Spread of matter over a neutron-star surface during disk accretion: Deceleration of rapid rotation

Abstract

The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of the Eddington one is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4–0.5 of the speed of light above the base of the spreading layer-a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to spin-up of the massive zone that extends to the “ocean“ made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This picture should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mechanism. Obviously, a more efficient mechanism for the dissipation of a strong azimuthal flow in the atmosphere should operate in X-ray bursters. We show that a giant solitary gravity wave in the atmosphere can lead to energy dissipation and to a sharp, abrupt decrease in azimuthal velocity in fairly rarefied atmospheric layers above the zone of explosive helium burning nuclear reactions. We discuss the reasons why this wave that has no direct analog in the Earth’s atmosphere and ocean appears and its stability. We pose the question as to whether neutron stars with massive atmospheres spun up to high velocities by accreting matter from a disk can exist among the observed Galactic X-ray sources.