Black hole spins in coalescing binary black holes

The possible formation mechanisms of massive close binary black holes (BHs) that can merge in the Hubble time to produce powerful gravitational wave bursts detected during advanced LIGO O1 and O2 science runs include the evolution from field low-metallicity massive binaries, the dynamical formation in dense stellar clusters and primordial BHs. Different formation channels produce different source distributions of total masses ${M_\mathrm{tot}}$ and effective spins $\chi_\mathrm{eff}$ of coalescing binary BHs. Using a modified \textsc{bse} code, we carry out extensive population synthesis calculations of the expected effective spin and total mass distributions from the standard field massive binary formation channel for different metallicities of BH progenitors (from zero-metal Population III stars up to solar metal abundance), different initial rotations of the binary components, stellar wind mass loss prescription, different BH formation models and a range of common envelope efficiencies. The stellar rotation is treated in two-zone (core-envelope) approximation using the effective core-envelope coupling time and with an account of the tidal synchronization of stellar envelope rotation during the binary system evolution. The results of our simulations, convolved with the metallicity-dependent star-formation history, show that the total masses and effective spins of the merging binary black holes detected during LIGO O1-O2 runs but the heaviest one (GW170729) can be simultaneously reproduced by the adopted BH formation models. Noticeable effective spin of GW170729 requires additional fallback from the rotating stellar envelope.Comment: 18 pages, 11 figures, accepted to MNRAS after taking into account star-formation rate history for comparison of the calculated BH-BH coalescences with observed systems, LIGO/Virgo GWTC-1 sources adde

Wind-accreting Symbiotic X-ray Binaries

We present a new model of the population of symbiotic X-ray binaries (SyXBs) that takes into account non-stationary character of quasi-spherical sub-sonic accretion of the red giant's stellar wind onto slowly rotating neutron stars. Updates of the earlier models are given, which include more strict criteria of slow NS rotation for plasma entry into the NS magnetosphere via Rayleigh-Taylor instability, as well as more strict conditions for settling accretion for slow stellar winds, with an account of variations in the specific angular momentum of captured stellar wind in eccentric binaries. These modifications enabled a more adequate description of the distributions of observed systems over binary orbital periods, NS spin periods and their X-ray luminosity in the $\sim 10^{32}-10^{36}$~erg s$^{-1}$ range and brought their model Galactic number into reasonable agreement with the observed one. Reconciliation of the model and observed orbital periods of SyXBs requires a low efficiency of matter expulsion from common envelopes during the evolution that results in the formation of NS-components of symbiotic X-ray systems.Comment: 11 pages, 4 figures, accepted in MNRA

Gravitational wave background from coalescing compact stars in eccentric orbits

Stochastic gravitational wave background produced by a stationary coalescing population of binary neutron stars in the Galaxy is calculated. This background is found to constitute a confusion limit within the LISA frequency band up to a limiting frequency \NUlim{}\sim 10^{-3} Hz, leaving the frequency window $\sim 10^{-3}$--$10^{-2}$ Hz open for the potential detection of cosmological stochastic gravitational waves and of signals involving massive black holes out to cosmological distances.Comment: 6 pages, 7 figure