1. Gamma-Ray Burst Working Group Co-conveners: Abe Falcone, Penn State, afalcone@astro.psu.edu afalcone@astro.psu.edu David A. Williams, UCSC, daw@scipp.ucsc.edu daw@scipp.ucsc.edu February 8, 2007

2. GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu2 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 AIROBICC) – 2.7  excess above 20 TeV from GRB 920925c Atkins et al. 2000; Atkins et al. 2003 (Milagrito) – 3  excess (post-trials) from GRB 970417a >650 GeV; no evidence from 53 other bursts 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 et al. 2006 (Whipple) – Limits >400 GeV from follow-up observations of 7 GRB 2002–2004 Jarvis et al. 2005 (STACEE) – No detection in follow-up observations of 8 GRB 2002–2005 Albert et al. 2006 (MAGIC) – Upper limits overlapping T90 for GRB 050713a and 050904; afterglow limits for 7 other bursts

3. February 8, 2007GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu3 Why so difficult? Many good talks yesterday describing high energy emission mechanisms for GRBs, often extending to TeV – Where is it? Pair opacity in source can prevent escape EBL absorption significant >100 GeV for most bursts Ground-based instruments with lowest energy thresholds have narrow field of view – Atmospheric Cherenkov telescopes: Whipple, STACEE, MAGIC, HESS, VERITAS Wide FoV instruments have thresholds which increase significantly with zenith angle – Air shower detectors: Milagro, ARGO-YBJ, Tibet AS-  No gold-plated burst yet to really reveal or constrain VHE emission

4. February 8, 2007GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu4 Inverse Compton scattering of keV/MeV emission should create GeV/TeV emission Fighting pair opacity in the burst itself Highest energy photons which escape related to bulk Lorentz factor Opacity less in external shock models –May help discriminate Ground-based needed for GRBs with very high (>500) Lorentz factor –Probe the most extreme of the GRBs What sensitivity is needed and what’s the best strategy for getting there? “Prompt” Emission

5. February 8, 2007GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu5 Early Afterglow (~T90–1000 s) Transition period from prompt emission to afterglow –Pair opacity declining as fireball expands Many possible processes could be in play –Continued internal shocks –External shock blast wave (i.e. nominal afterglow) –Reverse shock –Prolonged energy injection by central engine Delayed X-ray flares Hard component in GRB 941017 – 211 s duration Many possibilities for SSC and EC production of VHE  ’s VHE light curve of IC from X-ray flare depends on which photon and electron populations are involved Requirements on FOV and slew speed are somewhat relaxed, but bigger/faster gives more detections

6. February 8, 2007GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu6 Afterglow (>1000 s) Time frame of EGRET high energy emission from GRB 940217 –Understand the limits of mechanism External shock blast wave still present VHE afterglow related to shock parameters, e.g. –Ambient density –Magnetic field –Fraction of energy in electrons Continued X-ray flares Pion decay could be significant Less flux, but higher likelihood that slewing instrument will react fast enough

7. February 8, 2007GRB Working Group — afalcone@astro.psu.edu, daw@scipp.ucsc.edu7 Links to Other Science Lorentz Invariance Violation –Energy dependent delays of simultaneously emitted photons can limit (or measure) Lorentz invariance Energy dependent light curves? –Limited redshift range because of EBL absorption –Broad energy range, especially if delay between GLAST and TeV –Probably more credible during prompt phase –Does low energy really win by statistics? Cosmic Ray Acceleration –What role do GRBs play in production of UHE cosmic rays? –If significant UHE cosmic ray acceleration: p synchrotron p+γ initiated cascades inelastic n+p initiated cascades (dominant?) –Like blazars, it may be difficult to break degeneracy between IC and hadronic models Have the advantage of better constraints on Lorentz factor and smaller timescales/regions May still need neutrinos