Black hole spectroscopy by mode cleaning

At the final stage of the collision of two black holes, the newly formed remnant black hole rings at specific characteristic frequencies and decay rates. The ringdown gravitational waves are described by the superposition of these complex-valued, quasinormal modes (QNMs), which are determined solely by the remnant black hole's mass and spin according to the no-hair theorem. We propose a novel framework for ringdown analysis using a set of frequency-domain rational filters designed to remove any given QNMs. By cleaning the dominant fundamental mode in the data of a gravitational-wave event, we are able to probe weaker modes and subdominant effects. This new method provides a rigorous and powerful tool to analyze experimental data and interrogate the nature of gravity. In this talk, I will first introduce the filters and describe the Bayesian framework based on mode cleaning. Next, I will demonstrate the application of the method and uncover the first overtone in the ringdown of GW150914.