Accretion form a clumpy massive-star wind in Supergiant X-ray binaries

摘要

Supergiant X-ray Binaries (SgXB) host a compact object, often a neutron star(NS), orbiting an evolved O/B star. Mass transfer proceeds through the intenseline-driven wind of the stellar donor, a fraction of which is captured by thegravitational field of the NS. The subsequent accretion process onto the NS isresponsible for the abundant X-ray emission from SgXB. They also displaypeak-to-peak variability of the X-ray flux by a factor of a few 10 to 100,along with changes in the hardness ratios possibly due to varying absorptionalong the line-of-sight. We use recent radiation-hydrodynamic simulations ofinhomogeneities (aka clumps) in the non-stationary wind of massive hot stars toevaluate their impact on the time-variable accretion process. For this, we run3D hydrodynamic simulations of the wind in the vicinity of the accretor toinvestigate the formation of the bow shock and follow the inhomogeneous flowover several spatial orders of magnitude, down to the NS magnetosphere. Inparticular, we show that the impact of the wind clumps on the time-variabilityof the intrinsic mass accretion rate is severely tempered by the crossing ofthe shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation.We also account for the variable absorption due to clumps passing by theline-of-sight and estimate the final effective variability of the columndensity and mass accretion rate for different orbital separations. Finally, wecompare our results to the most recent analysis of the X-ray flux and thehardness ratio in Vela X-1.