Black hole ringdown: the importance of overtones

It is possible to infer the mass and spin of the remnant black hole of binary mergers by comparing the ringdown gravitational-wave signal to results from studies of perturbed Kerr spacetimes. Typically these studies are based on the fundamental quasinormal mode (QNM) of the dominant l=m=2 harmonic. Using the ringdown from numerical relativity simulations, we find that the fundamental mode alone is insufficient to recover the true underlying mass and spin, unless the analysis is started very late in the ringdown. Including higher overtones associated with this l=m=2 harmonic appears to resolve this issue, and provides an unbiased estimate of the true remnant parameters. Further, including overtones allows for the modeling of the ringdown signal for all times beyond the peak strain amplitude, with good agreement even before the peak amplitude, indicating the linear quasinormal regime has a start time much earlier than previously expected. Moreover, we explore the possibility of using the l=m=2 fundamental QNM and its first overtone as a two-mode test of general relativity, as opposed to the conventional approach of relying on the fundamental QNMs from two different angular harmonics.