Axial quasi-normal modes of neutron stars: Accounting for the superfluid in the crust

TL;DR

This study investigates how superfluid components in neutron star crusts influence axial quasi-normal modes, revealing significant effects on oscillation spectra and emphasizing the importance of microphysical details for astrophysical seismology.

Contribution

First analysis of global oscillations in relativistic stars with elastic crusts and superfluid components, highlighting the impact of superfluid entrainment on mode spectra.

Findings

Crust torsional modes are unaffected by gravity coupling.

Superfluid entrainment significantly alters oscillation spectra.

Microphysical properties are crucial for neutron star seismology.

Abstract

We present the results of the first study of global oscillations of relativistic stars with both elastic crusts and interpenetrating superfluid components. For simplicity, we focus on the axial quasi-normal modes. Our results demonstrate that the torsional crust modes are essentially unaffected by the coupling to the gravitational field. This is as expected since these oscillations are known to be weak gravitational-wave sources. In contrast, the presence of a loosely coupled superfluid neutron component in the crust can have a significant effect on the oscillation spectrum. We show that the entrainment between the superfluid and the crust nuclei is a key parameter in the problem. Our analysis highlights the need for a more detailed understanding of the coupled crust-superfluid at the microphysical level. Our numerical results have, even though we have not considered magnetised stars, some relevance for efforts to carry out seismology based on quasi-periodic oscillations observed in the tails of magnetar flares. In particular, we argue that the sensitive dependence on the entrainment may have to be accounted for in attempts to match theoretical models to observational data.