We have obtained snapshot images of an X-ray nova in M31 from Chandra ACIS-I and the Hubble Space Telescope ( HST ) Advanced Camera for Surveys ( ACS ) .
The Chandra position of the X-ray nova was R.A.=00:44:06.68 \pm 1.74 ^ { \prime \prime } , Dec.=+41:12:20.0 \pm 2.31 ^ { \prime \prime } .
A follow-up HST observation 24 days later revealed a source at R.A.=00:44:06.81 , Dec.=+41:12:24.0 that was B = 25.75 \pm 0.05 .
This optical source faded to B = 27.1 \pm 0.1 in 3 months .
During this time period , the X-ray flux decayed linearly from ( 3.6 \pm 0.2 ) \times 10 ^ { -4 } to < ( 6.9 \pm 0.09 ) \times 10 ^ { -5 } ct cm ^ { -2 } s ^ { -1 } .
The HST identification of an optical source in the same region experiencing an obvious drop in brightness in concert with the X-ray nova suggests that this optical source is the counterpart of the X-ray nova .
However , the precision of the X-ray position allows the possibility that the optical source is a nearby variable star .
We discuss the implications of both possibilities .