Accretion in wind-driven X-ray sources

The hydrodynamics of axisymmetric accretion flow in stellar wind-fed X-ray sources is investigated. Momentum deposition by radiation, as well as radiative heating and cooling by Compton and bremsstrahlung processes, has been included. The results of two-dimensional numerical simulations for mass accretion rates ranging from 0.02 to 0.4 of the Eddington value reveal a variety of behaviors. At low rates radiative effects are unimportant, and the accretion flow is nearly adiabatic. In this case the flow is steady. For intermediate accretion rates, radiative heating and cooling effects become important. The flow remains steady, but the accretion rate is significantly reduced from the Hoyle-Lyttleton estimate. At the highest mass accretion rates, radiative momentum transfer and energy exchange are important and lead to nonsteady behavior. For the parameters relevant to massive X-ray binary systems accreting at high rates, the results reveal that the accretion flows can be complex and time dependent. The typical time scale for variation of the accretion rate in such systems is comparable to the flow time from the accretion radius to the compact object. The temporal variability manifests itself as flarelike activity. These burstlike events are a consequence of the nonlinear development of dynamical instabilities in an optically thick accretion wake region. Such behaviors may be responsible for the presence of short-time-scale variable emission in the active state of hard X-ray transient sources.