Calibrating X-ray binary luminosity functions via optical reconnaissance II. The high-mass XLF and globular cluster population of X-ray binaries in the low star-forming spiral M81

TL;DR

This study uses HST data to identify and analyze the optical counterparts of X-ray binaries in M81, distinguishing between low-mass and high-mass systems in clusters and the field, revealing their properties and distributions.

Contribution

It provides the first detailed analysis of X-ray binary populations within globular clusters in a spiral galaxy, including classification reliability and host cluster characteristics.

Findings

Globular clusters hosting X-ray binaries are more massive and compact.

No correlation between cluster X-ray brightness and mass or density.

Field and cluster X-ray binaries show distinct properties.

Abstract

We characterize the optical counterparts to the compact X-ray source population within the nearby spiral galaxy M81 using multi-band Hubble Space Telescope (HST) imaging data. 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 $\leq$ 3 solar masses) within ancient globular clusters, as well as 42 candidate high-mass X-ray binaries (i.e. donor star mass $\geq$ 8 solar masses). To estimate the likelihood of misclassifications, we inject 4,000 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. We find that globular clusters that host X-ray binaries are on average more massive and more compact than globular clusters that do not. However, there is no apparent correlation between the X-ray brightness of the clusters and their masses or densities, nor are X-ray binary hosts more X-ray luminous than the general field population of low-mass X-ray binaries. This work represents one of the first in-depth analyses of the population of X-ray binaries within globular clusters in a spiral galaxy.