# Observational Black Hole Spectroscopy: A time-domain multimode analysis of GW150914

## TL;DR

This paper presents a novel time-domain Bayesian analysis of GW150914's ringdown spectrum, finding no conclusive evidence for multiple quasi-normal modes and establishing a framework for black hole spectroscopy.

## Contribution

It introduces the first comprehensive agnostic time-domain framework for observational black hole spectroscopy using gravitational wave data.

## Key findings

- No evidence for more than one quasi-normal mode in GW150914
- Cannot unambiguously identify a single mode from frequency and damping time
- Data cannot discriminate between l=2 and l=3 modes

## Abstract

The detection of the least damped quasi-normal mode from the remnant of the gravitational wave event GW150914 realised the long sought possibility to observationally study the properties of quasi-stationary black hole spacetimes through gravitational waves. Past literature has extensively explored this possibility and the emerging field has been named "black hole spectroscopy". In this study, we present results regarding the ringdown spectrum of GW150914, obtained by application of Bayesian inference to identify and characterise the ringdown modes. We employ a pure time-domain analysis method which infers from the data the time of transition between the non-linear and quasi-linear regime of the post-merger emission in concert with all other parameters characterising the source. We find that the data provides no evidence for the presence of more than one quasi-normal mode. However, from the central frequency and damping time posteriors alone, no unambiguous identification of a single mode is possible. More in-depth analysis adopting a ringdown model based on results in perturbation theory over the Kerr metric, confirms that the data do not provide enough evidence to discriminate among an $l=2$ and the $l=3$ subset of modes. Our work provides the first comprehensive agnostic framework to observationally investigate astrophysical black holes' ringdown spectra.