Probing chromatic onsets of gravitational wave overtones

The ringdown data of both GW150914 and GW190521\_074359 (GW190521r) show evidence supporting the presence of overtone. Previous studies all adopt a fundamental assumptions, which were 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 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 $\Delta t_0\leq 10M_f$ at high probabilities (i.e., $\geq 96.5\%$), where $M_f$ is the mass of the remnant black hole formed in the merger. Moreover, for GW190521r (GW150914) we have $\Delta t_0\geq 3M_f$ at $98.3\%$ ($69.5\%$) credibility, indicating the non-simultaneous excitation between the fundamental mode and the first overtone in the ringdown.