Searching for a Ringdown Overtone in GW150914

TL;DR

This paper reanalyzes GW150914 data to search for quasinormal mode overtones using a frequency-domain Bayesian approach, finding tentative evidence at about 1.8 sigma significance, with advantages over previous time-domain analyses.

Contribution

The authors introduce a frequency-domain Bayesian method that marginalizes over source sky position and start time, providing a new approach to overtone detection in gravitational wave data.

Findings

Tentative evidence for an overtone at 1.8 sigma significance.

Analysis favors a non-zero overtone amplitude, but with lower significance than previous studies.

Method improves robustness by marginalizing over key uncertainties.

Abstract

We reanalyze the GW150914 post-merger data searching for quasinormal modes beyond the fundamental, quadrupolar mode. There is currently an ongoing disagreement in the literature about whether, and to what extent, the data contains evidence for a quasinormal mode overtone. We use a frequency-domain approach to ringdown data analysis that was recently proposed by the authors. Our analysis has several advantages compared to other analyses performed mainly in the time domain; in particular, the source sky position and the ringdown start time are marginalized over (as opposed to simply being fixed) as part of a Bayesian ringdown analysis. We find tentative evidence for an overtone in GW150914, but at a lower significance than reported elsewhere. Our preferred analysis, marginalizing over the uncertainty in the time of peak strain amplitude, gives a posterior on the overtone amplitude peaked away from zero at $\sim 1.8\sigma$.