A Population of Neutron Star Ultraluminous X-ray Sources with A Helium Star Companion

TL;DR

This study identifies a significant population of neutron star ultraluminous X-ray sources with helium star companions in Milky Way-like galaxies, highlighting their characteristics and contribution to the ULX population.

Contribution

It introduces a systematic analysis of neutron star ULXs with helium star donors, emphasizing their prevalence and properties through combined population synthesis and stellar evolution models.

Findings

NS-helium star binaries can account for several ULXs in a galaxy.

Orbital periods are typically around 0.1 days, with some up to 100 days.

Helium star masses peak near 1 solar mass.

Abstract

It was recently proposed that a significant fraction of ultraluminous X-ray sources (ULXs) actually host a neutron star (NS) accretor. We have performed a systematic study on the NS ULX population in Milky Way-like galaxies, by combining binary population synthesis and detailed stellar evolution calculations. Besides a normal star, the ULX donor can be a helium star (the hydrogen envelope of its progenitor star was stripped during previous common envelope evolution) if the NS is accreting at a super-Eddington rate via Roche lobe overflow. We find that the NS$-$helium star binaries can significantly contribute the ULX population, with the overall number of about several in a Milky Way-like galaxy. Our calculations show that such ULXs are generally close systems with orbital period distribution peaked at $ \sim 0.1 $ day (with a tail up to $ \sim100 $ days), and the helium stars have relatively low masses distributing with a maximum probability at $ \sim 1M_{\odot} $.