Wind accretion in Supergiant X-ray binaries
The turbulent twilight of massive stellar binaries stars determines their final fate. In Supergiant X-ray binaries, mass transfer proceeds through the accretion of a fraction of the dense and fast wind from the evolved donor star. The accretor, generally a neutron star, both perturbs the stellar wind and provides an orbiting X-ray source to probe its internal structure: the characteristics of the flow constrain the X-ray emission and absorption along the line-of-sight, while the X-ray ionizing feedback alters the wind acceleration. In order to consistently monitor the flow from the stellar surface down to the accretor, we designed a multi-scale model which accounts for the main physical ingredients at stake. Using the MPI-AMRVAC finite volume code, we performed 3D numerical simulations to evaluate the mass and angular momentum accretion rates onto the compact object. We identified conditions favorable to the formation of a disc-like structure beyond the neutron star magnetosphere, in spite of the low angular momentum carried by the wind, and analyzed its properties. The impact of the presence of overdense regions in the wind (aka clumps) on the time variability is also investigated. Finally, I will confront these results to recent observations of the photometric and spectroscopic X-ray variability in Vela X-1, the archetype of Supergiant X-ray binary, and discuss the relevance of our model for hybrid wind - Roche lobe overflow accreting systems such as Cygnus X-1.