Spectrum and duration of delayed MeV-GeV emission of gamma-ray bursts in cosmic background radiation fields

We generally analyze prompt high-energy emission above a few hundred GeV due to synchrotron self-Compton scattering in internal shocks. However, such photons cannot be detected because they may collide with cosmic infrared background photons, leading to electron/positron pair production. Inverse Compton scattering of the resulting electron/positron pairs off cosmic microwave background photons will produce delayed MeV-GeV emission, which may be much stronger than a typical high-energy afterglow in the external shock model. We expand on the model of Cheng & Cheng by deriving the emission spectrum and duration in the standard fireball shock model. A typical duration of the emission is similar to10(3) s, and the time-integrated scattered photon spectrum is nu(-(p+6)/4), where p is the index of the electron energy distribution behind internal shocks. This is slightly harder than the synchrotron photon spectrum, nu(-(p+2)/2). The lower energy property of the scattered photon spectrum is dependent on the spectral energy distribution of the cosmic infrared background radiation. Therefore, future observations on such delayed MeV-GeV emission and the higher energy spectral cutoff by the Gamma-Ray Large Area Space Telescope (GLAST) would provide a probe of the cosmic infrared background radiation.