The gravitational wave spectrum of non-axisymmetric, freely precessing neutron stars

TL;DR

This paper calculates the second-order gravitational wave spectrum of precessing neutron stars, revealing a new spectral line that could be observed by advanced LIGO, enabling detailed insights into neutron star structure.

Contribution

It extends previous models by including second-order effects in wobble angle, predicting a new spectral line in gravitational wave emissions from precessing neutron stars.

Findings

Second-order spectral line emerges in gravitational wave spectrum.

Potential observability of the second-order line with advanced LIGO.

Measurement of wobble angle and star properties from spectral data.

Abstract

Evidence for free precession has been observed in the radio signature of several pulsars. Freely precessing pulsars radiate gravitationally at frequencies near the rotation rate and twice the rotation rate, which for rotation frequencies greater than $\sim 10$ Hz is in the LIGO band. In older work, the gravitational wave spectrum of a precessing neutron star has been evaluated to first order in a small precession angle. Here we calculate the contributions to second order in the wobble angle, and we find that a new spectral line emerges. We show that for reasonable wobble angles, the second-order line may well be observable with the proposed advanced LIGO detector for precessing neutron stars as far away as the galactic center. Observation of the full second-order spectrum permits a direct measurement of the star's wobble angle, oblateness, and deviation from axisymmetry, with the potential to significantly increase our understanding of neutron star structure.