Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M 31 and M 33

¹ University of Crete, Department of Physics, PO Box 2208, 71003 Heraklion, Greece e-mail: dh@physics.uoc.gr
² Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
³ IESL, Foundation for Research and Technology, 71110 Heraklion, Greece

Received: 24 October 2005
Accepted: 18 January 2006

Abstract

Context.Recent surveys of the Local Group spiral Galaxies M 31 and M 33 with XMM-Newton yielded a large number of X-ray sources.

Aims.As part of the effort to identify and classify the objects responsible for this X-ray emission, we have obtained optical spectra of the brightest optical counterparts of the identified X-ray sources, using the 1.3 m Skinakas Telescope. Most of these objects are foreground star candidates. The purpose of the present study is to confirm this identification and to explore the compatibility between the optical spectral classification and the observed X-ray properties of the sources.

Methods.We have obtained optical spectra for the 14 brightest optical counterparts of X-ray sources identified by XMM-Newton in the direction of M 31 and for 21 optical counterparts in the direction of M 33, using the 1.3 m Skinakas telescope in Crete, Greece.

Results.All of the M 31 sources and all but one of the M 33 sources were confirmed to be foreground stars, of spectral types between A and M. One of the stars is a late M dwarf with H_α emission, a flare star, also displaying strong X-ray variability. One of the M 33 sources (lying within the D25 ellipse) corresponds to a previously known background galaxy, LEDA 5899.