Novel Ringdown Tests of General Relativity with Black Hole Graybody Factors.

We present greyring, a new model for the postmerger signal in black-hole binary coalescences based on the graybody factor of the remnant. The model accurately reproduces the full frequency-domain ringdown signal for a large set of comparable-mass, aligned-spin numerical relativity waveforms, including the (ℓ,m)=(2,2), (3,3), and (4,4) modes. It achieves mismatches of order O(10^{-6}) for the dominant (2,2) mode, demonstrating that the graybody-factor parametrization provides a very accurate phenomenological description of the postmerger signal. Building on this model, we introduce a novel consistency test of strong gravity based on the graybody factor: the remnant mass and spin inferred from greyring can be compared with those obtained through standard black-hole spectroscopy. This agnostic test relies exclusively on the postmerger signal and does not require the inclusion of overtones or the choice of very early ringdown starting times, combining the advantages of inspiral-merger-ringdown consistency tests and traditional black-hole spectroscopy. We apply the test to GW250114 and find that the remnant mass and spin inferred from greyring are consistent with those measured from the full signal. Remarkably, the inferred parameters can be measured with a precision comparable to, or slightly better than, that achieved with standard black-hole spectroscopy. Our graybody-factor waveform model allows for new precision tests of strong gravity using the ringdown signal.