Persistent crust-core spin lag in neutron stars

TL;DR

This paper demonstrates that certain magnetic field configurations in neutron stars prevent the crust and core from maintaining long-term corotation, leading to a persistent spin lag with implications for magnetic field evolution.

Contribution

It reveals that axisymmetric magnetic fields with closed lines, like the twisted torus, fail to enforce crust-core corotation, challenging previous assumptions about magnetic enforcement mechanisms.

Findings

Closed field line configurations lead to persistent spin lag.

Crust-core corotation may require early deposition of toroidal fields into the crust.

Implications for neutron star magnetic field evolution and stability.

Abstract

It is commonly believed that the magnetic field threading a neutron star provides the ultimate mechanism (on top of fluid viscosity) for enforcing long-term corotation between the slowly spun down solid crust and the liquid core. We show that this argument fails for axisymmetric magnetic fields with closed field lines in the core, the commonly used `twisted torus' field being the most prominent example. The failure of such magnetic fields to enforce global crust-core corotation leads to the development of a persistent spin lag between the core region occupied by the closed field lines and the rest of the crust and core. We discuss the repercussions of this spin lag for the evolution of the magnetic field, suggesting that, in order for a neutron star to settle to a stable state of crust-core corotation, the bulk of the toroidal field component should be deposited into the crust soon after the neutron star's birth.