Instability of Superfluid Flow in the Neutron Star Core

TL;DR

This paper investigates the hydrodynamic stability of superfluid vortices in neutron star cores, revealing potential turbulence that may explain observed timing noise in neutron stars.

Contribution

It introduces a stability analysis of vortex creep in neutron star superfluids, showing conditions for turbulence onset and its possible observational implications.

Findings

Vortex creep induces unstable low-frequency modes.

Superfluid flow becomes unstable at wavelengths less than 10 meters.

Turbulence may account for neutron star timing noise.

Abstract

Pinning of superfluid vortices to magnetic flux tubes in the outer core of a neutron star supports a velocity difference of $\sim 10^5$ \cms\ between the neutron superfluid and the proton-electron fluid as the star spins down. Under the Magnus force that arises on the vortex array, vortices undergo {\em vortex creep} through thermal activation or quantum tunneling. We examine the hydrodynamic stability of this situation. Vortex creep introduces two low-frequency modes, one of which is unstable above a critical wavenumber for any non-zero flow velocity of the neutron superfluid with respect to the charged fluid. For typical pinning parameters of the outer core, the superfluid flow is unstable over wavelengths $\lambda\lap 10$ m and over timescales of $\sim (\lambda/{1 m})^{1/2}$ yr down to $\sim 1$ d. The vortex lattice could degenerate into a tangle, and the superfluid flow would become turbulent. We suggest that superfluid turbulence could be responsible for the red timing noise seen in many neutron stars, and find a predicted spectrum that is generally consistent with observations.