First Overtone Mode in the Ringdown Signal of GW231028

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 ($\mathrm{\ell }\mathrm{\mid }m\mathrm{\mid }n=221$) and the higher order ($\mathrm{\ell }\mathrm{\mid }m\mathrm{\mid }n=210$). The detection of the $221$ mode is statistically significant, achieving a Bayes factor of $\sim 189.2$ and an amplitude exclusion from zero at $>7\sigma$ credibility for an analysis beginning at $10M$ 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 $246.2_{-22.4}^{+22.3}{\mathrm{M}}_{\odot }$ and a final spin of $0.81_{-0.10}^{+0.07}$ (at $90\mathrm{\%}$ 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.