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

TL;DR

This paper investigates how wind clumps in supergiant X-ray binaries affect accretion variability onto neutron stars, using 3D hydrodynamic simulations to better understand observed X-ray flux fluctuations.

Contribution

It introduces detailed 3D hydrodynamic simulations of clumpy stellar winds impacting accretion processes in SgXB, improving understanding of variability mechanisms.

Findings

Wind clumps' impact on accretion variability is reduced by shock crossing.

Variable absorption due to clumps affects observed X-ray properties.

Results align with observed flux and hardness ratio variations in Vela X-1.

Abstract

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 intense line-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the NS. The subsequent accretion process onto the NS is responsible for the abundant X-ray emission from SgXB. They also display peak-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 absorption along the line-of-sight. We use recent radiation-hydrodynamic simulations of inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. For this, we run 3D hydrodynamic simulations of the wind in the vicinity of the accretor to investigate the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the NS magnetosphere. In particular, we show that the impact of the wind clumps on the time-variability of the intrinsic mass accretion rate is severely tempered by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line-of-sight and estimate the final effective variability of the column density and mass accretion rate for different orbital separations. Finally, we compare our results to the most recent analysis of the X-ray flux and the hardness ratio in Vela X-1.