2005ApJ...632.1086W

We present X-ray and optical observations of a new transient X-ray source in M31 first detected 2004 May 23 at R.A.=00^h43^m09{fs}940±0".65, decl.=41°23'32".49±0".66. The X-ray light curve shows two peaks separated by several months, reminiscent of many Galactic X-ray novae. The location and X-ray spectrum of the source suggest it is a low-mass X-ray binary. Follow-up HST ACS observations of the location both during and after the outburst provide a high-confidence detection of variability for one star within the X-ray position error ellipse. This star has ΔB~1 mag, and there is only a ~1% chance of finding such a variable in the error ellipse. We consider this star a good candidate for the optical counterpart of the X-ray source. The luminosity of this candidate provides a prediction for the orbital period of the system of 2.3^+3.7_–1.2days.