Detection of a Higher Harmonic Quasi-normal Mode in the Ringdown Signal of GW231123

Abstract The detection of multiple quasinormal modes, as expected in the ringdown phase of a black hole merger, will provide a direct test of general relativity in the strong-field regime and a way to characterize the remnant black hole. In this study, we present the first robust, multi-mode detection from the notable event GW231123. We identify the (2, 0, 0) quasinormal mode with very high statistical confidence (i.e., log10 (Bayes factor) ~5.3), initiating the analysis at 12M after the merger-a time that lies within the theoretically expected linear regime of the signal. Notably, this axisymmetric mode emerges as the dominant component, exhibiting substantially greater power than the canonical (2, 2, 0) mode. This observation indicates an unusual merger scenario, potentially resulting from a nearly head-on collision between two massive black holes. Moreover, the inferred remnant properties are consistent with the predictions of the no-hair theorem in general relativity.