Abstract
The properties of gamma-ray burst spectra in the 1 GeV-1 TeV range may elucidate our understanding of these sources. If bursts are indeed at cosmological distances, it is indisputable that they possess relativistic bulk motion with large Lorentz factors. Calculations of pair production transparency in bursts usually assume an infinite power-law source spectrum for simplicity. However, photons above the EGRET range can potentially interact with sub-MeV photons in such calculations. The observed breaks and spectral curvature seen by BATSE around 1 MeV in many bursts is thus important to include in the pair opacity internal to the source. We present our recent determinations of gamma-ray burst pair opacity using such non-power law spectral forms. The comparative depletion of photons below 1 MeV can generate broad absorption troughs (corresponding to significant opacity) in the 1 GeV-1 TeV range for sources located in the galactic halo. These distinctive signatures can differentiate between a cosmological or galactic origin of a given source. For cosmological bursts, spectral turnovers in the super-GeV range are possible, with their shape depending on source parameters, notably the distribution of photons within the source. Instruments like INTEGRAL, GLAST, Whipple and MILAGRO will play an important role in discriminating between these possibilities.
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