X-ray luminosity function of high-mass X-ray binaries: Studying the signatures of different physical processes using detailed binary evolution calculations

TL;DR

This study uses detailed binary evolution models to generate synthetic X-ray luminosity functions of high-mass X-ray binaries, revealing how various physical assumptions influence their shape and normalization, and comparing these to observations.

Contribution

It introduces a comprehensive binary population synthesis approach with detailed models to explore the effects of physical assumptions on XLFs, providing insights into binary evolution processes.

Findings

Synthetic XLFs show a break at ~10^38 erg/s, similar to observed XLFs.

Assumptions about supernova kicks, common-envelope physics, and transient behavior significantly affect XLF shape.

Overabundance of XRBs with black hole accretors in models compared to observations.

Abstract

The ever-expanding observational sample of X-ray binaries (XRBs) makes them excellent laboratories for constraining binary evolution theory. Such constraints can be obtained by studying the effects of various physical assumptions on synthetic X-ray luminosity functions (XLFs) and comparing to observed XLFs. In this work, we focus on high-mass XRBs (HMXBs) and study the effects on the XLF of various, poorly-constrained assumptions regarding physical processes such as the common-envelope phase, the core-collapse, and wind-fed accretion. We use the new binary population synthesis code POSYDON, which employs extensive pre-computed grids of detailed stellar structure and binary evolution models, to simulate the evolution of binaries. We generate 96 synthetic XRB populations corresponding to different combinations of model assumptions. The generated HMXB XLFs are feature-rich, deviating from the commonly assumed single-power law. We find a break in our synthetic XLF at luminosity $\sim 10^{38}$ erg s$^{-1}$, similar to observed XLFs. However, we find also a general overabundance of XRBs (up to a factor of $\sim$10 for certain model parameter combinations) driven primarily by XRBs with black hole accretors. Assumptions about the transient behavior of Be-XRBs, asymmetric supernova kicks, and common-envelope physics can significantly affect the shape and normalization of our synthetic XLFs. We find that less well-studied assumptions regarding the circularization of the orbit at the onset of Roche-lobe overflow and criteria for the formation of an X-ray emitting accretion disk around wind-accreting black holes can also impact our synthetic XLFs. Our study reveals the importance of large-scale parameter studies, highlighting the power of XRBs in constraining binary evolution theory.