We searched optical/UV/IR counterparts of seven supersoft X-ray sources ( SSS ) in M31 in the Hubble Space Telescope ( HST ) ‘ ‘ Panchromatic Hubble Andromeda Treasury ’ ’ ( PHAT ) archival images and photometric catalog .
Three of the SSS were transient , the other four are persistent sources .
The PHAT offers the opportunity to identify SSS hosting very massive white dwarfs ( WD ) that may explode as type Ia supernovae in single degenerate binaries , with magnitudes and color indexes typical of symbiotics , high mass close binaries , or systems with optically luminous accretion disks .
We find evidence that the transient SSS were classical or recurrent novae ; two likely counterparts 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 , \mid \dot { P } \mid \lesssim 0.82 \times 10 ^ { -11 } .
This limit can be reconciled with the rotation period of a WD accreting at high rate in a binary with a few-hours orbital period .
However , there is no luminous counterpart with color indexes typical of an accretion disk irradiated by a hot central source .
Adopting a semi-empirical relationship , the upper limit for the disk optical luminosity implies an upper limit of only 169 minutes for the orbital period of the WD binary .