Electromagnetic Torques, Precession and Evolution of Magnetic Inclination of Pulsars

TL;DR

This paper analytically models electromagnetic torques on neutron stars, including near-zone effects, and explores how these torques can cause pulsar precession and magnetic inclination evolution, explaining recent observational phenomena.

Contribution

It introduces a comprehensive analytical framework for electromagnetic torques, including inertial effects, and applies it to pulsar precession and inclination evolution.

Findings

Precession can explain magnetic inclination counter-alignment in pulsars.

Magnetic inclination angle decreases over the pulsar's spindown timescale.

Near-zone torques modify the star's effective moment of inertia.

Abstract

We present analytic calculations of the electromagnetic torques acting on a magnetic neutron star rotating in vacuum, including near-zone torques associated with the inertia of dipole and quadrupole magnetic fields. We incorporate these torques into the rotational dynamics of a rigid-body neutron star, and show that the effects of the inertial torque can be understood as a modification of the moment of inertia tensor of the star. We apply our rotational dynamics equation to the Crab pulsar, including intrinsic distortions of the star and various electromagnetic torques, to investigate the possibility that the counter-alignment of the magnetic inclination angle, as suggested by recent observations, could be explained by pulsar precession. We find that if the effective principal axis of the pulsar is nearly aligned with either the magnetic dipole axis or the rotation axis, then precession may account for the observed counter-alignment over decade timescales. Over the spindown timescale of the pulsar, the magnetic inclination angle always decreases.