# Nuclear timescale mass transfer in models of supergiant and ultra-luminous X-ray binaries

## TL;DR

This study models how the internal structure of supergiant stars affects their mass transfer behavior in X-ray binaries, revealing conditions that enable long-term mass transfer even with high mass ratios.

## Contribution

It introduces detailed stellar and binary evolution models showing that hydrogen/helium gradients can sustain nuclear timescale mass transfer in supergiant X-ray binaries.

## Key findings

- Hydrogen/helium gradients enable long-term mass transfer in SGXBs.
- Donor stars can adjust their radii to maintain stability despite high mass ratios.
- Results suggest new pathways for ULX formation and neutron star magnetic field origins.

## Abstract

We investigate how the proximity of supergiant donor stars to the Eddington-limit, and their advanced evolutionary stage, may influence the evolution of massive and ultra-luminous X-ray binaries with supergiant donor stars (SGXBs and ULXs). We construct models of massive stars with different internal hydrogen/helium gradients and different hydrogen-rich envelope masses, and expose them to slow mass loss to probe the response of the stellar radius. In addition, we compute the corresponding Roche-lobe overflow mass-transfer evolution with our detailed binary stellar evolution code, approximating the compact objects as point masses. We find that a hydrogen/helium gradient in the layers beneath the surface, as it is likely present in the well-studied donor stars of observed SGBXs, can enable nuclear timescale mass-transfer in SGXBs with a BH or a NS accretor, even for mass ratios in excess of 20. In our binary evolution models, the donor stars rapidly decrease their thermal equilibrium radius and can therefore cope with the inevitably strong orbital contraction imposed by the high mass ratio. Our results open a new perspective for understanding the large number of Galactic SGXBs, and their almost complete absence in the SMC. They may also offer a way to obtain more ULX systems, to find nuclear timescale mass-transfer in ULX systems even with neutron star accretors, and shed new light on the origin of the strong B-field in these neutron stars.