A Chandra Search for Periodic X-ray Sources in the Bulge of M31

Abstract: We present a systematic search for periodic X-ray sources in the bulge of M31, using ~ 2 Ms of archival Chandra observations spanning a temporal baseline of 16 years. Utilizing the Gregory-Loredo algorithm that is designed for photon-counting, phase-folded light curves, we detect seven periodic X-ray sources, among which four are newly discovered. Three of these sources are novae, the identified periods of which range between 1.3-2.0 hour and is most likely the orbital period. The other four sources are low-mass X-ray binaries, the identified periods of which range between 0.13-19.3 hour and are also likely orbital due to a clear eclipsing/dipping behavior in the light curve. We address implications on the X-ray binary population of the M31 bulge. Our study demonstrates the potential of using archival X-ray observations to systematically identify periodic X-ray sources in external galaxies, which would provide valuable information about the underlying exotic stellar populations.