The inner structure of an accretion disc around a magnetic neutron star

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Abstract
The effect of a magnetic binary neutron star on the inner structure of its accretion disc is considered. The stellar field penetrates the disc and toroidal field is created as a result of the vertical shearing motions, its magnitude being diffusion limited. If the disc’s magnetic diffusivity, ν. is treated as a function of ω, the distance from the stellar centre, with a radial length scale of ∼ω w, a self-consistent solution can be found for the inner region. In the orbital plane, the magnetic force has a small effect on the radial momentum, so the angular velocity is Keplerian. The vertical equilibrium is a balance between the gradients in azimuthal magnetic pressure and thermal pressure, while the inflow is caused by the transfer of angular momentum from the disc to the star, via magnetic stresses. Vertical equilibrium breaks down as the inflow speed becomes a significant fraction of the Keplerian speed, this occurring in the region of the Alfven radius based on radial free-fall. However, if ν is prescribed by a simple model of turbulence, or has a magnetic buoyancy origin, the disc becomes rapidly disrupted vertically where the azimuthal magnetic force starts to exceed that due to viscosity. This occurs at a distance from the star significantly larger than the free-fall Alfvén radius.

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