Quasi-spherical accretion in X-ray pulsars

Quasi-spherical accretion in wind-fed X-ray pulsars is discussed. At X-ray luminosities < 4� 10 36 erg/s, a hot convective shell is formed around the neutron star magnetosphere, and subsonic settling accretion regime sets in. In this regime, accretio n rate onto neutron star is determined by the ability of plasma to enter magnetosphere via Rayleigh-Taylor instability. A gas-dynamic theory of settling accretion is constructed taking into acc ount anisotropic turbulence. The angular momentum can be transferred through the quasi-static shell via large-scale convective motions initiating turbulence cascade. The angular velocity distr ibution in the shell is found depending on the turbulent viscosity prescription. Comparison with observations of long-period X-ray wind-fed pulsars shows that an almost iso-angular-momentum distribution is most likely realized in their shells. The theory explains long-term spin-down in wind- fed accreting pulsars (e.g. GX 1+4) and properties of short-term torque-luminosity correlati ons. The theory can be applied to slowly rotating low-luminosity X-ray pulsars and non-stationary accretion phenomena observed in some SFXTs.