Probing chromatic onsets of gravitational wave overtones

TL;DR

This study investigates whether the fundamental mode and first overtone in gravitational wave ringdowns are excited simultaneously, finding evidence that they may have different onset times, challenging previous assumptions.

Contribution

It relaxes the common assumption of simultaneous excitation of modes and probes the chromatic onsets of gravitational wave overtones in real data.

Findings

Bound the onset lag to be at least 5M_f for both events.

Results favor non-simultaneous excitation of modes.

Moderate evidence against the assumption of simultaneous mode onset.

Abstract

The ringdown data of both GW150914 and GW190521\_074359 (GW190521r) show evidence supporting the presence of overtone. Previous studies all adopt a fundamental assumption, 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 $\Delta t_0\geq 5M_f$ at probabilities of $\geq 94.7\%$, where $M_f$ is the mass of the remnant black hole formed in the merger. This result moderately favours the non-simultaneous excitation between the fundamental mode and the first overtone in the ringdown.