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Astron. Astrophys. 332, 189-193 (1998)

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Evidence for massive white dwarfs in the M 31 supersoft sources

P. Kahabka

Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 4 August 1997 / Accepted 28 November 1997


Simulating ROSAT PSPC hardness ratios HR1 and HR2 and count rates for NLTE white dwarf atmosphere spectra one finds that the 15 supersoft sources in the spiral galaxy M 31 contain hot [FORMULA] white dwarfs. If they undergo steady nuclear burning and if they are located close to the stability line then they harbor massive white dwarfs [FORMULA] in agreement with what has been found for the galactic supersoft sources. The recurrent supersoft transient RX J0045.4+4154 affords a very hot [FORMULA] white dwarf. If it is on the stability line then it has a white dwarf mass of [FORMULA]. Taking a mass distribution of steadily nuclear burning white dwarfs into account one expects that there exist at least about 650 supersoft sources in M 31. The spectral parameters of the so far most distant ([FORMULA]) supersoft source found in the galaxy NGC 55 imply a hot ([FORMULA]) and massive ([FORMULA]) white dwarf. The number of such sources seen in a galaxy at this distance is derived from a luminosity function of supersoft sources.

Key words: galaxies: individual: M 31 – galaxies: individual: NGC 55 – binaries: close – X-rays: stars – stars: evolution – white dwarfs

Send offprint requests to: ptk@astro.uva.nl

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998