RX J0042.3+4115: A stellar mass black hole binary identified in M 31

¹ The Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6BT, UK
² The Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK
³ Los Alamos National Laboratory, PO Box 1663, Mail Stop D436, NIS-2, Los Alamos, MN 87545, USA

Corresponding author: R. Barnard, r.barnard@open.ac.uk

Received: 6 February 2003
Accepted: 17 April 2003

Abstract

Four XMM-Newton X-ray observations of the central region of the Andromeda Galaxy (M 31) have revealed an X-ray source that varies in luminosity over ~ erg s^-1 between observations and also displays significant variability over time-scales of a few hundred seconds. The power density spectra of lightcurves obtained in the 0.3–10 keV energy band from the three EPIC instruments on board XMM-Newton are typical of disc-accreting X-ray binaries at low accretion rates, observed in neutron star binaries only at much lower luminosities (~ 10³⁶ erg s^-1). However X-ray binaries with massive black hole primaries have exhibited such power spectra for luminosities >10³⁸ erg s^-1. We discuss alternative possibilities where RX J0042.3+4115 may be a background AGN or foreground object in the field of view, but conclude that it is located within M 31 and hence use the observed power spectra and X-ray luminosities to identify the primary as a black hole candidate.