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 hundreds of 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 highenergy afterglow in the external shock model. We expand on the Cheng & Cheng model by deriving the emission spectrum and duration in the standard fireball shock model. A typical duration of the emission is ∼ 10 seconds, and the time-integrated scattered photon spectrum is ν, where p is the index of the electron energy distribution behind internal shocks. This is slightly harder than the synchrotron photon spectrum, ν. 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 GammaRay Large Area Space Telescope (GLAST) would provide a probe of the cosmic infrared background radiation. Subject headings: diffuse radiation — gamma rays: bursts — gamma rays: theory