Gamma-ray burst sources with a high luminosity can produce electron-positron pair cascades in their environment as a result of backscattering of a seed fraction of their original hard spectrum. The resulting spectral modifications offer the possibility of diagnosing not only the compactness parameter of the gamma-ray-emitting region but also the baryonic density of the environment external to the burst.
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Mészáros et al. (2001) studied this question.
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