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 \pm 0.11 ^ { \prime \prime } , Dec.=+41:17:03.37 \pm 0.11 ^ { \prime \prime } 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 \pm 0.5 ) \times 10 ^ { 37 } erg 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 .