Accretion onto magnetized neutron stars - The fate of sinking filaments

Abstract

An analysis is presented of plasma dynamics inside the magnetosphere of an accreting, magnetized neutron star, under the assumption of entry of spherically accreting plasma through hydromagnetic interchange flows (the 'Rayleigh-Taylor' instability). It is suggested that the falling blobs of diamagnetic plasma can be disrupted by the hydromagnetic Kelvin-Helmholtz instability with consequent diffusion of plasma onto field lines and flow onto polar caps. The inner radius at which the blobs are fully disrupted defines a plasmapause; high-density plasma exists between the plasmapause and the magnetopause. The dipole field line whose equatorial radius equals the plasmapause radius defines the outer edge of the polar caps, whose area increases with decreasing accretion rate. It is shown that in low luminosity sources, the polar cap area may approach the surface area of the star. The dependence of effective temperature on bolometric luminosity is estimated, and the possible relevance of the model to X-ray burst sources is discussed.