Monday

Modelling the end point of binary black hole mergers is a cornerstone of modern gravitational-wave astronomy. The ringdown signal, a superposition of quasinormal modes, provides a way to directly measure the remnant black hole mass and spin, independent of the rest of the gravitational wave signal. Extracting multiple quasinormal mode frequencies from the ringdown signal allows the remnant black hole to be studied in unprecedented detail.

By fitting ringdown models to numerical relativity waveforms, we can assess the model performance across the binary parameter space (initial black hole spins and mass ratio). This includes black hole binaries with misaligned spins, which we study for the first time. When fitting to these precessing systems, we find a much greater variation in the performance of the ringdown fits than in the aligned-spin case. This is true even when we consider the contribution to the ringdown signal from overtones, mirror modes, and higher harmonics (through the process of mode mixing). The ambiguity in the choice of a ringdown start time is a well known problem in ringdown analysis, and the challenge of finding a suitable ringdown start time for precessing systems is at least partly responsible for the greater variation in fit performance.

I will conclude by discussing some implications of these results for the ringdown analyses of LIGO/Virgo observations of high mass binary black holes.