Pair production absorption troughs in gamma-ray burst spectra: A potential distance discriminator

In order to explain the emergence of a high-energy continuum in gamma-ray bursts detected by EGRET, relativistic bulk motion with large Lorentz factors has recently been inferred for these sources regardless of whether they are of Galactic or cosmological origin. This conclusion results from calculations of internal pair production opacities in bursts that usually assume an infinite power-law source spectrum for simplicity, an approximation that is quite adequate for some bursts detected by EGRET. However, for a given bulk Lorentz factor Gamma, photons above the EGRET range can potentially interact with sub-MeV photons in such opacity calculations. Hence it is essential to accurately address the spectral curvature in bursts seen by BATSE and also treat the X-ray paucity that is inferred from low-energy fluxes observed in the X-ray band. In this paper we present the major properties induced in photon-photon opacity considerations by such spectral curvature. The observed spectral breaks around 1 MeV turn out to be irrelevant to opacity in cosmological bursts, but they are 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 troughs can arise at these energies for suitable bulk motion parameters Gamma. Such troughs are probably an unambiguous signature of a Galactic halo population and if observed by experiments such as Whipple, MILAGRO, and GLAST, would provide powerful evidence that such bursts are not at cosmological distances.