Bright X-Ray Transients in the Andromeda Galaxy Observed with Chandra and XMM-Newton

Abstract

We study three transient X-ray sources that were bright in the central region of the M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time to derive X-ray light curves of transient sources in M31 suitable for studying their variability on a timescale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral characteristics. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+411759 is probably a classical nova-like system containing a magnetized, rapidly spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 yr^-1. The rate of the bright hard X-ray transients (~5 yr^-1) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.