First overtone and higher-order modes 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 ( ℓ | m | n = 221 ) and the higher order ( ℓ | m | n = 210 ). The detection of the 221 mode is statistically significant, achieving a Bayes factor of ∼ 189.2 and an amplitude exclusion from zero at > 7 σ credibility for an analysis beginning at 10 M 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 M ⊙ and a final spin of 0 . 81 − 0.10 + 0.07 (at 90 % 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.