We have created 0.3-10?keV, 13?yr, unabsorbed luminosity lightcurves for 528 X-ray sources in the central 20' of M31. We have 174 Chandra observations spaced at ~1 month intervals due 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 SF of active galactic nuclei (AGN). We find 220 X-ray sources with luminosities 1035?erg?s–1 that have SFs with significantly more variability than the ensemble AGN SF, and which 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.
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