Thin accretion discs around millisecond X-ray pulsars

TL;DR

This paper models the structure of thin accretion discs around millisecond X-ray pulsars, highlighting the impact of internal dynamos on torque and spin variations.

Contribution

It introduces a self-similar model of accretion discs with internal dynamos around millisecond pulsars, showing enhanced torques compared to non-dynamo models.

Findings

Models with dynamos produce stronger torques.

Discs can extend closer to the neutron star.

Explains observed spin variations in millisecond pulsars.

Abstract

Millisecond x-ray pulsars have weak magnetic dipole moments of $\sim 10^{16}$\,T\,m$^3$ compared to ordinary X-ray pulsars with dipole moments of $10^{20}$\,T\,m$^3$. For this reason a surrounding accretion disc can extend closer to the neutron star, 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.