RX J0045.4+4154: A recurrent supersoft x-ray transient in M31

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Abstract

Using data extracted from the ROSAT archive we have discovered a recurrent supersoft X-ray transient RX J0045.4+4154 in M31. The first outburst began on 1992 February 2 and continued for at least 4 days, until the end of the observation sequence. A second outburst that lasted more than 6 days was seen to begin on 1993 January 7. The X-ray spectrum on both occasions yields a characteristic blackbody temperature of ~90 eV. For a range of plausible continuum models, the hydrogen column density is (0.8-1.5) x 10^21^ cm^-2^ and is consistent with the source being located in M31. This implies an unabsorbed 0.1-2.0 keV peak luminosity of ~10^38^ ergs s^-1^. This is the first recurrent X-ray transient to be found in M31 and is particularly notable because it is much softer than the bright X-ray transients seen in our Galaxy. The spectrum is characteristic of the supersoft class of X-ray sources, which are thought to be accreting white dwarfs that have a hydrogen-burning surface layer. A fit to a white dwarf model atmosphere gives a temperature of 10^6^ K, the hottest found so far. This high temperature is consistent with a white dwarf mass of 1.3-1.4 M_sun_, approaching the Chandrasekhar limit, and burning close to the nuclear stability limit.

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