Faint X-ray Binaries and Their Optical Counterparts in M31

Abstract

X-ray binaries (XRBs) are probes of both star formation and stellar mass, but more importantly remain one of the only direct tracers of the compact object population. To investigate the XRB population in M31, we utilized all 121 publicly available observations of M31 totalling over 1 Ms from $Chandr{a}^{\prime }s$ ACIS instrument. We studied 83 star clusters in the bulge using the year 1 star cluster catalogue from the Panchromatic Hubble Andromeda Treasury Survey. We found 15 unique star clusters that matched to 17 X-ray point sources within 1'' (3.8 pc). This population is composed predominantly of globular cluster low-mass XRBs, with one previously unidentified star cluster X-ray source. Star clusters that were brighter and more compact preferentially hosted an X-ray source. Specifically, logistic regression showed that the F475W magnitude was the most important predictor followed by the effective radius, while color (F475W$-$F814W) was not statistically significant. We also completed a matching analysis of 1566 HII regions and found 10 unique matches to 9 X-ray point sources within 3'' (11 pc). The HII regions hosting X-ray point sources were on average more compact than unmatched HII regions, but logistic regression concluded that neither the radius nor H$\alpha$ luminosity was a significant predictor. Four matches have no previous classification and thus are high-mass XRB candidates. A stacking analysis of both star clusters and HII regions resulted in non-detections, giving typical upper limits of $\approx 10^{32}$ erg s${}^{-1}$, which probes the quiescent XRB regime.

15 pages, 9 figures, 9 tables. Accepted for publication in The Astrophysical Journal

References in corpus (11)

• A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
• A Catalog of X-ray Point Sources from Two Megaseconds of Chandra Observations of the Galactic Center
• Populating the Galaxy with low-mass X-ray binaries
• Faint X-ray Sources in the Globular Cluster Terzan 5
• PHAT Stellar Cluster Survey I. Year 1 Catalog and Integrated Photometry
• A study of the population of LMXBs in the bulge of M31
• Distribution of High-Mass X-ray Binaries in the Milky Way
• The dynamical formation of LMXBs in dense stellar environments: globular clusters and the inner bulge of M31
• Low-mass X-ray binaries in the bulge of the Milky Way
• The Local Environment of Ultraluminous X-ray Sources Viewed by XMM Newton's Optical Monitor
• An X-ray spectral survey of the disc of M31 with XMM-Newton

Cited by in corpus (2)

• The Spatial Correlation of High Mass X-ray Binaries and Young Star Clusters in Nearby Star-Forming Galaxies
• Milky Way Globular Cluster Metallicity and Low-Mass X-ray Binaries: The Red Giant Influence