Do remnant black hole ringdown tones match Kerr at percent precision?

For merger remnants, measure the complex frequencies of at least two independent quasinormal modes, for example $(l,m,n) = (2,2,0)$ together with $(2,2,1)$, $(3,3,0)$ or $(4,4,0)$, and constrain the fractional deviations $\delta f$ and $\delta \tau$ from Kerr values for a common mass and spin. The target is $\mid\delta f\mid, \mid\delta \tau\mid$ below 0.01. An answer is the measured deviations with uncertainties from a catalog of events.

Multi-mode ringdown measurements with percent-level frequency and damping-time constraints from next-generation ground detectors or LISA.