1. Ehud Nakar California Institute of Technology Gamma-Ray Bursts and GLAST 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. Observations prompt emission Time Flux F Fluence ~ 10 -7 - 10 -4 erg/cm 2 Isotropic Energy ~ 10 50 -10 54 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  -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 Longs Collapsar (Woosley et al., …) (Review by Piran 05, Meszaros 06) Shorts A merger of compact binary ??? (Eichler et al 1989; …) (Review by Nakar 07) ?

6. Collimated Baryonic flow  em  E k Goodman 86’ Paczynski 86’ Shemi & Piran 90’, … The Fireball Model Prompt emission Poynting flux dominated flow E em >>E k Thompson 94’, Usov 94’, Katz 97’, Meszaros & Rees 97’, … Compact Source Internal Shocks 10 13 -10 15 cm synchrotron  -rays (Rees & Meszaros 94, …) EM instabilities  Particle acceleration (~10 16 cm) synchrotron  -rays Lyutikov & Blandford 02, Thompson 06

7. Baryonic flow External medium Relativistic ejecta Afterglow (in the fireball model) Reverse shock †† (~10 17 cm) X-rays Optical Radio Poynting flux dominated flow Magnetic ††† bubble X-rays Optical Radio Forward shock † (10 17 -10 18 cm) † 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)   decay, 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  >~100-300 (e.g., Lithwick & Sari 01) Short GRBs – Observatoins hint on a spectral cutoff (indication of particle acceleration cutoff???) around 300 keV implying  >~15 (Nakar 07) Detection of opacity spectral cutoff will provide a measurement of 

10. Synchrotron Self-Compton constraining the magnetic field strength SSC emission is predicted to dominate at GeV Afterglow observation indicate  e ~0.1 and  B ~10 -3 -10 -2 In the prompt emission  e >0.1,  B is not well constrained  e – fractional electron energy  B – fractional magetic field energy

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

12. E iso =10 54 erg, n=1 cm -3,  e =0.3,  B =0.01, z=1 Detectability of a very bright GRB by the LAT alone  jet =0.05 rad

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

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

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

16. C A B Ando, Nakar & Sari, in preparation Detection Rate (yr -1 ) >5 photons prompt A T90 15 B T90 20 C T90 10 Afterglow A 200 20 B 200 30 C 200 10

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 E p 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!