A Population of Ultraluminous X-ray Sources with An Accreting Neutron Star

TL;DR

This study models the formation and characteristics of neutron star ultraluminous X-ray sources (ULXs), revealing their potential significant contribution to the ULX population in galaxies like M82 and the Milky Way.

Contribution

It provides the first detailed population synthesis and evolution analysis of neutron star ULXs, highlighting their formation rates and distribution in different galaxy types.

Findings

Neutron star ULXs have a birthrate of about 10^-4 per year.

High-mass X-ray binaries dominate neutron star ULXs in M82-like galaxies.

Intermediate-mass X-ray binaries are prevalent in Milky Way-like galaxies.

Abstract

Most ultraluminous X-ray sources (ULXs) are believed to be X-ray binary systems, but previous observational and theoretical studies tend to prefer a black hole rather than a neutron star accretor. The recent discovery of 1.37 s pulsations from the ULX M82 X-2 has established its nature as a magnetized neutron star. In this work we model the formation history of neutron star ULXs in an M82- or Milky Way-like galaxy, by use of both binary population synthesis and detailed binary evolution calculations. We find that the birthrate is around $10^{-4}\, \rm yr^{-1}$ for the incipient X-ray binaries in both cases. We demonstrate the distribution of the ULX population in the donor mass - orbital period plane. Our results suggest that, compared with black hole X-ray binaries, neutron star X-ray binaries may significantly contribute to the ULX population, and high-mass and intermediate-mass X-ray binaries dominate the neutron star ULX population in M82- and Milky Way-like galaxies, respectively.