2012A&A...542A.120N

A very bright transient X-ray source, CXOM31 J004253.1+411422, was found by Chandra/HRC in the M31 bulge. We present Chandra, Swift, and Hubble Space Telescope (HST) observations of this source. Since this source is the brightest known X-ray transient in M31, we want to study its nature with Chandra and Swift. Comparing the results of Galactic transients and M31 transients can give a better understanding of the nature of extragalactic binaries. We fitted disk black body and power law models to X-ray data from Chandra and Swift. Follow-up HST/ACS imaging during and after the outburst revealed a transient optical counterpart. Our HST observations show an optical counterpart with optical magnitude B=23.91±0.08. Using the empirical relations between X-ray luminosity and absolute visual magnitude, we estimate the orbital period of the system is about ∼15h. Our optical identification of the brightest X-ray transient so far discovered in M31 suggests an orbital period of about 15h. The decay light curve is consistent with previous models of X-ray novae outbursts.