Astronomy & Astrophysics
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 use the Swift X-ray and opticaldata together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, andthereby to identify the source types. Results.The outburst of XMMU J004215.8+ 411924 lasted for about one month. The source had a hard power-law spectrum with aphoton index of 1.6. It was previously identiﬁed as a Be/X-ray binary based on the optical identiﬁcation with a star. However, we showthat with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFTJ004217.3+ 411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to athermal accretion disk with innermost temperature of ∼ 250 − 600 eV. The source was not seen in the optical, and the soft spectrumindicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed inthe optical. We detected it both in X-rays and UV with Swift ∼ half a year after the optical maximum, after which it decayed below theSwift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of50 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
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