Gravitational waves from compact binary coalescences provide unique opportunities to test general relativity (GR) in the strong-field regime. In particular, the merger phase, during which two compact objects finally coalesce, corresponds to the regime of the strongest gravitational fields accessible by direct observation and thus serves as a probe of the nonlinear nature of gravity. In this work, we test GR in the merger phase by analyzing GW150914 using a modified waveform proposed in Watarai et al. [Phys. Rev. D 109, 084058 (2024).], which parametrizes possible deviations from GR during this stage. Within this framework, the inferred deviation parameters can be translated into model-independent constraints on physically meaningful quantities. For GW150914, we find that the additional energy radiated in the merger phase is constrained to be ${0.26}_{-0.62}^{+0.75}\%$ of the total energy emitted over the entire coalescence predicted by GR, and the peak time shift due to beyond-GR effects is ${2.17}_{-9.90}^{+9.56}\mathrm{ms}$, both within the 90% credible interval. These two constraints serve as observational benchmarks for deviations in the nonlinear gravity regime, offering guidance for theoretical investigations of beyond-GR models.

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• Alternative gravity theories
• General relativity
• Gravitational waves