Pulsar anti-glitches: starquakes driven by magnetism?

Abstract

In the conventional starquake model of pulsar glitches, it is usually assumed that such events arise from fault slip induced by the self-gravity of compact objects. This inevitably decreases the moment of inertia, producing a glitch with an amplitude of only $\Delta\nu/\nu > 0$. However, an increasing number of anti-glitches ($\Delta\nu/\nu < 0$) have been observed in extremely magnetized pulsars, the magnetars, and this cannot be explained by that framework. In the present study, we hypothesis that magnetic stresses within a compact object can make for elastic deformations that trigger fault slipping, resulting in a ``magnetism-driven starquake'' when the local breaking threshold is exceeded. This process can then either decrease or increase the moment of inertia, naturally generating a glitch or an anti-glitch, respectively. With an order-of-magnitude calculation in this brief report, we present a simple relationship between the magnetic field $B$ and the amplitude $\Delta\nu/\nu$, which is consistent with the observational distribution of existing glitch and anti-glitch data. Further discoveries of glitch/anti-glitch events, alongside more quantitative models of elastic-magnetic stress coupling, would be welcome and could eventually provide clear tests for the hypothesis.