Vortex Pinning in Neutron Stars, Slip-stick Dynamics, and the Origin of Spin Glitches

TL;DR

This paper uses 3D simulations to investigate vortex pinning and unpinning in neutron star crusts, revealing mechanisms behind observed spin glitches and the complex dynamics of vortex motion.

Contribution

It introduces detailed dynamical simulations of vortex pinning in neutron stars, highlighting the two-state vortex motion and its role in spin glitches, with insights into vortex tangling and Kelvin wave excitation.

Findings

Pinning forces are sufficient to explain observed glitches.

Vortices exhibit slip-stick dynamics with two stable states.

Vortex tangling may occur due to repeated unpinning and repinning.

Abstract

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using 3-dimensional dynamical simulations. Strong pinning occurs for certain lattice orientations of an idealized, body-centered cubic lattice, and occurs generally in an amorphous or impure nuclear lattice. The pinning force per unit length is $\sim 10^{16}$ dyn cm$^{-1}$ for a vortex-nucleus interaction that is repulsive, and $\sim 10^{17}$ dyn cm$^{-1}$ for an attractive interaction. The pinning force is strong enough to account for observed spin jumps (glitches). Vortices forced through the lattice move with a slip-stick character; for a range of superfluid velocities, the vortex can be in either a cold, pinned state or a hot unpinned state, with strong excitation of Kelvin waves on the vortex. This two-state nature of vortex motion sets the stage for large-scale vortex movement that creates an observable spin glitch. We argue that the vortex array is likely to become tangled as a result of repeated unpinnings and repinnings. We conjecture that during a glitch, the Kelvin-wave excitation spreads rapidly along the direction of the mean superfluid vorticity and slower in the direction perpendicular to it, akin to an anisotropic deflagration.