Quasi-Spherical Subsonic Accretion onto Magnetized Neutron Stars

Abstract

A theory of quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is presented. In this regime, the accreted matter settles with subsonic velocities onto the rotating magnetosphere forming an extended quasi-spherical shell. The accretion rate in the shell is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability with account for cooling. This accretion regime may be established for moderate X-ray luminosities, corresponding to accretion rates $\dot M< \dot M^\dag \simeq 4\times 10^{16}$ g s⁻¹. For higher accretion rates a free-fall gap appears, due to strong Compton cooling of the flow above the magnetosphere, and accretion becomes highly non-stationary. Observations of spin-up and spin-down in equilibrium wind-fed X-ray pulsars with known orbital periods (like GX 301-2 and Vela X-1) enable the determination of the basic dimensionless model parameters and estimation of the neutron star magnetic field. In equilibrium pulsars with independently measured magnetic fields, the model enables the stellar wind velocity to be independently estimated. For non-equilibrium pulsars, there exists a maximum spin-down rate of the accreting neutron star. The model can also explain bright flares in Supergiant Fast X-ray Transients if stellar winds of the O-supergiant companions are magnetized.