A Spectroscopic Search for High Mass X-ray Binaries in M31

TL;DR

This study uses optical spectroscopy to identify and classify high-mass X-ray binary candidates and other X-ray sources in the Andromeda galaxy, revealing several new candidate systems and other astrophysical objects.

Contribution

It presents new spectroscopic data and identification of HMXB candidates in M31, including the first strong candidates with emission lines, expanding knowledge of X-ray source populations.

Findings

Identified 8 early-type star counterparts with hard X-ray spectra.

Discovered 3 strong HMXB candidates with emission lines.

Detected additional objects including cataclysmic variables, a supernova remnant, and globular cluster X-ray sources.

Abstract

We present new optical spectroscopy of 20 candidate counterparts of 17 X-ray sources in the direction of the M31 disc. By comparing the X-ray catalogue from the XMM-Newton survey of M31 with star catalogues from the Local Group Galaxy Survey, we chose counterpart candidates based on optical colour and X-ray hardness. We have discovered 17 counterpart candidates with spectra containing stellar features. Eight of these are early-type stars of O or B type in M31, with hard X-ray spectra, making them good HMXB candidates. Three of these eight exhibit emission lines, which we consider to be the strongest HMXB candidates. In addition, our spectra reveal two likely Galactic cataclysmic variables, one foreground M star, two probable LMXBs related to M31 globular clusters, one emission line region with an embedded Wolf-Rayet star, and one newly-discovered supernova remnant. Finally, two of the sources have stellar spectra with no features indicative of association with an X-ray source.