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Ehud Nakar California Institute of Technology

Gamma-Ray Bursts and GLAST . Ehud Nakar California Institute of Technology . GLAST at UCLA May 22. Outline GRBs: observations and model –very very brief overview Sources of GeV emission in GRBs Some physics probed by GLAST The Lorentz factor during the prompt emission

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Ehud Nakar California Institute of Technology

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  1. Gamma-Ray Bursts and GLAST Ehud Nakar California Institute of Technology GLAST at UCLA May 22

  2. Outline • GRBs: observations and model –very very brief overview • Sources of GeV emission in GRBs • Some physics probed by GLAST • The Lorentz factor during the prompt emission • The magnetic field strength • The jet structure • Predictions based on EGRET observations • Summary

  3. Flux F Time  Observations prompt emission • Fluence ~ 10-7- 10-4 erg/cm2 Isotropic Energy ~ 1050-1054 erg • Duration 0.01- 1000 s • Non-thermal spectrum (peaking at ~0.1-1Mev) • Highly variable temporal structure

  4. Afterglow Radio – optical – X-rays Following soft g-rays we observe: X-rays (minutes-weeks), optical emission (hours-months) radio emission (weeks-years) Fox et. al. ‘05

  5. Longs & shorts Kouveliotou et al. 1993 ? Shorts A merger of compact binary ???(Eichler et al 1989; …) (Review by Nakar 07) Longs Collapsar (Woosley et al., …) (Review by Piran 05, Meszaros 06)

  6. Goodman 86’ Paczynski 86’ Shemi & Piran 90’, … (Rees & Meszaros 94, …) Collimated Baryonic flow emEk synchrotron -rays Compact Source Internal Shocks 1013-1015cm EM instabilities  Particle acceleration (~1016 cm) Poynting flux dominated flow Eem>>Ek synchrotron -rays Lyutikov & Blandford 02, Thompson 06 Thompson 94’, Usov 94’, Katz 97’, Meszaros & Rees 97’, … The Fireball Model Prompt emission

  7. Reverse shock†† (~1017 cm) Poynting flux dominated flow X-rays Optical Radio Magnetic††† bubble Afterglow (in the fireball model) Relativistic ejecta X-rays Optical Radio Baryonic flow Forward shock† (1017-1018 cm) External medium †Meszaros & Rees 92… ††Meszaros & Rees 92; Katz 94; Sari & Piran 95… †††Luytikov & Blandford 02

  8. GeV-TeV photons • Gev-TeV photons are expected to result from • Inverse compton: • Comptonization of the self synchrotron emission (SSC) in the internal, external and reverse shocks (Meszaros et al 94, Waxman 97, Wei & Lu 99, Dermer et al, …) • IC of photons produced in one shock by electrons that are accelerated in another shock (e.g.,Pe’er & Waxman 04,Beloborodov 05. Wang et al. 2006, Fan & Piran 2006) • p0decay, proton synchrotron: • Expected to be fainter than IC component (e.g., Bottcher & Dermer 98, Totani 98, Bahcall & Meszaros 00, Zhang & Meszaros 01)

  9. GeV spectrum of the prompt emission Constraining the Lorentz factor High opacity to MeV photons is avoided by high Lorentz factor Long GRBs - assumption of high energy power-law spectrum up to Gev (supported by EGRET) implies G>~100-300 (e.g., Lithwick & Sari 01) Short GRBs – Observatoins hint on a spectral cutoff (indication of particle acceleration cutoff???) around 300 keV implying G>~15 (Nakar 07) Detection of opacity spectral cutoff will provide a measurement of G

  10. Synchrotron Self-Compton constraining the magnetic field strength SSC emission is predicted to dominate at GeV ee – fractional electron energy eB – fractional magetic field energy Afterglow observation indicate ee~0.1 and eB~10-3-10-2 In the prompt emission ee>0.1, eB isnot well constrained

  11. Orphan afterglows –probing the jet structure A collimated relativistic jets predict: On-axis orphan afterglow (Nakar & Piran ‘03) Typical GRB Off-axis orphan afterglow (Rhoads ‘97) Nakar & Piran 03 Extensive search for optical orphan afterglows didn’t detect any yet.GLAST has the potential to detect GeV orphans!

  12. Detectability of a very bright GRB by the LAT alone qjet=0.05 rad Eiso=1054 erg, n=1 cm-3, ee=0.3, eB=0.01, z=1

  13. Earth occultation EGRET GRBs Hurley et al 1994 EGRET detected about a dozen GRBs both during the prompt emission and the afterglow

  14. GeV detections by EGRET From Ph.D. thesis by Maria Magdalena Gonzalez Sanchez Prompt emission Afterglow

  15. SSC predicts (to first order) a linear relation between BATSE and EGRET fluences: FEGRET=10h·FBATSE where h distributed normally Afterglow Prompt emission Ando, Nakar & Sari, in preparation Likelihood contours for h distribution (m and s)

  16. B A C Ando, Nakar & Sari, in preparation

  17. Summary • EGRET observations guarantee GRB detections by the LAT • If the GeV emission source is synchrotron self-compton the predicted LAT detection rate is ~20 yr-1 • Determination of the MeV-GeV spectrum of the prompt emission: • will constrain (and maybe measure) the Lorentz factor • may shed light on electrons acceleration in short GRBs • will help to determine Ep in many bursts • The ratio of the GeV to MeV emission in the prompt and afterglow emission may constrain the magnetic field strength • LAT triggering may detect the long sought for orphan afterglows. • Simultaneous operation with Swift is very important

  18. Thanks!

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