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Clumpy wind accretion in Supergiant X-ray Binaries

Published online by Cambridge University Press:  30 December 2019

Ileyk El Mellah Open the ORCID record for Ileyk El Mellah [Opens in a new window] ,
Andreas A. C. Sander Open the ORCID record for Andreas A. C. Sander [Opens in a new window] ,
Jon O. Sundqvist  and
Rony Keppens
Ileyk El Mellah
Affiliation:
Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium email: [email protected]
Andreas A. C. Sander
Affiliation:
Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
Jon O. Sundqvist
Affiliation:
KU Leuven, Instituut voor Sterrenkunde, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Rony Keppens
Affiliation:
Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium email: [email protected]
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Abstract

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Supergiant X-ray Binaries host a compact object, generally a neutron star, orbiting an evolved O/B star. Mass transfer proceeds through the intense radiatively-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the neutron star. The subsequent accretion process onto the neutron star is responsible for the abundant X-ray emission from those systems. They also display variations in time of the X-ray flux by a factor of a few 10, along with changes in the hardness ratios believed to be due to varying absorption along the line-of-sight. We used the most recent results on the inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. We ran three-dimensional simulations of the wind in the vicinity of the accretor to witness the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the neutron star magnetosphere. In particular, we show that the impact of the clumps on the time-variability of the intrinsic mass accretion rate is severely damped 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 mass accretion rate for different orbital separations. These results are confronted to recent analysis of Vela X-1 observations with Chandra by Grinberg et al. (2017). It shows that clumps account well for time-variability at low luminosity but can not generate, per se, the high luminosity activity observed.

Keywords

accretionaccretion disksmethods: numericalhydrodynamicsstars: neutronX-rays: binariesstars: windsoutflowsstars: supergiantsplasmasstars: early-type
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 14 , Symposium S346: High-mass X-ray Binaries: Illuminating the Passage from Massive Binaries to Merging Compact Objects , August 2018 , pp. 34 - 39
Copyright
© International Astronomical Union 2019 

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