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tokens: the SED fitting used to determine spectral types for companion stars in HMXB candidates. In Section 5 we discuss our results in the context of previous studies, and in Section 6 we provide a brief summary of our results. We assume a Galactic column density, , and a photon index, Γ = 1.7 (Stiele et al. 2011 ), to convert count rates into absorbed energy flux. We assume a distance of 776 kpc to M31 (Dalcanton et al. 2012 ) for luminosity calculations. 2. Data In this study we employ data from NuSTAR , Chandra , and HST . We now describe each data set in more detail below. For an overview of the area observed by each telescope, see Figure 1 . Figure 1. Near-UV image of M31 from the Galaxy Evolution Explorer ( GALEX ; left; Gil de Paz et al. 2009 ) and three-color X-ray image of Chandra Field A (right), see Section 2.2 for more information on Field A data reduction. Magenta regions outline the area observed by PHAT. Green outlines the NuSTAR observed region, and cyan outlines the area observed by Chandra , with solid lines indicating ChandraPHAT observations and dashed lines indicating Field A observations. In the UV image, the 64 sources observed by NuSTAR and Chandra that fall within the PHAT footprint are marked with white crosses, and a 5′ circle outlines the densest inner bulge region of M31. In the X-ray image, all sources detected by wavdetect within Field A are marked with white ellipses (including sources that do not match NuSTAR sources or are outside the PHAT footprint, and thus are not presented in our sample in Table 1 ). 2.1. <italic>NuSTAR</italic> Data NuSTAR source catalogs and source classifications come from Wik et al. (2018, in preparation). Observations were taken between February and October 2015 covering the area outlined in green in Figure 1 with an average exposure time of ∼400 ks, and they were reduced using the nupipeline software. Sources previously observed with Chandra were used for astrometric alignment. For detailed information on data reduction, source detection, and source classification, see Wik et al. (2018, in preparation), who present the entire NuSTAR M31 survey. The NuSTAR observations cover the nucleus and inner disk regions of M31 at an energy range of 4–25 keV using the 4–6 keV, 6–12 keV, and 12–25 keV energy bands. The completeness of the NuSTAR observations starts to fall off at a luminosity of ∼3 × 10 36 erg s −1 and reaches zero at ∼2 × 10 36 erg s −1 . 2.2. <italic>Chandra</italic> Data Chandra data used in this study are comprised of two data sets: the ChandraPHAT data, a Chandra Large Project survey by B. Williams et al. (2018, submitted), and one additional Chandra field (obsid 18046, P.I. Hornschemeier), hereby referenced as Field A, that was reduced for this paper. The ChandraPHAT data set is comprised of seven Chandra pointings, each with a depth of about 50 ks. The Field A data consist of one Chandra pointing with a depth of 25 ks. The ChandraPHAT observations were taken in 2015 October and the