A period distribution of X-ray binaries observed in the central region of M31 with Chandra and HST

TL;DR

This study estimates orbital periods of 12 X-ray transients in M31 using Chandra and HST data, revealing a skew towards shorter periods compared to Milky Way transients, which informs galaxy evolution models.

Contribution

First to systematically estimate periods of extragalactic X-ray transients in M31, providing insights into their distribution and potential differences from Milky Way counterparts.

Findings

Periods range from <0.4 to 490 hours.

M31 transients skew towards shorter periods.

Statistical analysis suggests different period distributions from the Milky Way.

Abstract

Almost all Galactic black hole 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 black hole binaries also. Obtaining period estimates for extra-galactic 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 HST; 4\sigma B magnitude limits for optical counterparts are ~26--29, depending on crowding. We obtain period estimates for each transient for both neutron star and black hole accretors. Periods range from <0.4 to 490+/-90 hours (<0.97 to <175 hrs if all are BH systems). These M31 transients appear to be somewhat skewed towards 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 yield 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.