Evidence for massive white dwarfs in the M 31 supersoft sources

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 ≈ 3.0 − 4.5 × 105 K white dwarfs. If they undergo steady nuclear burning and if they are located close to the stability line then they harbor massive white dwarfs MWD ≈ 0.85−1.0M in agreement with what has been found for the galactic supersoft sources. The recurrent supersoft transient RX J0045.4+4154 affords a very hot≈ 8.4×105 K white dwarf. If it is on the stability line then it has a white dwarf mass of ≈ 1.26 M . 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 (D ∼ 1.3 Mpc) supersoft source found in the galaxy NGC 55 imply a hot (∼ 6× 105 K) and massive (MWD ∼ 1.1 M ) white dwarf. The number of such sources seen in a galaxy at this distance is derived from a luminosity function of supersoft sources.