Abstract

Following a merger, a perturbed black hole relaxes into its final Kerr state, in part by radiating quasinormal modes (QNMs) — oscillations with specific complex frequencies and angular structures predicted by black hole perturbation theory. QNMs are widely used in waveform modelling and underpin key tests of general relativity and the nature of compact objects, though challenges remain, particularly in avoiding overfitting through well-motivated mode selection. In this talk, I will introduce new techniques for detecting and analysing QNMs, including spatial mapping, which extracts their angular structure, and a fast Bayesian fitting approach that models numerical relativity noise with Gaussian processes, providing a robust alternative to least-squares methods. I will present results obtained using the latest high-fidelity Cauchy-characteristic evolution (CCE) simulations by the SXS collaboration and show how these methods provide insight into the role of overtones and nonlinear effects, both of which will be important for future gravitational wave observations.