Gamma-ray bursts

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Gamma-ray bursts

Gamma-ray bursts

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  • 1. Gamma-ray bursts Volodymyr Savchenko ISDC, Geneve 21.11.12
  • 2. Outline● What do we know about GRBs?● What there is more to learn about GRBs?● How are the GRBs useful?
  • 3. GRBs are brightFluence reaching 10-3 erg/cm2 hugely dominating the gamma-ray skyNumerous detectors may serve unintentionally as GRB detectors: as long as theyare out of atmosphereIn fact the first one to observe them was military satellite searching for the nuclearexplosions in 1967.
  • 4. Early theories First decades after the discovery, a number of theories were suggested Most of them appeared to be applicable in other objectsEssential to keep in mind nowadays!
  • 5. First indication of the luminosity scale● CGRO/BATSE: big dedicated detector (1991-2000) ~2500 GRBs: No correlation with the galactic plane or any local structures, suggesting extragalactic origin for the bulk of the events
  • 6. Afterglows: the breakthroughDedicated instrument had to be build to react to the prompt emissionby rotating the X-ray instrument.First implemented in Beppo-SAX immediately led to discovery of X-ray emission following GRB (Costa et al 1997), opening the wholenew field. Redshifted lines were observed in the afterglow, firmly establishing cosmological nature of the GRBs Lasting sometimes for months
  • 7. Afterglows: the breakthroughDedicated instrument had to be build to react to the prompt emissionby rotating the X-ray instrument.First implemented in Beppo-SAX immediately led to discovery of X-ray emission following GRB (Costa et al 1997), opening the wholenew field. Redshifted lines were The luminosity of the order of 1054 erg observed in the afterglow, on the time scale of about some seconds firmly establishing would suggest underlying cosmological nature of the gravitational source of energy GRBs for the bulk of the events Lasting sometimes for months
  • 8. Early spectra Peaks at ~1MeV Powerlaw (i.e. certainly non- thermal) both above and below the peak Phenomenological “Band model” is used to describe:Spectra of the bulk of the GRBs still contain roughly same amount of information
  • 9. Compatness problemFast variability suggests small region - ~<10msHigh luminosity in small emission region would cause pair production andthermalize the particles Very large optical depth But the observed spectrum is non-thermal!
  • 10. Compatness problemThe situation can be saved assuming the emission region is moving relativisticallyGamma-factor at least 100 is requiredThe most relativistic outflow known.
  • 11. Beaming● The isotropic equivalent of 1054 erg solely in gamma-rays would be hard to explain: probably the emission is beamed. Characteristic achromatic break in the afterglow light curve is a signature of beaming Another confirmation comes from the observation of late time radio scintilations Beaming of the order of 1-10 degrees is usually inferred It is possible that there are two components, differently beamed
  • 12. Emission mechanism● Thermal: expected, but observed non-thermal, can be a contribution● Electron synchrotron: requires non-thermal population of electrons● Electron Inverse Compton: requires target field● Proton synchrotron, pion decay: requires proton- loaded outflow
  • 13. The fireball modelThe most radiatively efficient process is the electron synchrotronNon-thermal population would have to be re-accelerated in the shocksRapid and violent “internal” shocks are responsible for the promptemissionMore regular external shock accounts for the afterglow
  • 14. Challenge to the fireball model● Prediction of the “synchrotron deathline” BATSE Preece at al 2000 Low-energy asymptote can not be harder than that of a single electron But it is. Swift/BAT Savchenko et al 2008
  • 15. Challenge to the fireball model● Prediction of the “synchrotron deathline” Preece at al 2000 A set of models were proposed to address the problem (modified synchrotron, inverse compton, thermal contribution), all with considerable issues The measurement itself was not considered quite reliable due to lack of systematically high precision at the most Low-energy asymptote can not be important low energy part of the spectrum harder than that of a single electron But it is. Savchenko et al 2008
  • 16. Not the true spectrum Another complication is that the spectrum is highly variable Evolution of spectral parameters of GRB 090902B Evolution of spectral parameters of GRB 080319Blow-energy slope peak energy seconds The measured spectra are averaged on the time scale larger then variability
  • 17. The true spectrum Another complication is that the spectrum is highly variable Evolution of spectral parameters of GRB 090902B Evolution of spectral parameters of GRB 080319Blow-energy slope peak energy Big detector is required to measure spectra below the variability scale. To access 1 ms one needs 10 m2 at 1 MeV 10.000 kg seconds The measured spectra are averaged on the time scale larger then variability
  • 18. Polarization of the MeV emission Polarization of sub-MeV photons can be measured by measuring direction of Compton-scattered electron Requires dedicated instrument or very careful analysis IKAROS: Solar sail with a GRB detector Strong and variable polarization?INTEGRALIBIS Yonetoku 2012 Gotz 2004 Would indicate ordered magnetic field in the emission region and non-thermal emission process. But further measurements are required.
  • 19. Polarization of the MeV emission Polarization of sub-MeV photons can be measured by measuring direction of Compton-scattered electron Requires dedicated instrument or very careful analysis IKAROS: Solar sail with a GRB detector Strong and variable polarization?INTEGRALIBIS only 3-4 sigma results Would indicate ordered magnetic field in the emission region and non-thermal emission process. But further measurements are required.
  • 20. Polarization of the MeV emission Polarization of sub-MeV photons can be measured by measuring direction of Compton-scattered electron Requires dedicated instrument or very careful analysis IKAROS: Solar sail with a GRB detector Strong and variable Dedicated instrument: POLAR polarization?INTEGRALIBIS In space soon Would indicate ordered magnetic field in the emission region and non-thermal emission process. But further measurements are required.
  • 21. Extension of the energy range: GeV● First observed only in a handful of cases by CGRO/EGRET● Fermi/LAT since 2008 has dramatically improved the quality of the measurements The emission correlates with the prompt at first but then extends for decades longer
  • 22. Extension of the energy range: GeV ● First observed only in a handful of cases by CGRO/EGRET ● Fermi/LAT detected 30 GeV-loud bursts in 4 yearsIts not yet clear what fraction of the bursts have GeV emission. The number of the bursts in LATis less then expected, but no HE cut off was so far observed, putting extreme limit of>1000 on the Lorentz factor.
  • 23. Extension of the energy range: GeV to TeVCurrent generation Cherenkov telescopes are barely able to perform GRB observationsMAGIC specificity was designed light – rapid – to follow GRBs. But no bright enough burstwas in the FoV. CTA will be major improvement. Might measure the cutoff due to pair production, study the decay of the emission in greater detail
  • 24. Extension of the energy range: keV The additional component extends also below the peak It modifies measurements of the low-energy slopes during the prompt phase In one case instead an inexplicable suppression is measured Very few quality measurements are available – X-ray instrument is hard to point promptly
  • 25. Extension of the energy range: keV The additional component extends also below the peak It modifies measurements of the low-energy slopes during the prompt phase In one case instead an inexplicable emission down measured Will measure prompt suppression is to 1 KeV (instead of 10 keV)! Very few quality measurements are available – X-ray instrument is hard to point promptly
  • 26. Extension of the energy rangeGeV-to-X-ray emission long after the prompt phase: INTEGRAL/ISGRI is very useful,but only if lucky 2012, in preparation
  • 27. Extension of the energy rangeGeV-to-X-ray emission long after the prompt phase: INTEGRAL/ISGRI is very useful,but only if lucky Missing instrument 2012, in preparation
  • 28. Extension of the energy range: optical Optical emission during the prompt phase of the GRB has been detected in few cases. The challenge is to start observation of a narrow-field optical instrument in time.It can be in fact extremely bright: reaching magnitude 5.3: stellar size object visible to anaked eye from redshift of 0.97!
  • 29. Extension of the energy range: optical Optical emission during the prompt phase of the GRB has been detected in few cases. The challenge is to start observation of a narrow-field optical instrument in time. Although no burst was seen simultaneously in GeV and optical energetics and MeV spectrum comparative evolution suggest that They might be of common origin A single powerlaw from 1 eV to 10 GeV probably carrying bulk of the GRB energyIt can be in fact extremely bright: reaching magnitude 5.3: stellar size object visible to anaked eye from redshift of 0.97!
  • 30. Extension of the energy range: optical To study the prompt GRB optical emission the telescope has to be extremely fast: react at <1 second. Currently available cases are due to extreme GRB duration, presence of a precursor or to pure luck UFFOThe slewing mirror telescope(SMT), can slew to Field of view of SVOM will be constantlytarget within 10 msec using MEMS (Micro-Electro- monitored by a group of optical telescopesMechanical Systems)
  • 31. Classification Two components? Large sample is required – large instrumentKouveliotou 1999
  • 32. Classification Using more then only the duration Three components?..
  • 33. ClassificationDifferent divisions?.. Large instrument: INTEGRAL/SPI-ACS Savchenko et al 2012
  • 34. Progenitors Two major classes: two kind of progenitorsCollapsar: hypernova – massive supernova Merging compact objects Supported by localization in the host galaxies
  • 35. Collapsar: direct confirmationIn some cases supernova was directly observed after a GRB – always longBut in two cases upper limit excluded supernova...
  • 36. Neutron star merger: directClose compact binary must emit gravitational waves, especially before merging LIGO: interferometerBulk of the short GRBs, if related to NS mergers, will be soon detectableFor local events the limit already is reached. No detection indicates that the originwas most likely not a merger.
  • 37. Tidal disruptionsTidal disruptions of small objects by stellar BH or stars bysupermassive black holes lead to similar phenomena.The difference can be seen in the afterglow.In several cases were directly identified, but contribution to the bulk ofthe events is not really knownIt may be seen as a reminder that a single event should may not representa population (although it is often tempting in the case of GRBs)
  • 38. Magnetar flaresReorganization of magnetic field in extremely magnetized neutron stars alsoleads to short strong bursts, sometimes confused with the GRBs.
  • 39. Unexplained burstsSome of the bursts lack any explanation
  • 40. GRBs to probe history of the universe GRBs are distant: have a potential The most distant single event is at redshift of 9.4 Population studies suggest that there may be till ~20
  • 41. GRBs as standard candlesThe idea is to deduce luminosity from the spectral parametersMost notably correlation between energy of the peak of the spectrum andthe luminosity is observed The correlation is probably driven by the Lorentz factor of the outflow
  • 42. GRBs as standard candles
  • 43. GRBs as probes star formationGRBs carry unique direct information about high-redshift stars Counting number of GRBs with redshift one can deduce the star formation history to unprecedented redshift Unbiased large sample is required
  • 44. GRBs line of sightAbsorption lines by different structures along the line of sight are observedand can be used to study the structure, similarly to Lyman-alpha forestDifferent elements can be probed, to higher redshiftAbsorption in X-ray probes ionized medium
  • 45. GRB as probes for vacuum dispersionDependency of speed of light on photon energy and polarization can be tested.Strong upper limits are set, especially if the polarization is measured.
  • 46. Conclusions● Mechanism of the MeV prompt emission is still not clear but major advances were made recently● Classification is gradually shaping out● Connections of the GRBs with cosmology are strengthening● New results are expected in the coming years