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A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

Strong magnetic field of accreting neutron stars (1014 G) is hard to probe by Xray spectroscopy but can be indirectly inferred from spin-up/spin-down measurement in X-ray pulsars. The existing observations of slowly rotating X-ray pulsars are discussed. It is shown that magnetic fields of neutron stars derived from these observations (or lower limits in some cases) fall within the standard 1012-1013 G range. Claims about the evidence for accreting magnetars are critically discussed in the light of recent progress in understanding of accretion onto slowly rotating neutron stars in the subsonic regime.

Observations of X-ray binaries indicate substantial half-thicknesses for the accretion disks in these systems (up to h/R ≈ 0.25, where h is the disk half-thickness and R its radius), while standard α accretion disks predict appreciably smaller half-thicknesses. We study the theoretical vertical structure of such disks using two independent numerical methods, and show that their maximum half-thicknesses in the subcritical regime cannot exceed h/R ≈ 0.1. We consider various reasons for the apparent increase in the disk thickness, the most probable of which is the presence of matter above the disk in the form of a hot corona that scatters hard radiation from the central source and inner parts of the disk. As a result, the observed thickness of the disk and the illumination of its outer parts effectively increase. This mechanism can also explain both the optical-to-X-ray flux ratio in these systems and the observed parameters of eclipsing X-ray binaries. [ABSTRACT FROM AUTHOR]

Explores some general physical relationships that must remain valid for precise calculations of the boundary layer (BL) structure. Differential equations included in the basic equations considered for the BL structure; Estimation on the turbulence in the BL; Comparison on the characteristics of turbulence in accretion disks and BL.

Proposes a model for the X-ray dips of the X-ray light curves of Her X-1. Origin of the pre-eclipse and type I anomalous dips; Mechanism underlying type II anomalous dips and post-eclipse recoveries.

Proposes a method of calculating the vertical structure of optically thick accretion discs. Search for the eigenvalues of the dimensionless parameters for a set of four ordinary differential equations with definite boundary conditions; Demonstration that the vertical structure of optically thick accretion disks can be satisfactorily described by polytropic models.