Supersoft X-ray Sources in M31

We report on luminous supersoft X-ray sources (SSSs) in M31. In a survey which was most sensitive to soft sources in four 8×8 regions covered by Chandra’s ACIS-S S3 CCD, we find 33 SSSs that appear to belong to M31. Two of these were first identified as SSSs through ROSAT observations. Two SSSs may be identified with symbiotics, and 2 with supernova remnants. SSSs in the disk are clustered near star-forming regions, possibly indicating that they are young; SSSs in the outer disk and halo are likely to be old systems. The largest density of bright SSSs is in the bulge; some of the bulge sources are close enough to the nucleus to be remnants of the tidal disruption of a giant by the massive central black hole. By using Chandra data in combination with ROSAT and XMM observations, we find most SSSs to be highly variable, fading from or brightening toward detectability on time scales of months. There is evidence for SSSs with low luminosities (∼ 1036 erg s−1). There is also evidence that some SSSs have spectral characteristics mirroring those of the Magellanic Cloud and Milky Way SSSs (k T ≤ 100 eV); we call these “classical” supersoft sources. Other SSSs may either have small (< 10%) hard components, or slightly higher temperatures (but still < 300 eV). We refer to these SSSs as quasi-soft sources. While white dwarf models may apply to M31 SSSs, some SSSs, including some of the quasi-soft sources, could be accreting intermediate-mass black holes, or neutron stars. Subject headings: galaxies: individual (M31) — X-rays: binaries — X-rays: galaxy