Conditions for accretion disc formation and observability of wind-accreting X-ray binaries. (arXiv:2108.03774v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Hirai_R/0/1/0/all/0/1">Ryosuke Hirai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mandel_I/0/1/0/all/0/1">Ilya Mandel</a>
We explore the effect of anisotropic wind driving on the properties of
accretion onto black holes in close binaries. We specifically focus on
line-driven winds, which are common in high-mass X-ray binaries. In close
binary systems, the tidal force from the companion star can modify the wind
structure in two different ways. One is the reduction of wind terminal velocity
due to the weaker effective surface gravity. The other is the reduction in mass
flux due to gravity darkening. We incorporate these effects into the so-called
CAK theory in a simple way and investigate the wind flow around the accretor on
the orbital scale. We find that a focused accretion stream is naturally formed
when the Roche lobe filling factor is $gtrsim0.8$-0.9, analogous to that of
wind Roche lobe overflow, but only when the velocity reduction is taken into
account. The formation of a stream is necessary to bring in sufficient angular
momentum to form an accretion disc around the black hole. Gravity darkening
effects reduce the amount of accreted angular momentum, but not enough to
prevent the formation of a disc. Based on these results, we expect there to be
a discrete step in the observability of high-mass X-ray binaries depending on
whether the donor Roche lobe filling factor is below or above $sim$0.8-0.9.
We explore the effect of anisotropic wind driving on the properties of
accretion onto black holes in close binaries. We specifically focus on
line-driven winds, which are common in high-mass X-ray binaries. In close
binary systems, the tidal force from the companion star can modify the wind
structure in two different ways. One is the reduction of wind terminal velocity
due to the weaker effective surface gravity. The other is the reduction in mass
flux due to gravity darkening. We incorporate these effects into the so-called
CAK theory in a simple way and investigate the wind flow around the accretor on
the orbital scale. We find that a focused accretion stream is naturally formed
when the Roche lobe filling factor is $gtrsim0.8$-0.9, analogous to that of
wind Roche lobe overflow, but only when the velocity reduction is taken into
account. The formation of a stream is necessary to bring in sufficient angular
momentum to form an accretion disc around the black hole. Gravity darkening
effects reduce the amount of accreted angular momentum, but not enough to
prevent the formation of a disc. Based on these results, we expect there to be
a discrete step in the observability of high-mass X-ray binaries depending on
whether the donor Roche lobe filling factor is below or above $sim$0.8-0.9.
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