On the magnetic braking law in black hole low-mass X-ray binaries

TL;DR

This study investigates the role of magnetic braking in the evolution of black hole low-mass X-ray binaries, using simulations to compare theoretical models with observed properties and exploring implications for ultracompact binary formation.

Contribution

It combines binary population synthesis with detailed evolution to evaluate magnetic braking laws in BH LMXBs, revealing differences from neutron star systems and explaining the scarcity of BH ultracompact X-ray binaries.

Findings

Magnetic braking laws with low efficiency match observations better.

Only a small fraction of BH LMXBs can evolve into ultracompact binaries.

No black hole ultracompact X-ray binaries have been observed so far.

Abstract

Magnetic braking (MB) plays an important role in the evolution of close low-mass X-ray binaries (LMXBs). It is also essential to the formation of ultracompact X-ray binaries (UCXBs). There have been lively investigations on the MB mechanism(s) in both single stars and close binaries including cataclysmic variables and neutron star (NS) LMXBs, but with diverse conclusions. In this paper we explore the effect of MB on the black hole (BH) LMXB evolution. We combine binary population synthesis with detailed binary evolution to obtain the expected properties of Galactic BH LMXB population. The simulated results are compared with the observational data including the BH mass, companion mass, companion temperature, orbital period, and mean accretion rate. Our results reveal that the MB laws with relatively low efficiency (i.e., RM12 and RVJ83) exhibit better agreement with observations, contrary to what was found for NS LMXBs. This raises the interesting question about whether MB really follows the same unified law in different types of binaries. We also predict that only a very small fraction ($\lesssim 2.5\%$) of BH LMXBs can evolve to be UCXBs. This explains why there is no BH UCXB discovered by far.