The Formation, Evolution and Parameters of Short-Period Low-Mass X-Ray Binaries with Black-Hole Components

TL;DR

This paper models the formation and evolution of short-period low-mass black-hole binaries, explaining their observational properties and the importance of angular momentum loss and common envelope ejection efficiency.

Contribution

It introduces a model that accounts for reduced angular momentum loss and disc truncation, aligning with observed properties of short-period LMBHBs and their transients.

Findings

Model reproduces observed secondary star temperatures and masses.

Mass-transfer rates match observations if discs are truncated.

High efficiency of common envelope ejection is required.

Abstract

We discuss the formation, evolution and observational parameters of the population of short-period ($<10$ hr) low-mass black-hole binaries (LMBHB). Their evolution is determined by the orbital angular momentum loss and/or nuclear evolution of the donors. All observed semidetached LMBHB are observed as soft X-ray transients (SXTs). The absence of observed short-period stable luminous X-ray sources with black holes and low-mass optical components suggests that upon RLOF by the donor, the angular-momentum losses are substantially reduced. The model with reduced angular-momentum loss reasonably well reproduces the masses and effective temperatures of the observed secondaries of SXTs. Theoretical mass-transfer rates in SXTs are consistent with those deduced from observations only if the accretion discs in LMBHB are truncated. The population of short-period LMBHB is formed mainly by systems which at RLOF had unevolved or slightly evolved donors (abundance of hydrogen in the center $> 0.35$). Our models suggest that a very high efficiency of common envelopes ejection is necessary to form LMBHB.