Hydrodynamics of rapidly rotating superfluid neutron stars with mutual friction

TL;DR

This paper investigates the oscillation spectrum, mutual friction effects, and hydrodynamical spin-up processes in rapidly rotating superfluid neutron stars using a linearized two-fluid model without Cowling approximation.

Contribution

It provides a detailed analysis of neutron star oscillations, mutual friction damping, and glitch dynamics with stratified and non-stratified models, including gravitational potential effects.

Findings

Identification of oscillation mode spectra including gravitational perturbations.

Quantification of mutual friction damping effects on superfluid modes.

Methodology for simulating pre-glitch states and gravitational-wave extraction.

Abstract

We study time evolutions of superfluid neutron stars, focussing on the nature of the oscillation spectrum, the effect of mutual friction force on the oscillations and the hydrodynamical spin-up phase of pulsar glitches. We linearise the dynamical equations of a Newtonian two-fluid model for rapidly rotating backgrounds. In the axisymmetric equilibrium configurations, the two fluid components corotate and are in beta-equilibrium. We use analytical equations of state that generate stratified and non-stratified stellar models, which enable us to study the coupling between the dynamical degrees of freedom of the system. By means of time evolutions of the linearised dynamical equations, we determine the spectrum of axisymmetric and non-axisymmetric oscillation modes, accounting for the contribution of the gravitational potential perturbations, i.e. without adopting the Cowling approximation. We study the mutual friction damping of the superfluid oscillations and consider the effects of the non-dissipative part of the mutual friction force on the mode frequencies. We also provide technical details and relevant tests for the hydrodynamical model of pulsar glitches discussed by Sidery, Passamonti and Andersson (2010). In particular, we describe the method used to generate the initial data that mimic the pre-glitch state, and derive the equations that are used to extract the gravitational-wave signal.