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 ob
servations of M31 totalling over 1 Ms from $it{Chandras}$ 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 $approx10^{32}$ erg s$^{-1}$, which probes the quiescent XRB regime.
The population of low-luminosity (< 10^35 erg/s) X-Ray Binaries (XRBs) has been investigated in our Galaxy and M31 but not further. To address this problem, we have used data from the Chandra X-Ray Observatory and the Hubble Space Telescope to invest
igate the faint population of XRBs in the grand-design spiral galaxy M51. A matching analysis found 25 star clusters coincident with 20 X-ray point sources within 1.5 (60 pc). From X-ray and optical color-color plots we determine that this population is dominated by high-mass XRBs. A stacking analysis of the X-ray data at the positions of optically-identified star clusters was completed to probe low-luminosity X-ray sources. No cluster type had a significant detection in any X-ray energy band. An average globular cluster had the largest upper limit, 9.23 x 10^34 erg/s, in the full-band (0.3 - 8 keV) while on average the complete sample of clusters had the lowest upper limit, 6.46 x 10^33 erg/s in the hard-band (2 - 8 keV). We determined average luminosities of the young and old star cluster populations and compared the results to those from the Milky Way. We conclude that deeper X-ray data is required to identify faint sources with a stacking analysis.
