Self-Shielding of X-rays and Gamma-Rays in Compact Sources

Abstract

It is generally supposed that when the "compactness" $l \equiv Lσ_T/(r m_e c^3)$ in photons above the pair-production threshold is large, few $γ$-rays can escape. We demonstrate that even when $l \gg 1$, if the high energy and low energy photons are produced in geometrically-separated regions, many of the $γ$-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 which 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 flow outward relativistically, forming a structure resembling a pair-dominated mildly relativistic jet.