Vortex Pinning in Neutron Stars, Slipstick Dynamics, and the Origin of Spin Glitches

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using three-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 similar to 10(16) dyn cm(-1) for a vortex-nucleus interaction that is repulsive and similar to 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 slipstick 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.

Automated summary

The study investigates the pinning and unpinning of superfluid vortices in the inner crust of a neutron star using three-dimensional simulations. It is found that certain lattice orientations lead to strong pinning, and this is generally observed in an amorphous or impure nuclear lattice. The pinning force is strong enough to explain observed glitches, and the two-state nature of vortex motion sets the stage for large-scale vortex movement. The vortex array is likely to become tangled due to repeated unpinning and repinning.