Ringdown spectroscopy of phenomenologically modified black holes

Abstract: The characteristic oscillations of black holes, as described by their quasinormal mode spectrum, play a fundamental role in testing general relativity with gravitational waves. The so-called parametrized QNM framework was introduced to predict perturbative changes in the spectrum due to small deviations from general relativity. In this work, we extend the framework to model time-domain signals and study the excitation of quasinormal modes from the time evolution of initial data. Specifically, we quantify whether the perturbative eigenvalue predictions agree with extracting excited quasinormal modes from such simulations. Addressing this issue is particularly important in the context of agnostic ringdown tests and the possible presence of spectral instabilities, which may diminish the promises of black hole spectroscopy. We find that the extracted quasinormal modes agree well with the perturbative predictions, underlining that these types of modifications can, in principle, be robustly tested from real observations. Moreover, we also report the importance of including late-time tails for accurate mode extractions. Finally, we provide a WKB-inspired analysis supporting the importance of the peak of the scattering potential and show robust scaling relations.