Discovery of an X-ray Nova in M31

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(h)44(m)06.(s)68 +/- 1.(s)74, decl. = +41degrees12'20."0 +/- 2."31 (J2000.0). A follow-up HST observation 24 days later revealed a source at R: A. = 00(h)44(m)06.(s) 81, decl. = +41degrees12'24."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) x 10(-4) to <(6.9 +/- 0.09) x 10(-5) counts 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.