Fast fossil rotation of neutron star cores

TL;DR

This paper proposes that neutron star cores can super-rotate relative to their crusts for thousands of years after birth, affecting magnetic and gravitational wave phenomena.

Contribution

It introduces the idea that stratification prevents core response to braking torque, leading to fossilized super-rotation in young neutron stars.

Findings

Core super-rotation can persist for over 1000 years.

Fossil core rotation impacts magnetic field generation.

Enhanced gravitational wave emission due to super-rotation.

Abstract

It is argued that the superfluid core of a neutron star super-rotates relative to the crust, because stratification prevents the core from responding to the electromagnetic braking torque, until the relevant dissipative (viscous or Eddington-Sweet) time-scale, which can exceed ~ 10^3 yr and is much longer than the Ekman timescale, has elapsed. Hence, in some young pulsars, the rotation of the core today is a fossil record of its rotation at birth, provided that magnetic crust-core coupling is inhibited, e.g. by buoyancy, field-line topology, or the presence of uncondensed neutral components in the superfluid. Persistent core super-rotation alters our picture of neutron stars in several ways, allowing for magnetic field generation by ongoing dynamo action and enhanced gravitational wave emission from hydrodynamic instabilities.