Identifying a black hole X-ray transient in M 31 with XMM-Newton and Chandra^*

¹ The Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6BT, UK e-mail: r.barnard@open.ac.uk
² The Department of Physics and Astronomy, The University of Leicester, Leicester LE1 7RH, UK

Received: 11 November 2003
Accepted: 6 May 2004

Abstract

Stochastic variability in two out of four XMM-Newton observations of XMMU J004303+4115 along with its power spectra and X-ray luminosities suggest a low-mass X-ray binary (LMXB) with a black hole primary. However, Chandra observations resolve the object into two point sources. We use data from 35 Chandra observations to analyse the contributions of each source, and attribute the variability to CXOM31 J004303.2+411528 (known as r2-3), which varies in intensity by a factor of ~100 between observations. We assume that the power density spectra of LMXBs are governed by the luminosity, and that the transition between types of power density spectra occurs at some critical luminosity in Eddington units, l_c, that applies to all LMXBs. We use results from these XMM-Newton observations and past results from the available literature to estimate this transition luminosity, and find that all results are consistent with in the 0.3–10 keV band. CXOM31 J004303.2+411528 exhibits a low accretion rate power density spectrum at a 0.3–10 keV luminosity of erg s^-1. Known stellar mass black holes have masses of 4–15 ; hence our observations of CXOM31 J004303.2+411528 are consistent with if it has a black hole primary.