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Volume 405, Number 2, July II 2003
Page(s) 505 - 511
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030638

A&A 405, 505-511 (2003)
DOI: 10.1051/0004-6361:20030638

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

R. Barnard1, J. P. Osborne2, U. Kolb1 and K. N. Borozdin3

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

(Received 6 February 2003 / Accepted 17 April 2003 )

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 ~ $1{-}3\times 10^{38}$ 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 >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.

Key words: X-rays: general -- galaxies: individual: M 31 -- X-rays: binaries -- black hole physics

Offprint request: R. Barnard, r.barnard@open.ac.uk

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