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arXiv · 0807.2949

Three X-ray transients in M31 observed with Swift

R. Voss·W. Pietsch·F. Haberl·H. Stiele·J. Greiner·G. Sala·D. H. Hartmann·D. Hatzidimitriou
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Abstract

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for 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 disk with innermost temperature of 250-600 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 M31 to estimate their rate, and we find a lower limit of 9/yr.

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R. Voss, W. Pietsch, F. Haberl, H. Stiele, J. Greiner, G. Sala, D. H. Hartmann, D. Hatzidimitriou. 2008-07-18. Three X-ray transients in M31 observed with Swift. https://doi.org/10.1051/0004-6361:20078970

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