How to adapt broad-band gravitational-wave searches for r-modes

TL;DR

This paper discusses how to modify broad-band gravitational-wave searches to detect r-modes in neutron stars, addressing unique polarization and amplitude conversion issues, and shows existing data can already be sensitive to these signals.

Contribution

It provides the necessary modifications and considerations to adapt current continuous gravitational-wave search methods for r-modes in neutron stars.

Findings

Existing data already have sensitivity to r-modes.

Key differences in polarization affect detection strategies.

Conversion from wave amplitude to r-mode amplitude is essential.

Abstract

Up to now there has been no search for gravitational waves from the r-modes of neutron stars in spite of the theoretical interest in the subject. Several oddities of r-modes must be addressed to obtain an observational result: The gravitational radiation field is dominated by the mass current (gravitomagnetic) quadrupole rather than the usual mass quadrupole, and the consequent difference in polarization affects detection statistics and parameter estimation. To astrophysically interpret a detection or upper limit it is necessary to convert the wave amplitude to an r-mode amplitude. Also, it is helpful to know indirect limits on gravitational-wave emission to gauge the interest of various searches. Here I address these issues, thereby providing the ingredients to adapt broad-band searches for continuous gravitational waves to obtain r-mode results. I also show that searches of existing data can already have interesting sensitivities to r-modes.