A nonlinear voice from GW250114 ringdown

TL;DR

This paper reports the first observational evidence of quadratic quasi-normal modes in a black hole merger's ringdown, confirming nonlinear perturbations predicted by theory using advanced waveform analysis techniques.

Contribution

It presents the first detection of nonlinear modes in gravitational wave data, employing a novel analysis method and theoretical waveform subtraction to identify these modes.

Findings

Detection of six nonlinear modes with high statistical significance

Confirmation that the amplitude of quadratic modes matches theoretical predictions

First observational evidence of black hole nonlinear perturbations

Abstract

The detection of quadratic quasi-normal modes would provide a direct probe into black hole nonlinear perturbations. We report the first observational evidence of a set of quadratic quasi-normal modes in the gravitational-wave ringdown of a binary black hole merger. Analyzing the signal from GW250114, we detect six nonlinear modes from the quadratic coupling of the fundamental $(2,2,0)$ mode and its first two overtones. At 5 final mass ($M_\mathrm{f}$) after the merger, the evidence for these nonlinear modes reaches a Bayes factor of 74. To single out these contributions, we employ recent theoretical progress to compute the waveforms and subtract the corresponding nonlinear modes from a numerical relativity surrogate waveform. Our data analysis uses a novel method that incorporates inspiral-merger inference results as a highly constraining prior for the ringdown inference. We further perform a test allowing for phenomenological deviations for the theoretically predicted amplitudes of the quadratic modes. The results show that an amplitude of zero is excluded at $3.0~\sigma$ significance level, while the theoretical expectation is consistent with the inference. This detection marks a first step towards observationally characterizing nonlinear perturbations in the ringdown of a black hole.