Thin accretion discs around millisecond X-ray pulsars

Abstract

Millisecond X-ray pulsars have weak magnetic dipole moments of ∼1016 T m3 compared to ordinary X-ray pulsars with dipole moments of 1020 T m3. For this reason a surrounding accretion disc can extend closer to the millisecond X-ray pulsars, and thus reach a higher temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong. We compute the self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal dynamo. Such models produce significantly stronger torques on the neutron star than models without dynamos, and can explain the strong spin variations in some millisecond X-ray pulsars.