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We use 2D (axisymmetric) and 3D hydrodynamic simulations to studyBondi-Hoyle-Lyttleton (BHL) accretion with and without transverse upstreamgradients. We mainly focus on the regime of high (upstream) Mach number, weakupstream gradients and small accretor size, which is relevant to neutron star(NS) accretion in wind-fed Supergiant X-ray binaries (SgXBs). We present asystematic exploration of the flow in this regime. When there are no upstreamgradients, the flow is always stable regardless of accretor size or Machnumber. For finite upstream gradients, there are three main types of behavior:stable flow (small upstream gradient), turbulent unstable flow without a disk(intermediate upstream gradient), and turbulent flow with a disk-like structure(relatively large upstream gradient). When the accretion flow is turbulent, theaccretion rate decreases non-convergently as the accretor size decreases. Theflow is more prone to instability and the disk is less likely to form thanpreviously expected; the parameters of most observed SgXBs place them in theregime of a turbulent, disk-less accretion flow. Among the SgXBs withrelatively well-determined parameters, we find OAO 1657-415 to be the only onethat is likely to host a persistent disk (or disk-like structure); this findingis consistent with observations.

Bondi-Hoyle-Lyttleton accretion in Supergiant X-ray binaries: stability and disk formation