Vortex Dynamics in the Neutron Star Inner Crust

Abstract We study the superfluid vortex motion in the neutron star inner crust through direct 3D simulations of the coupled dynamics of the vortex and the nuclear lattice. We demonstrate the pinning of an initially moving vortex to the lattice through the excitation of lattice vibrations and show that the efficiency of this process is higher for attractive than for repulsive nucleus–vortex interactions. We explore the unpinning of a vortex under the action of the applied Magnus force and find that it is influenced by multiple parameters, including the sign of the pinning force, the lattice orientation, composition, temperature, and the energy of the pinning to individual nuclei. In lattices with multiple grains, the unpinning transition is triggered inside the grains with weaker pinning, propagates along the vortex (mediated by the excited Kelvin waves), and crosses into grains with stronger pinning. This is likely to effectively decrease the critical force at which vortices unpin and to produce extended regions of unpinned vorticity. The shearing of the crust lattice (e.g., by a starquake) initiates the unpinning of the vortices that are crossing the slip plane. A close encounter of an unpinned vortex with a pinned vortex would cause the latter to unpin, perhaps initiating an unpinning avalanche of many vortices.