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

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 over1 3 10 38 erg s 1 between observations and also displays significant variabil- ity 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 (10 36 erg s 1 ). However X-ray binaries with mas- sive black hole primaries have exhibited such power spectra for luminosities>10 38 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.