Continuous gravitational wave emission from neutron stars with pinned superfluids in the core

TL;DR

This paper explores how pinned superfluid vortices in neutron star cores can generate detectable continuous gravitational waves, potentially dominating the spin-down process in millisecond pulsars.

Contribution

It introduces a model for gravitational wave emission caused by superfluid vortex pinning in neutron star cores, highlighting its significance in neutron star spin-down.

Findings

Mass quadrupole from pinning can be significant for gravitational wave emission.

Pinned superfluid vortices can sustain non-axisymmetric configurations.

Gravitational waves may dominate the spin-down torque in millisecond pulsars.

Abstract

We investigate the effect of a pinned superfluid component on the gravitational wave emission of a rotating neutron star. Pinning of superfluid vortices to the flux-tubes in the outer core (where the protons are likely to form a type-II superconductor) is a possible mechanism to sustain long-lived and non-axisymmetric neutron currents in the interior, that break the axial symmetry of the unperturbed hydrostatic configuration. We consider pinning-induced perturbations to a stationary corotating configuration, and determine upper limits on the strength of gravitational wave emission due to the pinning of vortices with a strong toroidal magnetic field of the kind predicted by recent magneto-hydrodynamic simulations of neutron star interiors. We estimate the contributions to gravitational wave emission from both the mass and current multipole generated by the pinned vorticity in the outer core, and find that the mass quadrupole can be large enough for gravitational waves to provide the dominant spindown torque in millisecond pulsars.