Parametrizing gravitational-wave polarizations

TL;DR

This paper reviews the formalism of gravitational wave polarizations, clarifies polarization angle definitions, explores signal parametrizations, and discusses their implications for data analysis, providing guidance for improved GW signal modeling and interpretation.

Contribution

It introduces a comprehensive review of GW polarization formalisms, clarifies polarization angle concepts, and evaluates various parametrizations for different GW analysis applications.

Findings

Identifies three distinct definitions of polarization angle.

Shows any GW signal can be expressed as a superposition of elliptical states.

Analyzes the suitability of different parametrizations for various GW data analysis scenarios.

Abstract

We review the formalism underlying the modeling of gravitational wave (GW) polarizations, and the coordinate frames used to define them. In the process, we clarify the notion of "polarization angle" and identify three conceptually distinct definitions. We describe how those are related and how they arise in the practice of GW data analysis, explaining in detail the relevant conventions that have become the LIGO-Virgo standard. Furthermore, we show that any GW signal can be expressed as a superposition of elliptical (i.e., fully-polarized) states, and examine the properties and possible parametrizations of such elementary states. We discuss a variety of common parametrizations for fully-polarized modes, and compute Jacobians for the coordinate transformations relating them. This allows us to examine the suitability of each parametrization for different applications, including unmodeled or semimodeled signal reconstructions. We point out that analyses parametrized directly in terms of the plus and cross mode amplitudes will tend to implicitly favor high signal power, and to prefer linearly-polarized waves along a predefined direction; this makes them suboptimal for targeting face-on or face-off sources, which will tend to be circularly polarized. We discuss alternative parametrizations, with applications extending to continuous waves, ringdown studies, and unmodeled analyses like BayesWave. Code and additional material are made available in https://github.com/maxisi/gwpols.