Observational black hole spectroscopy: A time-domain multimode analysis of GW150914

The detection of the least damped quasinormal mode from the remnant of the gravitational wave event GW150914 realized the long-sought possibility of observationally studying the properties of quasistationary 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 characterize the ringdown modes. We employ a pure time-domain analysis method, which infers from the data the time of transition between the nonlinear and quasilinear regimes of the postmerger emission in concert with all other parameters characterizing the source. We find that the data provide no evidence for the presence of more than one quasinormal 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.