Chandra,Swift, and HST studies of the CXOM31 J004253.1+411422

Abstract

Context. A very bright transient X-ray source, CXOM31 J004253.1+411422, was found by Chandra/HRC in the M31 bulge. We present Chandra, Swift ,a ndHubble Space Telescope (HST) observations of this source. Aims. 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. 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 M31 suggests an orbital period of about 15 h. The decay light curve is consistent with previous models of X-ray novae outbursts.