We report evidence for nonlinear modes in the ringdown stage of the gravitational waveform
produced by the merger of two comparable-mass black holes. We consider both the …

The gravitational wave strain emitted by a perturbed black hole (BH) ringing down is
typically modeled analytically using first-order BH perturbation theory. In this Letter, we show …

Black hole spectroscopy is the program to measure the complex gravitational wave
frequencies of merger remnants, and to quantify their agreement with the characteristic …

We analyze GW150914 postmerger data to understand if ringdown overtone detection
claims are robust. We find no evidence in favor of an overtone in the data after the waveform …

We study extreme mass-ratio binary systems in which a stellar mass compact object spirals
into a supermassive black hole surrounded by a scalar cloud. Scalar clouds can form …

In general relativity, when two black holes merge they produce a rotating (Kerr) black hole
remnant. According to perturbation theory, the remnant emits “ringdown” radiation; a …

It has been shown, via specific examples and a pseudospectrum analysis, that the black
hole quasinormal spectra are unstable. The implication of such a result for gravitational …

The excitation of quadratic quasinormal modes is an important nonlinear phenomenon for a
Kerr black hole ringing at a specific linear mode. The amplitude of this second-order effect is …

Extracting quasinormal modes from compact binary mergers to perform black hole
spectroscopy is one of the fundamental pillars in current and future strong-gravity tests …

We propose two frequency-domain filters to analyze ringdown signals of binary black hole
mergers. The first rational filter is constructed based on a set of (arbitrary) quasinormal …