Pair Production and Radiation Effects in Clouds Illuminated by Gamma Ray Sources

Many classes of gamma-ray sources, such as gammaray bursts, blazars, Seyfert galaxies, and galactic black hole sources are surrounded by large amounts of gas and dust. X-rays and gamma-rays that traverse this material will be attenuated by Compton scattering and photoelectric absorption. One signature of an intervening scattering cloud is radiationhardening by electrons that have been scattered and heated by the incident radiation, as illustrated by a Monte Carlo calculation. Compton scattering provides backscattered photons that will attenuate subsequent gamma rays through γγ pair-production processes. We calculate the pair efficiency for a cloud illuminated by gamma-ray burst radiation. An analytic calculation of the flux of X-rays and gamma rays Thomson scattered by an intervening cloud is presented. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electronpositron pairs that can be detected within ∼ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy could reveal gamma-ray sources embedded in dense clouds, or sites of past GRB explosions.