Analyzing black-hole ringdowns II: data conditioning

TL;DR

This paper examines how data conditioning methods like filtering and downsampling affect black hole ringdown gravitational wave analysis, emphasizing the importance of careful implementation to avoid systematic errors and improve analysis accuracy.

Contribution

It identifies potential issues with current data conditioning techniques and proposes alternative methods that operate identically on data and models to preserve posterior distributions.

Findings

Aggressive filtering can skew posterior distributions.

Preferred anti-alias filtering preserves data structure better.

Long data segments may be necessary for overlapping noise features.

Abstract

Time series data from observations of black hole ringdown gravitational waves are often analyzed in the time domain by using damped sinusoid models with acyclic boundary conditions. Data conditioning operations, including downsampling, filtering, and the choice of data segment duration, reduce the computational cost of such analyses and can improve numerical stability. Here we analyze simulated damped sinsuoid signals to illustrate how data conditioning operations, if not carefully applied, can undesirably alter the analysis' posterior distributions. We discuss how currently implemented downsampling and filtering methods, if applied too aggressively, can introduce systematic errors and skew tests of general relativity. These issues arise because current downsampling and filtering methods do not operate identically on the data and model. Alternative downsampling and filtering methods which identically operate on the data and model may be achievable, but we argue that the current operations can still be implemented safely. We also show that our preferred anti-alias filtering technique, which has an instantaneous frequency-domain response at its roll-off frequency, preserves the structure of posterior distributions better than other commonly used filters with transient frequency-domain responses. Lastly, we highlight that exceptionally long data segments may need to be analyzed in cases where thin lines in the noise power spectral density overlap with central signal frequencies. Our findings may be broadly applicable to any analysis of truncated time domain data with acyclic boundary conditions.