Supersoft X-ray sources in M31

Three of the SSSs were transient; the other four are persistent sources. The PHAT offers the opportunity to identify SSSs hosting very massive white dwarfs (WDs) that may explode as Type Ia supernovae in single degenerate binaries, with magnitudes and colour indexes typical of symbiotics, high-mass close binaries, or systems with an optically luminous accretion disc.

We find evidence that the transient SSSs were classical or recurrent novae; two probable counterparts that we identified are probably symbiotic binaries undergoing mass transfer at a very high rate. There is a candidate accreting WD binary in the error circle of one of the persistent sources, r3-8. In the spatial error circle of the best-studied SSS in M31, r2-12, no red giants or AGB stars are sufficiently luminous in the optical and UV bands to be symbiotic systems hosting an accreting and hydrogen-burning WD.