Non-linear viscous saturation of r-modes

TL;DR

This paper introduces a new non-linear bulk viscosity mechanism that can saturate r-modes in neutron stars, leading to efficient spin-down by damping oscillations and emitting gravitational waves.

Contribution

It proposes a novel non-linear bulk viscosity mechanism that enhances damping of r-modes, providing a new way to explain neutron star spin evolution.

Findings

Enhanced damping due to non-linear bulk viscosity can saturate r-modes.

Saturation occurs at amplitudes suitable for efficient spin-down.

Implications for gravitational wave emission from neutron stars.

Abstract

Pulsar spin frequencies and their time evolution are an important source of information on compact stars and their internal composition. Oscillations of the star can reduce the rotational energy via the emission of gravitational waves. In particular unstable oscillation modes, like r-modes, are relevant since their amplitude becomes large and can lead to a fast spin-down of young stars if they are saturated by a non-linear saturation mechanism. We present a novel mechanism based on the pronounced large-amplitude enhancement of the bulk viscosity of dense matter. We show that the enhanced damping due to non-linear bulk viscosity can saturate r-modes of neutron stars at amplitudes appropriate for an efficient spin-down.