Volume 489, Number 2, October II 2008
|Page(s)||707 - 711|
|Section||Stellar structure and evolution|
|Published online||30 July 2008|
Three X-ray transients in M 31 observed with Swift
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany e-mail: email@example.com
2 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
3 University of Crete, Department of Physics, PO Box 2208, 71003 Heraklion, Greece
Accepted: 21 July 2008
Aims. The purpose of this study is to find transient X-ray sources in M 31 and to investigate and classify their nature.
Methods. Three X-ray transients were observed with Swift. For each of the three X-ray transients, we used the Swift X-ray and optical data with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, thereby identifying the source types.
Results. The outburst of XMMU J004215.8+411924 lasted about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that, with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disc with an innermost temperature of ~ eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black-hole low-mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift ~half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M 31 to estimate their rate, and we find a lower limit of 9 yr-1.
Key words: galaxies: individual: M 31 / X-rays: binaries / X-rays: galaxies
© ESO, 2008
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