, and most of them show enhanced abundances of Mg as well, confirming the presence of ejecta in W28’s NE shell. Emission from ejecta is still strong enough to be resolved by XMM, not expected from old SNRs like W28. Results for Northeastern Shell
- 14. Results from Chandra Data Image of the Chandra central chip with point sources moved
- 15. Results from Chandra Data Three-thermal model best fits the central spectra. The component of ~1.8keV was also observed by ROSAT and ASCA (2002), but two thermal only. The third thermal component may correspond to reflected shock by (molecular) cloud. Further analysis is needed to determine whether the reflected shock model fitted the data of the central region. Image of the Chandra central chip with point sources moved Three thermal components Z also observed by ROSAT & ASCA
- 16. Results from Chandra DataChandra image with point sources moved Power-law component was detected in R12. Γ
- 17. Constraint on Non-thermal X-ray Radiation in Northeastern Shell The multi-band spectra of the Fermi-LAT source at the NE boundary of SNR W28 (taken from Abdo et al. 2010). Fermi H.E.S.S EGRET
- 18. We also attempt to estimate the mass of the progenitor star associated with W28 by: 1. calculating the ejecta masses of overabundant elements 2. comparing our values to the well-known results (Thieleman et al. 1996). The progenitor star’s mass: 8 M⊙ < M < 13 M⊙ There should be a neutron star near the remnant’s center. Mass of Progenitor Star associated with W28 Our values are 10 times smaller than that for 13M⊙
- 19. Identifying Background Point Sources and Potential candidates for Neutron star The CCD image of W28 from Chandra • Five possible candidates were identified (blue circles). • But determination of their flux is needed to find out the strong candidates.
- 20. Summary 1. New thermal components detected in both the NE shell and the central region of W28. 2. Confirmation of high temperature plasma of ~1.8 keV at W28’s center. 3. First detection of ejecta with enhanced abundances of O and Mg in W28’s NE shell . 4. First detection of powerlaw components in both W28’s NE and S regions. 5. Determination of lower limit of flux of non-thermal X-ray for the NE shell’s broadband modeling. 6. First identification of possible candidates of neutron star associated with W28.
- 22. Results from Chandra Data O Chandra image with point sources moved
- 23. Results from Chandra Data Oxygen line, not detected in other Chandra’s regions we investigated, was detected in both R1 and R2. However I had trouble to determine the best fits for R1 and R2 due to the background spectrum dominant at energies E > 3keV. Considering consistency, the fits with solar abundances are the preferred ones. Chandra image with point sources moved Chandra image with point sources moved
- 24. Ejecta Injection of ions to shock Efficiency of particle acceleration Amplification of magnetic field in shock by CRs Particle acceleration or re-acceleration Collision between ejecta and dense cloud Reverse shock Modification of shock by CRs ? Energy spectrum of CRs SNR Evolution ? ?
- 25. We use data from X-ray Multi-mirror Mission (XMM-Newton). (above) XMM-Newton satellite and (left) its X-ray telescope’s basic structure.
- 26. CCDs of MOS1 (left) and MOS2 (right) cameras CCDs of pn camera European Photon Imaging Camera (EPIC)(EPIC)
- 27. Chandra X-ray Observatory (above) Chandra satellite and (left) its X-ray telescope’s basic structure.
- 28. Chandra’s Advanced CCD Imaging Spectrometer (ACIS)
Editor's Notes
- Welcome everyone. In the beginning My project as you see from the title is about studying supernova remnant in X-ray band. The remnant I investigated is called SNR W28. Basically, my project’s purpose is to attempt to detect non-thermal radiation associated with particle acceleration in W28, and study the plasma properties of W28.
- Because I don’t expect everyone here familiar with supernova remnant, it is better to talk about what SNR actually is. As the name tells us, the SNR was produced by a supernova explosion occurring at the end of the life of a massive star. The mixture of ejected stellar material from the supernova and the surrounding interstellar medium, compressed by the forward shock, forms the supernova remnant.
- According to the standard evolution, the supernova remnant will undergo three main phases when ejecta and forward shock are expanding outward to interstellar medium. The first phase is called free expansion phase, in which the ejecta is sweeping the surrounding gas and expanding without deceleration. Meanwhile we can see the shell of ISM formed at the boundary due to compression by the forward shock. As long as the mass of interstellar medium swept by ejecta is comparable to the ejecta mass, then the SNR evolves into adiabatic phase, in which ejecta slows down significantly and merge into forwad shock, energy loss is due to adiabatic expansion only. This phase only last for thousands of years but it is believed that efficient particle acceleration can occur by the strong forward shock during the adiabatic phase. Once the temperature becomes lower than a million Kelvin, and cooling by radiation is efficient, then the SNR enters the radiative phase. No efficient particle acceleration is expected because of slow forward shock. When the velocity of the expanding ejecta drops to the typical value of ISM, then the ejecta totally merge with ISM, and the remnant disappears.
- Astronomers used to classify SNRs into four types. Shell-type is the common one which has non-thermal X-ray emitting shell near its boundary, as shown by the classical example Cas A. Second type is called Crab-like in which we can see non-thermal X-ray or radio radiation from a fast spinning neutron star, called a pulsar, at the remnant's center. If the remnants appear as shell-like and Crab-like at the same time, we usually called them composite-type. The latest type is called mixed-morphology SNR. Mixed-morphology SNRs have radio emitting shell instead of non thermal X-ray shells like shell-type, and their central brightness is associated with thermal X-ray radiation emission coming from SNR material instead of non-thermal emission from pulsars. Also, uniform temperature profile is observed in mixed-morphology SNR.
- My target, W28, belongs to the mixed-morphology SNR. It is one of oldest remnants in the Galaxy, and it is likely that W28 has evolved into its radiative phase. The image at top right-hand corner is the radio image of W28. We can see the bright radio shell near the boundary. The red contour lines represent the X-ray emission observed by ROSAT in 2002. From the previous X-ray observation with ROSAT and ASCA, there is single thermal component
- My project was initially motivated by gamma-ray emission detected by H.E.S.S. near the northeastern region of W28. Gamma-ray emission, as we know, can be produced through IC scattering or non-thermal bremsstrahlung both associated with electron acceleration, or neutral pion decay associated with proton acceleration. In order to determine which kind of particle acceleration mainly contribute to the gamma-ray emission, we need detection of non-thermal X-ray emission from the northeastern shell for broadband modeling
- The data we used was downloaded from the public data archive. All useful data is contained in event files. We need to inspect the event file and check if solar flare exists in the event files by looking at their light curves. If the event files are all right, we extract the spectra of regions form the event file and do the background subtraction to eliminate any contribution of background noise to the spectra. Then load the spectra into XSPEC, provided by NASA, for spectral analysis and spectral fitting.
- For the northeastern shell, I investigated 10 regions. On the other hand, I studied 8 regions from Chandra data. I used the models available in XSPEC to fit the spectrum of a region to derive its temperature and abundances of different elements.
- For the northeastern shell, I was able to best fit all the regions using two thermal X-ray emission models. I am going to just show you the resultant spectra of several regions. You can see they are almost the same. The interesting thing is that we detect oxygen emission line which is unable to be detected by previous generation of X-ray telescopes before XMM and Chandra.
- The table on the left side shows the best fit parameters of several regions. The spectra of all regions in the northeastern shell are best fitted by two thermal components, different to the previous observation with ROSAT and ASCA, in which they observed single thermal component only.
- Although we could not detect powerlaw component in any region from XMM data for braodband modeling, we could still estimate the constraint on the non-thermal X-ray radiation. To do so, I intentionally add a powerlaw component with a photon index fixed to be the value of 2.2, then fine-tune the normalization of the powerlaw component unit 3 sigma confidence was achieved. Then the resultant photon flux we determined in this way would be the upper limit of photon flux of the non-thermal X-ray at the NE shell. The upper limit we found is about 0.2eVcm-2s-1 at around 1keV.
- The XMM observatory contains three EPIC detectors: two MOS cameras and a pn camera. All three cameras can detect photons with energy between 0.15 and 15 keV. Each EPIC detector has the field of view (FOV) of 30 arcmin and energy resolution of 0.15keV at 1keV.
- ACIS is composed of 10 planar, 1024 times 1024 pixel CCDs. Each CCD provides 8.4’times8.4’ observational view on the sky. Six of them forms a linear array and the rest is arranged as a square array. Up to 6 ACIS CCDs in any possible combination can be operated simultaneously. In our case, I0,I2 and S2-S5 CCDs were used during the observation.