1. Very High Energy Transient Extragalactic Sources: GRBs David A. Williams Santa Cruz Institute for Particle Physics University of California, Santa Cruz October 20, 2005

2. "Towards the Future" Workshop — DAW2 Two Classes of Bursts Short, Hard Bursts 19 sr -1 yr -1 Prompt emission too short to catch with slewed response May be “nearby” z = 0.16, 0.225?, 0.258 Delayed X-ray flare from 050724 ~200 seconds later Long, Soft Bursts 38 sr -1 yr -1 14% longer than 100 s 28% longer than 63 s z ~ 1, but wide range z = 0.0085 to 6.3 Several types of delayed emission Milagro/HAWC 2 sr yr annually VERITAS 3 x 10 -4 sr yr annually

3. October 20, 2005"Towards the Future" Workshop — DAW3 Redshift Distribution Klose, http://www.mpe.mpg.de/~jcg/grbrsh.html Redshifts established for 32 bursts (as of 2004) 1.3 < z < 2.5 difficult– spectra lack suitable lines These are all long duration (>2 s) bursts

4. October 20, 2005"Towards the Future" Workshop — DAW4 High-Energy  -Rays Hurley et al., Nature 372, 652 (1994) Ulysses data BATSE data EGRET data GRB 940217 18 GeV!

5. October 20, 2005"Towards the Future" Workshop — DAW5 High-Energy  -Rays II EGRET saw >100 MeV emission from three other bursts dN/dE ~ E -1.95 up to 10 GeV Limited exposure so such emission may be typical Longer duration, hard component in GRB 941017 (Gonzalez et al 2003) and 980923 (Gonzalez 2004)

6. October 20, 2005"Towards the Future" Workshop — DAW6 VHE  -Rays from GRB 970417a? Astrophysical Journal Letters 533, L119 (2000) Astrophysical Journal 583, 824 (2003) Evidence for TeV emission from GRB 970417a seen by Milagrito Probability of background fluctuation 1.5 x 10 -3 (3  ) More luminosity at TeV energies than MeV (Lack of) EBL absorption implies GRB must be close, so total energy released is not unusually large – z~0.1, then E  < 700 GeV, so L < 5 x 10 51 ergs – z~0.03, then E  < 10 TeV, so L < 1 x 10 49 ergs

7. October 20, 2005"Towards the Future" Workshop — DAW7 VHE  -Rays from GRB? Amenomori et al. 1996 (Tibet AS-  – 6  excess at ~10 TeV in stacked analysis of 57 BATSE GRB Padilla et al. 1998 (HEGRA) – 2.7  excess above 20 TeV from GRB 920925c Atkins et al. 2005; Saz Parkinson 2005 (Milagro) – Limits >100 GeV from 33 GRB during 2000–2003 Connaughton et al. 1997 (Whipple) – Limits >250 GeV from follow-up observations of 9 BATSE bursts Horan 2005 (Whipple) – Limits >400 GeV from follow-up observations of 10 GRB 2002–2004 Jarvis et al. 2005 (STACEE) – No detection in follow-up observations of 8 GRB 2002–2005 Bastieri et al. 2005 (MAGIC) – Upper limit starting 40 s after onset, overlapping T90 for 30 s, of GRB 050713a

8. October 20, 2005"Towards the Future" Workshop — DAW8 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) 10 7 cm Central engine injects 10 51 ergs over 1 second Expanding ,e +,e - plasma at 10 10 K Variability can produce multiple thin shells

9. October 20, 2005"Towards the Future" Workshop — DAW9 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) 10 13 cm Baryons are accelerated by radiation pressure until they share a common bulk Lorentz factor  ~300 with the plasma Fireball coasts 1000 s / 5 ms

10. October 20, 2005"Towards the Future" Workshop — DAW10 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) Photon number density and energy in expanding fireball has dropped to the point that fireball is optically thin: photons escape Most of the energy remains in the bulk motion 10 12 –10 13 cm 100–1000 s / 0.5–5 ms

11. October 20, 2005"Towards the Future" Workshop — DAW11 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) Typical radius at which shells with different  overtake each other, creating “internal” shocks 5 x 10 14 cm 10 4 s / 0.05 s

12. October 20, 2005"Towards the Future" Workshop — DAW12 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) Spreading radius at which variations in  within the shell cause it to broaden 3 x 10 15 cm 10 5 s / 0.5 s

13. October 20, 2005"Towards the Future" Workshop — DAW13 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) Deceleration radius – the fireball is decelerated by the ambient medium “External” shock propagates into the ambient medium “Reverse” shock propagates into fireball shell 3 x 10 16 cm 10 5 s / 5 s

14. October 20, 2005"Towards the Future" Workshop — DAW14 The Fireball Model Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004) Thick shell results if initial energy injection is longer Takes longer to decelerate and radius is larger

15. October 20, 2005"Towards the Future" Workshop — DAW15 External Shock Model Predictions Dermer & Chiang, AIP Conf. Proc. 515, 225 (2000) For burst at z ~ 1, d L ~ 10 28 cm, log 10 [ F ] = 47.4 corresponds to Crab flux (c) at log 10 [ ] = 25.4 (100 GeV) c c

16. October 20, 2005"Towards the Future" Workshop — DAW16 Internal Shocks–The Bad News Razzaque, Meszaros & Zhang, Astrophys. J. 613, 1072 (2004) Emission is at smaller radius Fireball is denser— Typically opaque above 10–100 GeV

17. October 20, 2005"Towards the Future" Workshop — DAW17 Flux for burst at z = 1 Includes EBL absorption a la Malkan & Stecker 2001 for z = 1 B IG = 10 -20 G B IG = 10 -17 G Reprocessing by EBL–Better News Razzaque, Meszaros & Zhang, Astrophys. J. 613, 1072 (2004) c c 10 4 s 10 6 s 10 2 s 50 s

18. October 20, 2005"Towards the Future" Workshop — DAW18 Fluxes for burst at z = 1 (But no EBL absorption) Different values of Density of surrounding medium Magnetic field Fraction of energy in e ± Afterglow Emission Zhang & Meszaros, Astrophys. J. 559, 110 (2001) c c c T0 1 m 1 h 1 d 1 mo

19. October 20, 2005"Towards the Future" Workshop — DAW19 c Color curves are IC from afterglow (external shock) electrons in scenarios with different Total energy Density of surrounding medium Bulk Lorentz factor Magnetic field constrained to fit the EGRET data Black curve is model based on rapid variability in prompt emission Modeling GRB 941017 Pe’er & Waxman, Astrophys. J. Lett. 603, L1 (2004) Spectrum between 100 and 200 s after start of burst

20. October 20, 2005"Towards the Future" Workshop — DAW20Summary Solid evidence for prompt and afterglow emission from EGRET up to 10 GeV No firm VHE signal from a burst yet Low threshold desirable –Elude EBL absorption in transit –Elude self-absorption in source Prompt emission –Potentially quit strong (many Crab) once below absorption cutoffs –Can determine burst parameters, e.g.  B from intrinsic cutoff –Short bursts can only be caught with large (~sr) field of view –Tail of longer bursts can be caught with slewed responses of ≤1 minute Afterglow emission –Can be quit strong (~Crab or more) initially. How long is it interesting to follow? –Can be as valuable for determining burst properties, e.g. GRB 941017 –Accessible to slewed instruments