We have obtained snapshot images of a transient X-ray source in M31 from Chandra ACIS-I and the Hubble Space Telescope ( HST ) Advanced Camera for Surveys ( ACS ) .
The Chandra position of the X-ray nova was R.A.=00:42:56.038 \pm 0.08 ^ { \prime \prime } , Dec.=+41:12:18.50 \pm 0.07 ^ { \prime \prime } .
The transient was active for at least 6 months .
Previous observations set an upper limit before the X-ray outburst , demonstrating variability by a factor of > 100 and confirming the transient nature of the source .
For the first 6 months after the initial detection , the X-ray luminosity was \sim 6 \times 10 ^ { 37 } erg s ^ { -1 } ; it then decayed to < 5 \times 10 ^ { 36 } erg s ^ { -1 } over the following 2 months .
An HST observation 29 days after the initial X-ray detection revealed a source at R.A.=00:42:56.042 , Dec.=+41:12:18.45 that was B = 24.52 \pm 0.07 .
This optical source faded to B = 24.95 \pm 0.08 in 9 months .
The HST identification of an optical source at the same position as the X-ray source , fading in concert with the X-ray source , indicates that this optical source is the counterpart of the X-ray transient .
The lack of high-mass stars in the region suggests this source is a low-mass X-ray binary , and the X-ray and optical luminosities provide a rough orbital period estimate of 8 ^ { +12 } _ { -5 } days for the system .