Discovery of an X-Ray Nova in M31

Abstract

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^m06.68s 68+/-1.74s, decl.=+41^deg12^'20.0"+/-2.31" (J2000.0). A follow-up HST observation 24 days later revealed a source at R.A.=00^h44^m06.81s, decl.=+41^deg12^'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.