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. = 00h44m06.ˢ68 ± 1.ˢ74, decl. = +41°12′20.″0 ± 2.″31 (J2000.0). A follow-up HST observation 24 days later revealed a source at R.A. = 00h44m06.ˢ81, 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) × 10-4 to <(6.9 ± 0.09) × 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.