Supersoft X-ray sources in M31

As the most massive galaxy in the Local Group the early-type spiral M31 is close enough to be studied in detail at various wavelengths. A ROSAT PSPC mosaic of 6 contiguous pointings with an exposure time of 25 ksec each was performed in July 1991 (first M31 survey). A raster pointing of 80 observations with 2.5 ksec exposure time each covering the whole M31 disk was made in July/August 1992 and January 1993 (second M31 survey). A multi-step detection algorithm involving a maximum likelihood technique as final step was applied to the individual pointings of the first survey with a likelihood threshold of 10. A total of 396 X-ray sources were detected within the field of view of about 6.3 deg 2 (Supper et al. 1996). For each source several quantites are determined such as the position, the total number of background subtracted counts, the corresponding countrate and two hardness ratios for a crude spectral characterisation. CCD photometry has been performed in four passbands (BVRI) of the entire optical disk of M31 in September 1990 and September 1991 using the McGrawHill 1.3m telescope of the Michigan-Dartmouth-MIT Observatory at Kitt Peak (Magnier et al. 1992, Haiman et al. 1993). The observations have typical completeness limits of (22.3,22.2,22.2,20.9). Different selection criteria on these data sets allow the extraction of different classes of objects. Of particular interest are likely Galactic foreground stars which have been identified with the criteria V_ 0.4 (predominantly main sequence and giant stars).