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

## TL;DR

This study uses hydrodynamic simulations to analyze accretion flows in Supergiant X-ray binaries, revealing conditions for stability, turbulence, and disk formation, with implications for observed systems.

## Contribution

It systematically explores the flow regimes in BHL accretion relevant to neutron star binaries, highlighting the impact of upstream gradients on flow stability and disk formation.

## Key findings

- Flow is stable without upstream gradients.
- Turbulent flows with upstream gradients often lack disks.
- Most observed SgXBs are in turbulent, disk-less regimes.

## Abstract

We use 2D (axisymmetric) and 3D hydrodynamic simulations to study Bondi-Hoyle-Lyttleton (BHL) accretion with and without transverse upstream gradients. We mainly focus on the regime of high (upstream) Mach number, weak upstream gradients and small accretor size, which is relevant to neutron star (NS) accretion in wind-fed Supergiant X-ray binaries (SgXBs). We present a systematic exploration of the flow in this regime. When there are no upstream gradients, the flow is always stable regardless of accretor size or Mach number. 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, the accretion rate decreases non-convergently as the accretor size decreases. The flow is more prone to instability and the disk is less likely to form than previously expected; the parameters of most observed SgXBs place them in the regime of a turbulent, disk-less accretion flow. Among the SgXBs with relatively well-determined parameters, we find OAO 1657-415 to be the only one that is likely to host a persistent disk (or disk-like structure); this finding is consistent with observations.