Calibrating X-ray binary luminosity functions via optical reconnaissance I. The case of M83

TL;DR

This study introduces a novel optical classification method for X-ray binaries in galaxy M83, revealing distinct luminosity function shapes and contributions from different donor mass classes, with implications for understanding X-ray source populations.

Contribution

The paper presents a new approach using optical data to classify X-ray binaries and models their luminosity functions with Schechter functions, improving upon previous X-ray based classifications.

Findings

High-mass XRBs show a marginal exponential cutoff in their XLF.

Low- and intermediate-mass XRBs are consistent with a single power-law XLF.

Low- and possibly intermediate-mass XRBs contribute 20-50% to the total XLF.

Abstract

Building on recent work by Chandar et al. (2020), we construct X-ray luminosity functions (XLFs) for different classes of X-ray binary (XRB) donors in the nearby star-forming galaxy M83 through a novel methodology: rather than classifying low- vs. high-mass XRBs based on the scaling of the number of X-ray sources with stellar mass and star formation rate, respectively, we utilize multi-band Hubble Space Telescope imaging data to classify each Chandra-detected compact X-ray source as a low-mass (i.e. donor mass <~ 3 solar masses), high-mass (donor mass >~ 8 solar masses) or intermediate-mass XRB based on either the location of its candidate counterpart on optical color-magnitude diagrams or the age of its host star cluster. In addition to the the standard (single and/or truncated) power-law functional shape, we approximate the resulting XLFs with a Schechter function. We identify a marginally significant (at the 1-to-2 sigma level) exponential downturn for the high-mass XRB XLF, at logLx ~ 38.48^{+0.52}_{-0.33} (in log CGS units). In contrast, the low- and intermediate-mass XRB XLFs, as well as the total XLF of M83, are formally consistent with sampling statistics from a single power-law. Our method suggests a non-negligible contribution from low- and possibly intermediate-mass XRBs to the total XRB XLF of M83, i.e. between 20 and 50%, in broad agreement with X-ray based XLFs. More generally, we caution against considerable contamination from X-ray emitting supernova remnants to the published, X-ray based XLFs of M83, and possibly all actively star-forming galaxies.