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.=00:42:56.038 ±0.08, Dec.=+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 ∼6×10 erg s; it then decayed to < 5×10 erg s 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±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 this source is a low-mass X-ray binary, and the X-ray and optical luminosities provide a rough orbital period estimate of 8 −5 days for the system. Subject headings: X-rays: binaries — galaxies: individual (M31)