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

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 to 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 ∼0.1 day (with a tail up to ∼100 days), and the helium stars have relatively low masses distributing with a maximum probability at ∼1 M ⊙ .