X-rays from spherical accretion onto degenerate dwarfs

Abstract

Spherically symmetric accretion onto degenerate dwarfs is reconsidered. The problem is divided into three parameter regimes on the basis of the accretion rate. In the regime of highest accretion rates, the flow is optically thick. For the middle regime calculations are extended to include Compton cooling of the shocked material by thermal radiation from the photosphere. If the accreted material is hydrogen-rich and burns, this effect greatly reduces the efficiency of X-ray production. In the lowest regime of accretion rate, the shock occurs far from the star, and the accretional energy is radiated through a thick layer. Accreting degenerate dwarfs are expected to be characterized by very hard X-ray spectra, which are nearly independent of other variations in the properties of the source. Applications of these results include arguments against the degenerate dwarf hypothesis for Sco X-1 and 3U 0900-40, and suggested observations of DQ Her.