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 <sup>h</sup> 44 <sup>m</sup> 0.6 <sup>S</sup> .68 ± 1 <sup>S</sup> .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 <sup>h</sup> 44 <sup>m</sup> 06 <sup>s</sup> .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) × 10 <sup>-4</sup> to <(6.9 ± 0.09) × 10 <sup>-5</sup> counts cm <sup>-2</sup> s <sup>-1</sup> . 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. © 2005, The American Astronomical Society. All rights reserved.