Calibrating X-ray binary LFs via opt. data. II. M81

By comparing the optical luminosities and colors measured for candidate donor stars and host clusters to stellar and cluster evolutionary models, respectively, we estimate the likely masses and upper age limits of the field and cluster X-ray binaries.

We identify 15 low-mass X-ray binaries (i.e., donor star mass <~3M_{sun}_) within ancient globular clusters, as well as 42 candidate high-mass X-ray binaries (i.e., donor star mass >~8M_{sun}_). To estimate the likelihood of misclassifications, we inject 4000 artificial sources into the HST mosaic image and conclude that our classifications of globular clusters and high-mass X-ray binaries are reliable at the >90% level.