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

¹ Institut d'Astrophysique de Paris, UMR 7095 CNRS, UPMC Univ Paris 06, 98bis Bd Arago, 75014 Paris, France
² Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatniskaya Str., 119017 Moscow, Russia e-mail: lry@inasan.ru
³ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

Received: 29 February 2008
Accepted: 30 May 2008

Abstract

We consider evolutionary models for the population of short-period (P_orb h) 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 () with a hydrogen abundance in the center at the Roche-lobe overflow (RLOF). Longer period (P_orb day) SXTs might descend from systems with initial donor masses of about 1  and . 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.