Tkachenko modes as sources of quasiperiodic pulsar spin variations

TL;DR

This paper investigates Tkachenko waves in neutron star interiors, showing their potential to explain long-term pulsar spin variations through their damping properties and frequencies.

Contribution

It introduces a detailed analysis of Tkachenko modes considering mutual friction and viscosity, linking these modes to observed pulsar timing variations.

Findings

Tkachenko modes can be weakly damped with small superfluid-normal fluid coupling.

Oscillation frequencies of these modes match observed pulsar spin variation periods.

Mode damping depends on shear viscosity and coupling strength.

Abstract

We study the long wavelength shear modes (Tkachenko waves) of triangular lattices of singly quantized vortices in neutron star interiors taking into account the mutual friction between the superfluid and the normal fluid and the shear viscosity of the normal fluid. The set of Tkachenko modes that propagate in the plane orthogonal to the spin vector are weakly damped if the coupling between the superfluid and normal fluid is small. In strong coupling, their oscillation frequencies are lower and are undamped for small and moderate shear viscosities. The periods of these modes are consistent with the observed ~100-1000 day variations in spin of PSR 1828-11.