The final stage of a binary black hole merger is ringdown , in which the system is described by a Kerr black hole with quasinormal mode perturbations .
It is far from straightforward to identify the time at which the ringdown begins .
Yet determining this time is important for precision tests of the general theory of relativity that compare an observed signal with quasinormal mode descriptions of the ringdown , such as tests of the no-hair theorem .
We present an algorithmic method to analyze the choice of ringdown start time in the observed waveform .
This method is based on determining how close the strong field is to a Kerr black hole ( Kerrness ) .
Using numerical relativity simulations , we characterize the Kerrness of the strong-field region close to the black hole using a set of local , gauge-invariant geometric and algebraic conditions that measure local isometry to Kerr .
We produce a map that associates each time in the gravitational waveform with a value of each of these Kerrness measures ; this map is produced by following outgoing null characteristics from the strong and near-field regions to the wave zone .
We perform this analysis on a numerical relativity simulation with parameters consistent with GW150914- the first gravitational wave detection .
We find that the choice of ringdown start time of 3 \mathrm { ms } after merger used in the GW150914 study ( ) to test general relativity corresponds to a high dimensionless perturbation amplitude of \sim 7.5 \times 10 ^ { -3 } in the strong-field region .
This suggests that in higher signal-to-noise detections , one would need to start analyzing the signal at a later time for studies that depend on the validity of black hole perturbation theory .

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