Hard X-rays from accretion disk boundary layers

Abstract

ACCRETION disks1,2 are found in many astrophysical objects, ranging from newly formed stars and mass-transferring binary systems to quasars and other active galactic nuclei. An important feature of accretion disks is the boundary layer—the interface between the disk and the accreting objects—where up to half the accretion luminosity may be liberated. The lack of a satisfactory description of the flow and thermal structure of this layer has long been a handicap when modelling disk spectra. Here we report numerical solutions of a model of thin accretion disks around a central white dwarf which includes a self-consistent description of the boundary layer. We find two distinct kinds of solution depending on the mass accretion rate Ṁ. At high rates, we find optically thick boundary layers whose radial width and peak temperature decrease with decreasing Ṁ, but when the accretion rate falls below a critical value, the boundary layer becomes optically thin, and the width and temperature increase dramatically. Our results provide an explanation for the hard X-rays observed3 in cataclysmic variables, particularly at low Ṁ. It should be possible to extend our analysis to other accretion-disk systems.