Around 200 new X-ray binary IDs from 13 years of Chandra observations of the M31 center

Abstract

We have created 0.3--10 keV, 13 year, unabsorbed luminosity lightcurves for 528 X-ray sources in the central 20' of M31. We have 174 Chandra observations spaced at ~1 month intervals thanks to our transient monitoring program, deeper observations of the M31 nucleus, and some public data from other surveys. We created 0.5--4.5 keV structure functions (SFs) for each source, for comparison with the ensemble structure function of AGN. We find 220 X-ray sources with luminosities > ~1E+35 erg/s that have SFs with significantly more variability than the ensemble AGN SF, and are likely X-ray binaries (XBs). A further 30 X-ray sources were identified as XBs using other methods. We therefore have 250 probable XBs in total, including ~200 new identifications. This result represents great progress over the ~50 XBs and ~40 XB candidates previously identified out of the ~2000 X-ray sources within the D_25 region of M31; it also demonstrates the power of SF analysis for identifying XBs in external galaxies. We also identify a new transient black hole candidate, associated with the M31 globular cluster B128.

Accepted for publication in ApJ; 20 pages, 7 figures; Tables 1 and 2 continue after the references (8 pages)

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• Identification of the Hard X-ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31
• Discovery of periodic dips in the brightest hard X-ray source of M31 with EXTraS
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• The Spatial Correlation of High Mass X-ray Binaries and Young Star Clusters in Nearby Star-Forming Galaxies
• Faint X-ray Binaries and Their Optical Counterparts in M31
• CXO J004318.8+412016, a steady supersoft X-ray source in M 31
• A Chandra Study of The Stellar X-ray Emissivity of Globular Clusters in M\,31 Bulge
• A multi-wavelength approach to classifying transient events in the direction of M31
• XMM-Newton and Chandra observations of the M31 globular cluster black hole candidate XB135: a heavyweight contender cut down to size
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