Interpreting the AXP 1E 2259+586 antiglitch as a change in internal magnetization

TL;DR

This paper models the internal magnetic field decay in magnetars to explain the 2012 anti-glitch in AXP 1E 2259+586, revealing that complex magnetic configurations can produce larger anti-glitches with lower field decay.

Contribution

It introduces a self-consistent twisted torus magnetic field model in non-barotropic equilibrium, expanding understanding of magnetic field decay effects on anti-glitch phenomena.

Findings

Pure dipolar fields require larger toroidal decay for anti-glitch

Quadrupolar components lower the decay threshold needed

Predicted maximum anti-glitch sizes relate to ellipticity changes

Abstract

The sudden spin-down event ('anti-glitch') observed in AXP 1E 2259+586 on 2012 April 21 was arguably caused by a decay of its internal toroidal magnetic field component, which turns a stable prolate configuration into an unstable one. We refine previous models of this process by modelling the star's magnetic field self-consistently as a 'twisted torus' configuration in non-barotropic equilibrium (which allows us to explore a greater range of equilibrium configurations). It is shown that, if the star's magnetic field is purely dipolar, the change in the toroidal field strength required to produce an anti-glitch of the observed size can be ~ 10 times larger than previously calculated. If the star has a quadrupolar magnetic field component, then an anti-glitch of similar magnitude can be produced via a decay of the quadrupole component, in addition to a decay of the toroidal component. We show that, if the quadrupole component decays, the minimum initial toroidal field strength and the change in toroidal field strength needed to produce the observed anti-glitch are lower than in the pure dipole twisted torus. In addition, we predict the maximum anti-glitch sizes, assuming that they are caused by a change in ellipticity, in four glitching magnetars and discuss the implications for energetics of accompanying X-ray bursts.