The evolution of symbiotic X-ray binaries in the Galaxy is studied by the population synthesis method. We show that allowance for the nonstationarity of the regime of quasi-spherical subsonic...

A model of a population of ultraluminous X-ray sources with magnetized neutron stars (NULXs) in a spiral galaxy with a star formation history as in the thin disk of the Milky Way is constructed by the hybrid population synthesis method. First, based on analytical approximations (BSE code), we compute a set of close binary systems (CBSs), potential precursors of NULXs, and, then, the evolution with mass accretion onto magnetized neutron stars (NSs) is computed by the MESA evolutionary code. The accretion rate onto NSs and the X-ray luminosity of sources are calculated for the models of sub- or supercritical disks and disks with advection. During accretion onto magnetized NSs, super-Eddington luminosities \(L_{\textrm{X}}>10^{38}\) erg s\({}^{-1}\) are reached already at the subcritical stage, when the energy release at the inner boundary of the disk defined by the NS magnetosphere is sub-Eddington one. Our calculations show that the standard evolution of CBSs, given the peculiarities of accretion onto magnetized NSs, allows the observed properties of NULXs (X-ray luminosities, NS spin periods, CBS orbital periods, and optical component masses) to be quantitatively explained without requiring additional model assumptions about the collimation of X-ray emission from NSs with a high observed super-Eddington luminosity. In a model galaxy with a star formation rate \({\sim}5M_{\odot}\) yr\({}^{-1}\) there can be several NULXs. The detection of a powerful wind from NULXs with \(L_{\textrm{X}}\sim 10^{41}\) erg s\({}^{-1}\) may suggest supercritical accretion onto magnetized NSs.

HD 49798/RX J0648.0–4418 is a peculiar X-ray binary with a hot subdwarf (sdO) mass donor. The nature of the accreting compact object is not known, but its spin period P = 13.2 s and $$\dot{P} =-2.15 \times 10^{-15}$$s s⁻¹, prove that it can be only either a white dwarf or a neutron star. The spin-up has been very stable for more than 20 years. We demonstrate that the continuous stable spin-up of the compact companion of HD 49798 can be best explained by contraction of a young white dwarf with an age ∼2 Myrs. This allows us to interpret all the basic parameters of the system in the framework of an accreting white dwarf. We present examples of binary evolution which result in such systems. If correct, this is the first direct evidence for a white dwarf contraction on early evolutionary stages.

Context. Stellar evolution theory predicts the existence of helium-core remnants of the primary components of intermediate-mass close binaries that lost most of their hydrogen-helium envelopes due to the mass exchange. These remnants are expected to be observed as hot helium-rich stars with masses of (1−7) M_⊙, located in the area of the Hertzsprung-Russell diagram between OB subdwarfs and Wolf-Rayet stars. While several thousands of such stripped helium stars are expected to exist in the Galaxy, none of them have been identified so far. Aims. We aim to provide comprehensive predictions of the numbers and fundamental properties of stripped helium stars and their binary companions in the Galaxy. This is a necessary first step to guide observations, to enable a comparison between binary evolution models and realistic stellar populations, and to determine the feedback parameters of stripped helium stars in the Galaxy. Methods. We expanded the previously considered space of parameters describing close binary systems producing stripped helium stars and applied a population synthesis based on a grid of evolutionary models computed by the code MESA, using a spin-dependent algorithm for the treatment of mass and angular momentum loss from the system. Results. We show that the number of Galactic binaries hosting (1−7) M_⊙ He-stars is ≃20 000 and that it steeply declines with an increase in the He-star mass (≃3000 with mass ≳2 M_⊙). The decisive factor that defines the low number of stripped He-stars is runaway mass-loss after Roche lobe overflow (RLOF) by primary components of the binaries, which leads to the formation of common envelopes and the further merger of components. This effect strongly restricts the initial ranges of masses of components of the progenitors of stripped stars and orbital periods. In addition, stripped helium stars are much less numerous than expected, since a significant fraction of binaries in which the primaries have masses less than (5−7) M_⊙ produce subdwarfs with masses ≲1 M_⊙. Our calculations show that the overwhelming majority of helium stars reside in binaries with an early-type companion star and can be identified neither by the UV excess nor by emission features. The large periods of a significant fraction of binaries hosting stripped stars (≳several hundred days) also hamper their discovery.

We study the effect of the neutron star spin–kick velocity alignment observed in young radio pulsars on the coalescence rate of binary neutron stars.