First Overtone Mode in the Ringdown Signal of GW231028
arXiv:2509.08657 · doi:10.1103/114x-tj8m
Abstract
The properties of a remnant black hole can be probed by analyzing the gravitational waves emitted during its ringdown phase. This signal provides a direct test of general relativity in the strong-field regime. In this study, we apply both a time-domain F-statistic framework and full Bayesian time-domain sampling to the ringdown of GW231028\_153006. We report decisive evidence for multimode content, specifically identifying both the first overtone () and the higher order (). The detection of the mode is statistically significant, achieving a Bayes factor of and an amplitude exclusion from zero at credibility for an analysis beginning at postpeak. These findings are rigorously validated against the numerical relativity injection SXS:BBH:1282, which confirms that such multimode features are physically expected for a remnant with the inferred high spin and mass ratio. The inclusion of the overtone mode allows for precise constraints on the remnant's properties, yielding a redshifted final mass of and a final spin of (at credibility), consistent with full inspiral-merger-ringdown predictions. A test of the no-hair theorem, enabled by this robust multimode detection, shows consistency with general relativity.
arXiv comment22 pages, 15 figures; published in Phys.Rev.Research
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