Evolutionary models of short-period soft X-ray transients: comparison with observations

TL;DR

This paper models the evolution of short-period low-mass black-hole binaries and compares predictions with observations, highlighting the importance of magnetic braking, disc truncation, and initial donor properties.

Contribution

It introduces evolutionary models with reduced magnetic braking and disc truncation, explaining observed properties of short-period SXTs without requiring hydrogen-depleted donors.

Findings

Models match observed donor masses and temperatures.

Mass-transfer rates align with observations assuming disc truncation.

Short-period SXTs originate from systems with unevolved or slightly evolved donors.

Abstract

We consider evolutionary models for the population of short-period (<10 hr) low-mass black-hole binaries (LMBHBs) and compare them with observations of soft X-ray transients (SXTs). We show that assuming strongly reduced magnetic braking (as suggested by us before for low-mass semidetached binaries) the calculated masses and effective temperatures of secondaries are encouragingly close to the observed masses and effective temperatures (as inferred from their spectra) of donor stars in short-period LMBHBs. Theoretical mass-transfer rates in SXTs are consistent with the observed ones if one assumes that accretion discs in these systems are truncated (``leaky''). We find that the population of short-period SXTs is formed mainly by systems which had unevolved or slightly evolved main-sequence donors ($M_2 \lesssim 1.2M_\odot$) with a hydrogen abundance in the center $X_c > 0.35$ at the Roche-lobe overflow (RLOF). Longer period (0.5 - 1) day) SXTs might descend from systems with initial donor masses of about 1 $M_\odot$ and $X_c < 0.35$. Thus, one can explain the origin of short period LMBHB without invoking donors with cores almost totally depleted of hydrogen. Our models suggest that, unless the currently accepted empirical estimates of mass-loss rates by winds for massive O-stars and Wolf-Rayet stars are significantly over-evaluated, a very high efficiency of common-envelope ejection is necessary to form short-period LMBHBs.