Quasinormal modes of spontaneous scalarized Kerr black holes

Abstract: Recent studies reveal that Kerr black holes can undergo spontaneous scalarization via an appropriate coupling between a scalar field and the Gauss-Bonnet term. This mechanism introduces observable signatures in the quasinormal mode spectrum, which characterizes the ringdown phase of gravitational waves. In this work, we present, for the first time, the quasinormal modes of rotating scalarized black holes within the framework of Einstein-scalar-Gauss-Bonnet theory, without resorting to perturbative expansion of either the coupling constant or the black hole spin. Using spectral decomposition, we numerically construct both the background solution and the perturbation functions, reformulating the calculation of quasinormal modes as a generalized eigenvalue problem. Our results pave the way for advanced ringdown tests of spontaneous scalarization using gravitational wave data.