Gravitational radiation back-reaction from f-modes on neutron stars

TL;DR

This paper investigates how gravitational radiation affects the spin of neutron stars through f-mode oscillations, revealing an unexpected doubling of angular momentum radiated compared to initially excited mode angular momentum.

Contribution

It provides a novel analysis of radiation reaction on neutron star spin, focusing on Kelvin modes and revealing a surprising angular momentum transfer mechanism.

Findings

Excitation of a mode with angular momentum δJ results in radiating 2δJ to infinity.

The star ends up with a bulk angular momentum of -δJ.

Implications for neutron star spin-down and angular momentum budgets.

Abstract

The problem of the gravitational radiation damping of neutron star fundamental ($f$) mode oscillations has received considerable attention. Many studies have looked at the stability of such oscillations in rapidly rotating stars, calculating the growth/decay rate of the mode amplitude. In this paper, we look at the relatively neglected problem of the radiation reaction on the spin of the star. We specialise greatly to the so-called Kelvin modes: the modes of oscillation of (initially) non-rotating incompressible stars. We find the unexpected result that the excitation of a mode of angular momentum $\delta J$ on an initially non-rotating star ends up radiating an angular momentum $2 \delta J$ to infinity, leaving the star itself with a bulk angular momentum of $-\delta J$. This result is interesting in itself, and also will have implications for the angular momentum budgets of spinning down neutron stars, should such modes be excited.