2012ApJ...756...32B

Almost all Galactic black hole (BH) binaries with low-mass donor stars are transient X-ray sources; we expect most of the X-ray transients observed in external galaxies to be BH binaries also. Obtaining period estimates for extragalactic transients is challenging, but the resulting period distribution is an important tool for modeling the evolution history of the host galaxy. We have obtained periods, or upper limits, for 12 transients in M31, using an updated relation between the optical and X-ray luminosities. We have monitored the central region of M31 with Chandra for the last ∼12 years, and followed up promising transients with the Hubble Space Telescope; 4σ B-magnitude limits for optical counterparts are 26-29, depending on crowding. We obtain period estimates for each transient for both neutron star and BH accretors. Periods range from <0.4 to 490±90 hr (<0.97 to <175 hr if all are BH systems). These M31 transients appear to be somewhat skewed toward shorter periods than the Milky Way (MW) transients; indeed, comparing the M31 and MW transients with survival analysis techniques used to account for some data with only upper limits yields probabilities of ∼0.02-0.08 that the two populations are drawn from the same distribution. We also checked for a correlation between orbital period and distance from the nucleus, finding a 12% probability of no correlation. Further observations of M31 transients will strengthen these results.