Multiwavelength search for counterparts of supersoft X-ray sources in M31

We searched for the optical/UV/IR counterparts of seven supersoft X-ray sources (SSSs) in M31 in the Hubble Space Telescope (HST) `Panchromatic Hubble Andromeda Treasury' (PHAT) archival images and photometric catalogue. 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. This SSS has a known modulation of the X-ray flux with a 217.7 s period, and we measured an upper limit on its derivative, namely mid dot{P}mid ≲ 0.82 × 10^{-11}. This limit can be reconciled with the rotation period of a WD accreting at a high rate in a binary with an orbital period of a few hours. However, there is no luminous counterpart with colour indexes typical of an accretion disc irradiated by a hot central source. Adopting a semi-empirical relationship, the upper limit for the disc optical luminosity implies an upper limit of only 169 min for the orbital period of the WD binary.