X-rays beware: the deepest Chandra catalogue of point sources in M31

This study represents the most sensitive Chandra X-ray point source catalogue of M31. Using 133 publicly available Chandra ACIS-I/S observations totalling ∼1 Ms, we detected 795 X-ray sources in the bulge, north-east, and south-west fields of M31, covering an area of ≈0.6 deg², to a limiting unabsorbed 0.5-8.0 keV luminosity of ∼10³⁴ erg s^-1. In the inner bulge, where exposure is approximately constant, X-ray fluxes represent average values because they were determined from many observations over a long period of time. Similarly, our catalogue is more complete in the bulge fields since monitoring allowed more transient sources to be detected. The catalogue was cross-correlated with a previous XMM-Newton catalogue of M31's D₂₅ isophote consisting of 1948 X-ray sources, with only 979 within the field of view of our survey. We found 387 (49 per cent) of our Chandra sources (352 or 44 per cent unique sources) matched to within 5 arcsec of 352 XMM-Newton sources. Combining this result with matching done to previous Chandra X-ray sources we detected 259. new sources in our catalogue. We created X-ray luminosity functions (XLFs) in the soft (0.5-2.0 keV) and hard (2.0-8.0 keV) bands that are the most sensitive for any large galaxy based on our detection limits. Completeness-corrected XLFs show a break around ≈1.3 × 10³⁷ erg s^-1, consistent with previous work. As in past surveys, we find that the bulge XLFs are flatter than the disc, indicating a lack of bright high-mass X-ray binaries in the disc and an aging population of low-mass X-ray binaries in the bulge.