Pair production opacities for gamma-ray burst spectra in the 10 GeV–1 TeV range

Abstract

Recent calculations of pair production opacities for gamma-ray bursts detected by EGRET have suggested that relativistic bulk motion with large Lorentz factors must exist in these sources, whether of galactic or cosmological origin, to permit the emergence of a high energy continuum. The extant calculations usually assume an infinite power-law source spectrum for simplicity, an approximation that is quite adequate for most bursts detected by EGRET. However, for a given bulk Lorentz factor, photons above the EGRET range can potentially interact with sub-MeV photons in pair production opacity calculations, so it is important to accurately include the spectral curvature in bursts seen by BATSE. In this paper we modify our previous work to include such spectral forms, well-represented by broken power-laws, determining opacity as a function of energy for different source Lorentz factors Γ. The comparative depletion of low energy photons below 1 MeV turns out to be immaterial to considerations of bulk motion in cosmological bursts, but is crucial to estimates of source transparency in the 1 GeV – 1 TeV range for sources located in the galactic halo. We find that broad absorption dips can arise at these energies for suitable expansion parameters. Whipple, MILAGRO and GLAST will play an important role in the observation of the presence or absence of such distinctive structure in GRB spectra.