Axisymmetric magneto-plastic evolution of neutron-star crusts

TL;DR

This paper investigates how neutron-star crusts evolve magnetically when they reach failure, revealing complex plastic flow effects that influence magnetic field behavior and potentially explain magnetar bursts.

Contribution

It introduces a model of magnetic field evolution in neutron-star crusts that includes plastic flow beyond crustal failure, extending previous theories.

Findings

Plastic flow can enhance the Hall effect in magnetic evolution.

Differences in magnetic behavior are more pronounced in the toroidal field.

Crustal failure impacts magnetar burst phenomena and spindown evolution.

Abstract

Magnetic field evolution in neutron-star crusts is driven by the Hall effect and Ohmic dissipation, for as long as the crust is sufficiently strong to absorb Maxwell stresses exerted by the field and thus make the momentum equation redundant. For the strongest neutron-star fields, however, stresses build to the point of crustal failure, at which point the standard evolution equations are no longer valid. Here, we study the evolution of the magnetic field of the crust up to and beyond crustal failure, whence the crust begins to flow plastically. We perform global axisymmetric evolutions, exploring different types of failure affecting a limited region of the crust. We find that a plastic flow does not simply suppress the Hall effect even in the regime of a low plastic viscosity, but it rather leads to non-trivial evolution -- in some cases even overreacting and enhancing the impact of the Hall effect. Its impact is more pronouced in the toroidal field, with the differences on the poloidal field being less substantial. We argue that both the nature of magnetar bursts and their spindown evolution will be affected by plastic flow, so that observations of these phenomena may help to constrain the way the crust fails.