Nature of donors in ultra-luminous X-ray binaries powered by neutron stars

TL;DR

This paper investigates the donor star properties in neutron star-powered ultraluminous X-ray sources by developing a new luminosity-period relation and comparing it with stellar evolution models.

Contribution

It introduces a theoretical $L_{X}-P_{orb}$ relation for ULXs with neutron stars to infer donor star characteristics from observed data.

Findings

Donor in NGC5907 ULX-1 is a 6-12 M_sun star near main sequence.

Models for M82 X-2 and NGC7793 P-13 align with previous studies.

ULX systems with neutron stars tend to have relatively massive donor stars.

Abstract

In this study, we examine the properties of donor stars of the three recently discovered ultraluminous X-ray sources (ULXs) powered by rotating neutron stars. For this purpose, we constructed a theoretical relation between the X-ray luminosity ($L_{\rm{X}}$) and the orbital period ($P_{\rm{orb}}$) suitable for ULXs with neutron stars. By using this new $L_{\rm{X}} - P_{\rm{orb}}$ relation, we attempt to determine the currently unknown nature of donor stars in ULXs associated with neutron stars. Especially, comparing the observed properties with the stellar evolution tracks, we suggest that the donor star in the NGC5907 ULX-1 system is a moderately massive star with $6 - 12 \rm{M}_{\odot}$, just departing from the main sequence phase. The results of our models for the other two ULX systems (M82 X-2 and NGC7793 P-13) are consistent with those in previous studies. Although there are only a few samples, observed ULX systems with neutron stars seems to involve relatively massive donors.