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 $\ell |m|n=200$ mode, with a ${\log }_{10}$(Bayes factor) of $5.3$, commencing at $12M$ after the peak amplitude--a time well within the accepted linear regime. This two-mode analysis yields a redshifted final mass of $305.6_{-47.3}^{+35.7}{M}_{\odot }$ and a final spin of $0.84_{-0.14}^{+0.07}$ 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.

14 pages, 8 figures