The nature of the X-ray pulsar in M31: an intermediate mass X-ray binary?

TL;DR

This paper investigates a unique X-ray binary in M31, suggesting it is an intermediate mass system based on observed properties, orbital data, and stellar evolution models, challenging typical classifications.

Contribution

It identifies the first candidate intermediate mass X-ray binary in M31 by analyzing orbital and stellar parameters against theoretical models.

Findings

Donor star likely exceeds 1.5 solar masses

System's properties are inconsistent with low-mass X-ray binaries

Magnetic field estimates support intermediate mass donor hypothesis

Abstract

Recently the first finding of a spin period of an accreting neutron star in M31 is reported. The observed spin period is 1.2 s and it shows 1.27 d modulations due to orbital motion. From the orbital information, the mass donor could not be a giant massive star. On the other hand, the observed properties are quite odd for typical low mass X-ray binaries. In this study, we compare observed binary parameters with theoretical models given by a stellar evolution track and make a restriction on the possible mass range of the donor. According to the standard stellar evolution model, the donor star should be larger than 1.5 solar mass, and this suggests that this system is a new member of a rare category, intermediate mass X-ray binary. The magnetic field strength of the neutron star suggested by spin-up/down tendency in this system supports the possibility of intermediate mass donor.