An X-ray Transient and Optical Counterpart in the M31 Bulge

We have obtained snapshot images of a transient X-ray source in M31 from Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The Chandra position of the X-ray nova was R.A.=00:42:56.038 +/-0.08'', Dec.=+41:12:18.50 +/-0.07''. The transient was active for at least 6 months. Previous observations set an upper limit before the X-ray outburst, demonstrating variability by a factor of >100 and confirming the transient nature of the source. For the first 6 months after the initial detection, the X-ray luminosity was ~6 X 10$^{37}$ erg/s; it then decayed to