An X-Ray Transient and Optical Counterpart in the M31 Bulge

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. = 00h42m56.ˢ038 ± 0.″08, decl. = +41°12′18.″50 ± 0.″07. 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 ~6 × 1037 ergs s-1; it then decayed to <5 × 1036 ergs s-1 over the following 2 months. An HST observation 29 days after the initial X-ray detection revealed a source at R.A. = 00h42m56.ˢ042, decl. = +41°12′18.″45 that was B = 24.52 ± 0.07. This optical source faded to B = 24.95 ± 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 that this source is a low-mass X-ray binary, and the X-ray and optical luminosities provide a rough orbital period estimate of 8 days for the system.