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

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 $\sim 1M_{\odot}$.

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