Implications of magnetar non-precession

TL;DR

This paper explores the internal properties of magnetars by analyzing their rotational behavior and lack of precession, suggesting either extreme crustal stresses or the presence of a pinned superfluid component.

Contribution

It provides a novel interpretation linking magnetar precession observations to their internal structure, proposing two possible states of their interior.

Findings

Magnetars show no evidence of free precession despite complex magnetic geometries.

Either magnetar crusts are near their elastic yield limit or a pinned superfluid exists inside.

Stronger magnetic field magnetars would be nearly aligned or orthogonal rotators if no superfluid is pinned.

Abstract

The objects known as anomalous X-ray pulsars and soft gamma repeaters are commonly identified with magnetars, neutron stars with ultrastrong magnetic fields. The rotational history of these objects has, so far, revealed no evidence of free precession. At the same time these objects do not generally appear to have magnetic axes nearly parallel or orthogonal to their spin axes. In this paper we show that the combination of these two observations, together with simple rigid-body dynamics, leads to non-trivial predictions about the interior properties of magnetars: either (i) elastic stresses in magnetar crusts are close to the theoretical upper limit above which the crustal matter yields or (ii) there is a "pinned" superfluid component in the magnetar interior. As a potentially observable consequence of these ideas we point out that, in the case of no pinned superfluidity, magnetars of stronger magnetic field strength than those currently observed would have to be nearly aligned/orthogonal rotators.