Probing chromatic onsets of gravitational wave overtones

The ringdown data of both GW150914 and GW190521_074359 (GW190521r) show evidence supporting the presence of an overtone. All of the previous studies adopt a fundamental assumption, which was motivated more by convenience than by first principles, that the first overtone and the fundamental mode share a same onset. In this work, for the first time, we relax such an assumption, and we aim to probe the possible chromatic onsets of these two components within the GW150914 and GW190521r ringdown data. For both events, we bound the onset lags to be Delta t(0) >= 5M(f) at probabilities of >= 94.7%, where M-f is the mass of the remnant black hole formed in the merger. This result moderately favors the nonsimultaneous excitation between the fundamental mode and the first overtone in the ringdown.