Emission of cosmological gamma-ray bursts in the GeV–TeV energy domain

Abstract

The observed sub-MeV radiation from gamma-ray bursts (GRBs) arises most likely from synchrotron emission of a relativistic shock. We estimate the magnetic field strength and the Lorentz factor of accelerated electrons required by the synchrotron mechanism. We find that these values are furthermore sufficient to cause significant emission due to inverse Compton scattering. The Comptonized photons typically have energies well above 1 TeV, so that they are strongly absorbed by infrared background radiation and cannot be observed from a source at cosmological distance. On the other hand, the ultra-high energy radiation may be absorbed in the close vicinity of the source due to interaction with ultraviolet and soft X-ray quanta originating from the same source and scattered in the ambient medium. In this case, almost all the energy initially contained in ultra-high energy radiation is reprocessed into a softer spectral range corresponding to the two-photon absorption threshold. Both the presence and the absence of absorption/reprocessing of the ultra-high energy photons places a rather strong limit on the density of interstellar medium near the GRB source and the Lorentz factor in fireball of a gamma-ray burst. We discuss all types of GRB emission in GeV–TeV spectral range, including 100 GeV emission of GRB fireballs, which originates from the decay of neutral pions.