Stepping Up Superradiance Constraints on Axions

Abstract: Light feebly-coupled bosonic particles can efficiently extract the rotational energy of rapidly spinning black holes on sub-astrophysical timescales via a phenomenon known as black hole superradiance. In the case of light axions, the feeble self-interactions of these particles can lead to a non-linear coupled evolution of many superradiant quasi-bound states, dramatically altering the rate at which the black hole is spun down. In this work, we extend the study of axion superradiance to higher order states, solving for the first time the coupled evolution of all states with $n \leq 5$ in the fully relativistic limit (with $n$ being the principle quantum number). Using a Bayesian framework, we re-derive constraints on axions using the inferred spins of solar mass black holes, demonstrating that previously adopted limit-setting procedures have underestimated current sensitivity to the axion decay constant $f_a$ by around one order of magnitude, and that the inclusion to higher order states allows one to reasonably capture the evolution of typical high-spin black holes across a much wider range of parameter space, thereby allowing constraints to be extended to more massive axions. We conclude with an extensive discussion on the systematics associated with spin inference from x-ray observations.