Optical Constraints on an X-ray Transient Source in M31

We have detected a transient X-ray source in the M31 bulge through a continuing monitoring campaign with the Chandra ACIS-I camera. The source was detected at R.A. = 00h42m33.ˢ428 ± 0.″11, decl. = +41°17′03.″37 ± 0.″11 in only a single observation taken 2004 May 23. Fortuitous optical HST ACS imaging of the transient location prior to the X-ray outburst, along with follow-up HST ACS imaging during and after the outburst, reveals no transient optical source brighter than B(equivalent) = 25.5. The location of the source and its X-ray properties suggest that it is a low-mass X-ray binary (LMXB). Assuming the transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9 ± 0.4) × 1037 ergs s-1 and the upper limit on the optical luminosity provide a prediction of ≤1.6 days for the orbital period of the binary system.