Wind-fed black-hole HMXB accretion-disk observability

Determine what fraction of wind-fed black-hole high-mass X-ray binaries form accretion disks and consequently produce detectable X-ray emission by incorporating the dependence of disk formation on the donor-star Roche-lobe filling factor.

Background

The synthetic population identifies wind-fed black-hole high-mass X-ray binaries without imposing an accretion-disk formation criterion. However, disk formation is expected to depend critically on the donor’s Roche-lobe filling factor, with significant X-ray emission expected only above a threshold near 0.8–0.9.

Consequently, the modeled number of observable black-hole high-mass X-ray binaries may be overestimated, potentially contributing to the discrepancy between theoretical populations and the small observed sample. Resolving this issue requires explicitly modeling disk formation and its effect on detectability.

References

An additional source of uncertainty concerns the observability of wind-accreting BH-HMXBs. \citet{Hirai2021} show that the formation of an accretion disk-required for significant X-ray emission, depends critically on the Roche-lobe filling factor of the donor star, $f$, with disk formation expected only when $f\gtrsim 0.8-0.9$. Since our current model does not impose this condition, a fraction of the wind-fed BH-HMXBs in our synthetic population may not produce detectable X-ray emission, which could partially account for the known discrepancy between large theoretical BH-HMXB populations and the small number of observed systems.

— Linking high-mass X-ray binaries to binary compact object mergers in Milky Way-like galaxies  (2608.23945 - Cádiz et al., 25 Aug 2026) in Section 4, “Observational and Model Uncertainties”