Pinned vortex hopping in a neutron star crust

TL;DR

This paper studies how superfluid vortices in neutron star crusts move and unpin, suggesting that vortex avalanches could explain pulsar glitches, with motion behavior depending on crust density and stellar parameters.

Contribution

It provides a detailed analysis of vortex scattering and mean free path, supporting the idea that vortex avalanches can trigger pulsar glitches, and explores how stellar parameters influence vortex motion regimes.

Findings

Vortex unpinning can lead to avalanches or creep depending on parameters.

Avalanches are more likely in higher density crust regions.

Stellar parameters may switch vortex motion between creep and avalanche regimes.

Abstract

The motion of superfluid vortices in a neutron star crust is at the heart of most theories of pulsar glitches. Pinning of vortices to ions can decouple the superfluid from the crust and create a reservoir of angular momentum. Sudden large scale unpinning can lead to an observable glitch. In this paper we investigate the scattering of a free vortex off a pinning potential and calculate its mean free path, in order to assess whether unpinned vortices can skip multiple pinning sites and come close enough to their neighbours to trigger avalanches, or whether they simply hop from one pinning site to another giving rise to a more gradual creep. We find that there is a significant range of parameter space in which avalanches can be triggered, thus supporting the hypothesis that they may lie at the origin of pulsar glitches. For realistic values of the pinning force and superfluid drag parameters we find that avalanches are more likely in the higher density regions of the crust where pinning is stronger. Physical differences in stellar parameters, such as mass and temperature, may lead to a switch between creep-like motion and avalanches, explaining the different characteristics of glitching pulsars.