Magnetically-driven crustquakes in neutron stars

TL;DR

This paper investigates how magnetic field decay induces stresses leading to crustquakes in neutron stars, potentially explaining phenomena like glitches and magnetar flares, with a focus on the role of magnetic field configurations and energy release.

Contribution

It derives a criterion for crust-breaking due to magnetic field evolution and analyzes strain patterns, highlighting the significance of toroidal fields and energy release mechanisms.

Findings

Crust is most prone to break if the magnetic field has a strong toroidal component.

Energy released in crustquakes is independent of magnetic field strength.

Magnetar flares could be powered solely by crustal energy release.

Abstract

Crustquake events may be connected with both rapid spin-up `glitches' within the regular slowdown of neutron stars, and high-energy magnetar flares. We argue that magnetic field decay builds up stresses in a neutron star's crust, as the elastic shear force resists the Lorentz force's desire to rearrange the global magnetic-field equilibrium. We derive a criterion for crust-breaking induced by a changing magnetic-field configuration, and use this to investigate strain patterns in a neutron star's crust for a variety of different magnetic-field models. Universally, we find that the crust is most liable to break if the magnetic field has a strong toroidal component, in which case the epicentre of the crustquake is around the equator. We calculate the energy released in a crustquake as a function of the fracture depth, finding that it is independent of field strength. Crust-breaking is, however, associated with a characteristic local field strength of $2.4\times 10^{14}$ G for a breaking strain of $0.001$, or $2.4\times 10^{15}$ G at a breaking strain of $0.1$. We find that even the most luminous magnetar giant flare could have been powered by crustal energy release alone.