High-energy photon emission in the early afterglow of GRBs

We consider the emission within the fireball model framework of very high energy, similar to 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 similar to 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, epsilon(B). It ranges from similar to 10(-7) ergs cm(-2) s(-1) in the case of typical ISM density and epsilon(B) less than or similar to 10(-4), to 10(-10) ergs cm(-2) s(-1) in the case of a source surrounded by a wind and epsilon(B) similar to 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 similar to 1 keV, similar to 1 GeV, and sub-TeV observations will allow determination of both epsilon(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 high-energy protons may contribute significantly in the wind case to the luminosity of high-energy 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.