Disc structure and angular momentum transfer in X-ray binary pulsars

Abstract

Abstract The angular momentum exchange between a binary magnetic neutron star and its accretion disc is considered. In a previous paper it was shown that if the disc's magnetic diffusivity,η, is due to buoyancy or turbulence, disruption occurs where the magnetic extraction of angular momentum starts to dominate that due to viscous advection. The disruption radius is significantly larger than one half the spherical Alfvén radius, often previously used for the disc's inner edge. The consequences of the increased hole radius are investigated in the present paper, the solutions for the two forms of η being compared. The magnetic and accretion torques acting on the neutron star are calculated, and expressions are found for the equilibrium period at which they cancel. Expressions are also derived for the observed quantity |[Pdot]|/P, these being related to the disruption radius and stellar spin rate. It is shown that the azimuthal viscous and magnetic forces can become comparable in a small region beyond the corotation radius, as P approaches P eq. The consequent local breakdown of the disc might explain the erratic period behaviour observed in some X-ray binary pulsars. The results are shown not to depend strongly on uncertainties in the magnitude of the vertical shear.