Chandra, Swift, and HST studies of the CXOM31 J004253.1+411422: Very bright X-ray transient in M 31

Context. A very bright transient X-ray source, CXOM 31 J004253.1+411422, was found by Chandra/HRC in the M 31 bulge. We present Chandra, Swift, and Hubble Space Telescope (HST) observations of this source. Aims. Since this source is the brightest known X-ray transient in M 31, we want to study its nature with Chandra and Swift. Comparing the results of Galactic transients and M 31 transients can give a better understanding of the nature of extragalactic binaries. Methods. 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. Results. 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 ~15 h. Conclusions. Our optical identification of the brightest X-ray transient so far discovered in M 31 suggests an orbital period of about 15 h. The decay light curve is consistent with previous models of X-ray novae outbursts.