High-luminosity accretion in wind-driven binary X-ray sources

Abstract

The transfer of mass onto accreting X-ray sources in binaries, when the mass flow is fueled by a radiation-driven wind from a companion supergiant, is studied with emphasis on the effect of the X-ray feedback on the wind. Photoionization by the X-ray raises the ionization stage of the wind, which diminishes the radiation line force and quenches the wind acceleration beyond a well-defined Stromgren surface. The resulting slowed wind leads to a high accretion rate and X-ray luminosity. A simple self-consistent model is constructed to describe the accretion of the wind which fuels the X-rays and the X-rays' feedback which ionizes the wind. The model yields several self-consistent solutions. Besides the low-luminosity solution corresponding to the accretion from the wind with velocity at the secondary's location, a solution with small wind velocity and high accretion rate is also found. The high-luminosity solutions agree with observations of Cen X-3 and SMC X-1 to within a factor of 3. This resolves the difficulties of previous simple models whose predicted X-ray luminosities for these sources are orders of magnitude lower than the observations.