ar X iv : a st ro - p h / 05 06 31 2 v 1 1 4 Ju n 20 05 Optical Constraints on an X - ray Transient Source in M 31

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.=00:42:33.428± 0.11, Dec.=+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 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±0.5)×10 erg s and the upper-limit on the optical luminosity provide a prediction of <1.6 days for the orbital period of the binary system. Subject headings: X-rays: binaries — binaries: close — X-rays: stars — galaxies: individual (M31)