Contribution from Nonlinear Quasi-normal Modes in GW250114

Abstract: We report evidence for nonlinear gravitational effects in the ringdown signal of gravitational wave event GW250114. Using Bayesian inference, we find that the inclusion of a nonlinear quasi-normal mode (220Q), a second-order harmonic predicted by general relativity, is statistically favored over the standard linear model (440 mode) when analyzing the post-merger oscillations. Specifically, models incorporating the 220Q mode yield higher Bayes factors than those including only the linear 440 mode, and produce remnant black hole parameters (mass and spin) more consistent with full numerical relativity simulations. This suggests that nonlinear mode coupling contributes significantly to the ringdown phase, opening a new avenue to probe strong-field gravity beyond linear approximations.