2005ApJ...620..723W

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. The Chandra position of the X-ray nova was R.A.=00^h44^m06{fs}68±1{fs}74, decl.=+41°12'20".0±2".31 (J2000.0). A follow-up HST observation 24 days later revealed a source at R.A.=00^h44^m06{fs}81, decl.=+41°12'24".0 that was B=25.75±0.05. This optical source faded to B=27.1±0.1 in 3 months. During this time period, the X-ray flux decayed linearly from (3.6±0.2)x10^–4 to <(6.9±0.09)x10^–5 counts/cm2/s. 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.