Population Synthesis of Black Hole X-Ray Binaries

TL;DR

This study models the population and evolution of black hole X-ray binaries, comparing predictions with observations to estimate their numbers and characteristics in the Milky Way and similar galaxies.

Contribution

It combines binary population synthesis and detailed evolution calculations to predict the properties and numbers of BH XRBs, providing insights into their formation and observational signatures.

Findings

Wind-fed XRBs are very rare, with fewer than 100 expected.

Most RLO XRBs are low-mass systems, with hundreds in the Milky Way.

Approximately a dozen ULXs with BHs may exist in a Milky Way-like galaxy.

Abstract

We present a systematic study of the X-ray binaries (XRBs) containing a black hole (BH) and a non-degenerate companion, in which mass transfer takes place via either capturing the companion's wind or Roche lobe overflow (RLO). As shown in our previous work that focusing on the formation and evolution of detached BH binaries, our assumed models relevant to BH's progenitors predicted significantly different binary properties \citep{sl19}. In this paper, we further follow the evolutionary paths of the BH systems that appearing as XRBs. By use of both binary population synthesis and detailed binary evolution calculations, we can obtain the potential population of BH XRBs. Distributions at the current epoch of various binary parameters have been computed. The observed sample of wind-fed XRBs can be well reproduced under assumption of all our models. The wind-fed XRBs are expected to be so rare ($ \lesssim 100 $) that only a couple of such systems have been detected. Comparison of known RLO XRBs with the calculated distributions of various binary parameters indicates that only the models assuming relatively small masses for BH progenitors can roughly match the observations. Accordingly we estimate that there are hundreds of RLO XRBs in the Milky Way, of which the majority are low-mass XRBs. The RLO systems may become ultraluminous X-ray sources (ULXs) if the BH accretes at a very high rate, we expect that about a dozen ULXs with a BH accretor may exist in a Milky Way$-$like galaxy.