Superradiance constraints from GW231123

Gravitational wave observations have recently revealed with high significance, and high precision, the existence of O(100)M⊙ rapidly rotating black holes, allowing gravitational wave events to be used for the first time to probe unexplored axion parameter space using the phenomenon known as black hole superradiance. Here, we present new limits on axions using the binary black hole merger event GW231123, whose constituent black holes are among the fastest spinning observed with gravitational waves to date. We demonstrate that the most viable binary formation channels lead to conservative constraints on axion masses μ∼[0.6-5]×10-13 eV and decay constants fΦ≳1014 GeV, extending existing superradiance constraints derived using x-ray observations to yet lower axion masses.