A Synoptic X-Ray Study of M31 with the Chandra High Resolution Camera

Abstract

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of ~2.5 yr and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term light curves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well fitted by a power law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X>10³⁷ ergs s^-1. We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up Hubble Space Telescope (HST) WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W=22.3+/-0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 M_solar black hole X-ray nova with an orbital period of 23⁺⁵⁴_-16 days. In the other case, the optical counterpart varies between F336W=20.82+/-0.06 and 21.11+/-0.02 mag. Ground-based and HST observations show that this object is bright (V=18.8+/-0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.