GW231123 ringdown: interpretation as multimodal Kerr signal

GW231123 boasts some of the strongest evidence to date for the clear presence of multiple quasinormal modes in the ringdown of an LVK signal. This allows for unprecedented experimental constraints of the Kerr metric — we demonstrate that quasinormal mode fits to GW231123 provide sub-10% constraints of the Kerr ringdown frequency spectrum at the 90% credible level. Our best quasinormal mode fits to this signal also return significantly higher remnant mass and spin estimates than any available inspiral-merger-ringdown models, which may alter our belief regarding whether the binary components of GW231123 lie in the pair-instability supernova mass gap.