Self-shielding of X-rays and gamma rays in compact sources

It is generally supposed that when the ''compactness'' l = L sigma(T)/(rm(e)c(3)) in photons above the pair-production threshold is large, few gamma-rays can escape. We demonstrate that even when l much greater than 1, if the high-energy and low-energy photons are produced in geometrically separated regions, many of the gamma-rays can, in fact, escape. Pair production along a thin surface separating the two sources creates enough Compton optical depth to deflect most of the low-energy photons away from the high-energy ones. Those few low-energy photons that penetrate the shielding surface are reduced in opacity by advection to large distance and small density, by relativistic beaming along the inner edge of the surface, and by Compton upscattering to higher energies inside the surface. The pairs in this surface how outward relativistically, forming a structure resembling a pair-dominated mildly relativistic jet.