The Start Time of Ringdown

Abstract

We determine the convergence of quasi-normal modes sums for a point-particle plunging into a Schwarzschild black hole. While a dynamical amplitude model reproduces the waveform after the particle crosses the bounce radius, a constant amplitude sum converges only after the start time $t_S$, which corresponds to the advanced time at which the particle hits the central singularity. For a plunge from the ISCO, $t_S \simeq 9.4 M$ after the peak of the $22$ mode. After $t_S$, dynamical and constant amplitudes models are equivalent; our analysis also implies that any "direct wave" must reduce to the late-time tail after $t_S$.