Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31. II

TL;DR

This study identifies optical counterparts of X-ray sources in M 31 using spectroscopy, revealing most are foreground late-type stars in the Milky Way, with some ambiguous cases and others inconsistent with stellar spectra.

Contribution

First optical spectroscopic identification of counterparts for X-ray sources in M 31, clarifying their nature as mostly foreground stars.

Findings

Most X-ray sources are associated with late-type stars in the Milky Way.

Two sources have ambiguous optical counterparts.

Some sources exhibit X-ray properties inconsistent with late-type stellar spectra.

Abstract

A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded a large number of X-ray sources. This is the second in a series of papers with the aim of identifying the optical counterparts of these X-ray sources. We have obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece. For 17 of the 20 X-ray sources, we have identified the optical counterpart as a normal late type star (of type F or later) in the foreground (i.e. in the Milky Way). For two more sources there were two possible optical counterparts in each case, while two more objects have X-ray properties that are not compatible with the spectral characteristics of late type non-flaring stars.