Accretion from a clumpy massive-star wind in supergiant X-ray binaries

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 p...

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Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2018-04, Vol.475 (3), p.3240-3252
Main Authors: El Mellah, I, Sundqvist, J O, Keppens, R
Format: Article
Language:English
Subjects:
Absorption
Accretion
Binary stars
Clumps
Deposition
Emission
Gravitational fields
Hardness
Hot stars
Line of sight
Magnetospheres
Mass transfer
Neutron flux
Neutrons
Radiation
Stellar evolution
Stellar winds
Variability
X ray binaries
X ray stars
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Summary: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 on to 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 10–100, 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 (a.k.a. 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.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/stx3211