Detection of a Higher Harmonic Quasi-normal Mode in the Ringdown Signal of GW231123
Abstract
The ringdown phase of a gravitational wave signal from a binary black hole merger offers a unique laboratory for testing general relativity in the strong-field regime and probing the properties of the final remnant black hole. In this study, we analyze the ringdown of GW231123 and find strong evidence for a multimode quasinormal spectrum. Our analysis employs two time-domain methodologies: a full Bayesian inference and an enhanced F-statistic framework, which we extend to enable the calculation of Bayesian evidence and the reconstruction of posterior distributions for all model parameters. We report a statistically significant detection of the |m|n=200 mode, with a 10(Bayes factor) of 5.3, commencing at 12\,M after the peak amplitude--a time well within the accepted linear regime. This two-mode analysis yields a redshifted final mass of 305.6+35.7-47.3M and a final spin of 0.84+0.07-0.14 at 90\% credibility, from a ringdown signal with a network signal-to-noise ratio of approximately 14.5. Furthermore, a test of the no-hair theorem performed using the two detected modes reveals no deviation from the predictions of general relativity. These results highlight the power of the F-statistic methodology to uncover nuanced features in gravitational wave signals, thereby providing novel insights into the fundamental properties of black holes.
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