We have examined a probable soft X-ray transient source in the M31 bulge at R.A.=0:42:41.814 \pm 0.08 ^ { \prime \prime } , Dec. = 41:16:35.86 \pm 0.07 ^ { \prime \prime } .
On the three occasions we observed the source , its spectrum was soft ( kT _ { in } \approx 1 keV ) .
The brightest detection of the source was 2004 July 17 with a 0.3–7 keV luminosity of \sim 5 \times 10 ^ { 37 } erg s ^ { -1 } .
The only previous detection of the source was in 1979 by the Einstein observatory .
The multiple detections over 25 years suggest the duty cycle of the source is in the range 0.02–0.06 .
Coordinated HST ACS imaging before , during , and after the outburst revealed no variable optical source within the position errors of the X-ray source .
The optical data place a firm upper limit on the brightness of the counterpart of the X-ray outburst of B > 24.7 , suggesting the binary has a period \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 5.2 days .
The X-ray spectrum and lack of bright stars at the source location indicate the source was a soft transient event occurring in a low-mass X-ray binary , making this source a good black hole candidate in M31 .