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 GeVdue to synchrotron self-Compton scattering in internal shocks. However, suchphotons cannot be detected because they may collide with cosmic infraredbackground photons, leading to electron/positron pair production.Inverse-Compton scattering of the resulting electron/positron pairs off cosmicmicrowave background photons will produce delayed MeV-GeV emission, which maybe much stronger than a typical high-energy afterglow in the external shockmodel. We expand on the Cheng & Cheng model by deriving the emission spectrumand duration in the standard fireball shock model. A typical duration of theemission is ~ 10^3 seconds, and the time-integrated scattered photon spectrumis nu^{-(p+6)/4}, where p is the index of the electron energy distributionbehind internal shocks. This is slightly harder than the synchrotron photonspectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photonspectrum is dependent on the spectral energy distribution of the cosmicinfrared background radiation. Therefore, future observations on such delayedMeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray LargeArea Space Telescope (GLAST) would provide a probe of the cosmic infraredbackground radiation.Comment: 5 pages, accepted for publication in Ap