The radial disc structure around a magnetic neutron star : analytic and semi -

The radial structure of a thin accretion disc is calculated in the presence of a central dipole magnetic eld aligned with the rotation axis. The problem is treated using a modiied expression for the turbulent magnetic diiusion, which allows the angular momentum equation to be integrated analytically. The governing algebraic equations are solved iteratively between 1 ? 10 4 stellar radii. An analytic approximation is provided that is valid near the disruption radius at about one hundred stellar radii. At that point, which is approximately 60% of the Alfv en radius and typically about 30% of the corotation radius, the disc becomes viscously unstable. This instability results from the fact that both radiation pressure and opacity due to electron scattering become important. This in turn is a consequence of the magnetic eld which leads to an enhanced temperature in the inner parts. This is because the magnetic eld gives rise to a strongly enhanced vertically integrated viscosity, so that the viscous torque can balance the magnetic torque.