High-Energy Photon Emission in the Early Afterglow of GRBs

We consider the emission within the fireball model framework of very high energy, ~1 GeV to >1 TeV photons, on a minute timescale, during the onset of fireball deceleration due to interaction with surrounding medium. Our time-dependent numerical model includes exact treatment of electron synchrotron emission, inverse Compton scattering, pair production, and evolution of electromagnetic cascades (initiated by pair production or photoproduction of pions). We find the following: (1) The 1-10 GeV flux is not sensitive to model parameters and is ~10^-7 ergs cm^-2 s^-1 for z = 1 bursts, well within the detection capabilities of GLAST. (2) The sub-TeV flux depends on the surrounding medium density and on the fraction of thermal energy carried by the magnetic field, (epsiolon)B. It ranges from ~10^-7 ergs cm^-2 s^-1 in the case of typical ISM density and (epsiolon)B <~10^-4, to 10^-10 ergs cm^-2 s^-1 in the case of a source surrounded by a wind and (epsilon)B ~10^-0.5. (3) The sub-TeV flux is detectable by high-energy (gamma)-ray experiments such as HESS, MAGIC, Milagro, and VERITAS. (4) Combined ~1 keV, ~ 1 GeV, and sub-TeV observations will allow determination of both (epsiolon)B and the ambient medium density. (5) The spectra depend only weakly on the spectral index of the energy distribution of the accelerated electrons. Pion production energy loss of highenergy protons may contribute significantly in the wind case to the luminosity of highenergy photons. However, it is difficult to distinguish in this case between the electron and proton contributions, since the spectral shape is determined primarily by the energy dependence of the pair production optical depth.