Pair-induced spectral changes and variability in compact X-ray sources

Abstract

Inverse Compton scattering of ultraviolet photons by GeV electrons produces γ-rays which in turn create electron–positron pairs if the source is sufficiently compact. The pairs modify the emergent radiation spectrum through their own inverse Compton scattering and through thermal Comptonization after they have cooled to sub-relativistic temperatures. Recent calculations of spectral reprocessing under these conditions are extended to situations in which the Thomson optical depth of the pair plasma exceeds unity, and to demonstrate time-dependent behaviour explicitly. The relevance of our results to X-ray observations of active galactic nuclei, binary X-ray sources and γ-ray bursters is discussed briefly.