Null-stream-based Bayesian Unmodeled Framework to Probe Generic Gravitational-wave Polarizations

TL;DR

This paper introduces a Bayesian null-stream framework to analyze gravitational-wave data, enabling the detection of various polarization modes without prior source localization, thus testing general relativity against alternative theories.

Contribution

It develops a basis formulation for null streams that can probe generic polarization states in gravitational waves without explicit basis modeling.

Findings

Framework can distinguish pure and mixed polarizations in simulated data.

Effective in constraining polarization content with 3-detector network.

Proposes a method to test for orthogonal polarization components.

Abstract

We present a null-stream-based Bayesian unmodeled framework to probe generic gravitational-wave polarizations. Generic metric theories allow six gravitational-wave polarization states, but general relativity only permits the existence of two of them namely the tensorial polarizations. The strain signal measured by an interferometer is a linear combination of the polarization modes and such a linear combination depends on the geometry of the detector and the source location. The detector network of Advanced LIGO and Advanced Virgo allows us to measure different linear combinations of the polarization modes and therefore we can constrain the polarization content by analyzing how the polarization modes are linearly combined. We propose the basis formulation to construct a null stream along the polarization basis modes without requiring modeling the basis explicitly. We conduct a mock data study and we show that the framework is capable of probing pure and mixed polarizations in the Advanced LIGO-Advanced Virgo 3-detector network without knowing the sky location of the source from electromagnetic counterparts. We also discuss the effect of the presence of the uncaptured orthogonal polarization component in the framework, and we propose using the plug-in method to test the existence of the orthogonal polarizations.