Spectral and variability constraints on compact sources

Abstract

The spectra of compact sources which radiate significantly above 511 keV may be modified by the electron-positron pairs created by photon-photon collisions. The Thomson opacity due to pairs may easily exceed unity in sources such as active galactic nuclei; most of the observed luminosity is likely to be due to mildly relativistic pairs. The high-energy spectral cut-off is inversely related to source luminosity. General constraints on variability involving rest-mass to energy conversion efficiency, geometry and the effects of pairs are discussed. Spectral relativistic corrections may be large, even in an isotropic source.