The X-ray source populations within galaxies are typically difficult to identify and classify from X-ray data alone .
We are able to break through this barrier by combining deep new Chandra ACIS-I observations with extensive Hubble Space Telescope imaging from the Panchromatic Hubble Andromeda Treasury of the M31 disk .
We detect 373 X-ray sources down to 0.35-8.0 keV flux of 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } over 0.4 square degrees , 170 of which are reported for the first time .
We identify optical counterpart candidates for 188 of the 373 sources , after using the HST data to correct the absolute astrometry of our Chandra imaging to 0.1 ^ { \prime \prime } .
While 58 of these 188 are associated with point sources potentially in M31 , over half ( 107 ) of the counterpart candidates are extended background galaxies , 5 are star clusters , 12 are foreground stars , and 6 are supernova remnants .
Sources with no clear counterpart candidate are most likely to be undetected background galaxies and low-mass X-ray binaries in M31 .
The hardest sources in the 1 - 8 keV band tend to be matched to background galaxies .
The 58 point sources that are not consistent with foreground stars are bright enough that they could be high mass stars in M31 ; however , all but 8 have optical colors inconsistent with single stars , suggesting that many could be background galaxies or binary counterparts .
For point-like counterparts , we examine the star formation history of the surrounding stellar populations to look for a young component that could be associated with a high mass X-ray binary ( HMXB ) .
About one third of the point sources are not physically associated with a young population , and are therefore more likely to be background galaxies .
For the 40 point-like counterpart candidates associated with young populations , we find that their age distribution has two peaks at 15-20 Myr and 40-50 Myr .
If we only consider the 8 counterpart candidates with typical high-mass main sequence optical star colors , their age distribution peaks mimic those of the sample of 40 .
Finally , we find that intrinsic faintness , and not extinction , is the main limitation for finding further counterpart candidates .