Ultraluminous X-ray sources with He star companions

TL;DR

This study uses stellar evolution modeling to explore how neutron star and helium star binaries can form ultraluminous X-ray sources, identifying key parameters and estimating their occurrence rate in our galaxy.

Contribution

It provides the first detailed parameter space and formation pathway for NS+He star systems as ULX progenitors, supported by population synthesis.

Findings

NS+He star systems with specific mass and orbital period ranges can produce ULXs.

Estimated Galactic rate of NS ULXs with He donors is approximately 1.6-4.0×10^{-4} per year.

There are about 7-20 detectable NS ULXs with He donors in the Galaxy.

Abstract

Ultraluminous X-ray sources (ULXs) are non-nuclear point-like objects observed with extremely high X-ray luminosity that exceeds the Eddington limit of a $\rm10\,M_\odot$ black hole. A fraction of ULXs has been confirmed to contain neutron star (NS) accretors due to the discovery of their X-ray pulsations. The donors detected in NS ULXs are usually luminous massive stars because of the observational biases. Recently, the He donor star in NGC 247 ULX-1 has been identified, which is the first evidence of a He donor star in ULXs. In this paper, we employed the stellar evolution code MESA to investigate the formation of ULXs through the NS+He star channel, in which a He star transfers its He-rich material onto the surface of a NS via Roche-lobe overflow. We evolved a large number of NS+He star systems and provided the parameter space for the production of ULXs. We found that the initial NS+He star systems should have $\rm\sim 0.7-2.6 \, M_\odot$ He star and $\rm \sim 0.1-2500\, d$ orbital period for producing ULXs, eventually evolving into intermediate-mass binary pulsars. According to binary population synthesis calculations, we estimated that the Galactic rate of NS ULXs with He donor stars is in the range of $\sim1.6-4.0\times10^{-4}\,{\rm yr}^{-1}$, and that there exist $\sim7-20$ detectable NS ULXs with He donor stars in the Galaxy.