From the 1991 ROSAT PSPC M31 X-ray point source catalog a sample of 26 candidate supersoft sources has been derived using one of the selection criteria or , and assuming that the observed count rate is in agreement with the expected steady-state luminosity. For these candidates absorbing hydrogen column densities, effective temperatures and white dwarf masses (assuming the sources are on the stability line of atmospheric nuclear burning) are derived. The observed white dwarf mass distribution of supersoft sources in M31 appears to be constrained to masses . The whole population of supersoft sources in M31 is estimated accordingly to be at least 1000 and at most 10,000 taking a theoretical white dwarf mass distribution and a double exponential scale height distribution for the gas and the source distribution into account and under the assumption that the observationally derived sample is restricted to white dwarf masses above . This range of a population has to be compared with the range of a population of 800-5000 sources as predicted from population synthesis calculations. We find the source population scale height to be 300 pc for a scale height for the gas of 150-600 pc. This is consistent with a young stellar population. Assuming a life time as a steadily nuclear burning white dwarf (a supersoft source) of and that all supersoft sources with masses in excess of are progenitors of supernovae of type Ia, a SN Ia rate of is inferred for M31 based on these progenitors. Supersoft sources then comprise 20-100% of the SNe Ia progenitors for a total estimated SN Ia rate of .
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999