THE RADIAL DISC STRUCTURE AROUND A MAGNETIC NEUTRON-STAR - ANALYTIC AND SEMIANALYTIC SOLUTIONS

The radial structure of a thin accretion disc is calculated in the pre sence of a central dipole magnetic field aligned with the rotation axi s. The problem is treated using a modified expression for the turbulen t magnetic diffusion, which allows the angular momentum equation to be integrated analytically. The governing algebraic equations are solved iteratively between 1 and 10(4) stellar radii. An analytic approximat ion is provided that is valid near the disruption radius at about 100 stellar radii. At that point, which is approximately 60 per cent of th e Alfven radius and typically about 30 per cent of the corotation radi us, the disc becomes viscously unstable. This instability results from the fact that both radiation pressure and opacity caused by electron scattering become important. This in turn is a consequence of the magn etic field which leads to an enhanced temperature in the inner parts. This is because the magnetic field gives rise to a strongly enhanced v ertically integrated viscosity, so that the viscous torque can balance the magnetic torque.