Calibrating X-Ray Binary Luminosity Functions via Optical Reconnaissance. I. The Case of M83
Qiana Hunt, Elena Gallo, Rupali Chandar, Paula Johns Mulia, Angus Mok, Andrea Prestwich, Shengchen Liu
Abstract
Abstract Building on recent work by Chandar et al., 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- versus high-mass XRBs based on the scaling of the number of X-ray sources with stellar mass and star formation rate, respectively, we utilize multiband Hubble Space Telescope imaging data to classify each Chandra-detected compact X-ray source as a low-mass (i.e., donor mass ≲3 M ⊙ ), high-mass (donor mass ≳8 M ⊙ ), 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 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 σ -to-2 σ level) exponential downturn for the high-mass XRB XLF, at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">ℓ</mml:mi> <mml:mo>≃</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>38.48</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.33</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.52</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (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.