On choosing the start time of binary black hole ringdown

TL;DR

This paper introduces an algorithmic method to determine the optimal start time of black hole ringdown signals by assessing Kerrness, which is crucial for accurate tests of general relativity using gravitational wave data.

Contribution

The authors develop a gauge-invariant, local measure of Kerrness to identify the onset of ringdown in gravitational wave signals, improving the precision of black hole perturbation analyses.

Findings

The method maps Kerrness measures to waveform times using null characteristics.

Application to GW150914 shows the initial ringdown start time used was at high perturbation amplitude.

Later start times may be necessary for more accurate tests in high signal-to-noise detections.

Abstract

The final stage of a binary black hole merger is ringdown, in which the system is described by a Kerr black hole with quasinormal mode perturbations. It is far from straightforward to identify the time at which the ringdown begins. Yet determining this time is important for precision tests of the general theory of relativity that compare an observed signal with quasinormal mode descriptions of the ringdown, such as tests of the no-hair theorem. We present an algorithmic method to analyze the choice of ringdown start time in the observed waveform. This method is based on determining how close the strong field is to a Kerr black hole (Kerrness). Using numerical relativity simulations, we characterize the Kerrness of the strong-field region close to the black hole using a set of local, gauge-invariant geometric and algebraic conditions that measure local isometry to Kerr. We produce a map that associates each time in the gravitational waveform with a value of each of these Kerrness measures; this map is produced by following outgoing null characteristics from the strong and near-field regions to the wave zone. We perform this analysis on a numerical relativity simulation with parameters consistent with GW150914- the first gravitational wave detection. We find that the choice of ringdown start time of $3\,\mathrm{ms}$ after merger used in the GW150914 study to test general relativity corresponds to a high dimensionless perturbation amplitude of $ \sim 7.5 \times 10^{-3}$ in the strong-field region. This suggests that in higher signal-to-noise detections, one would need to start analyzing the signal at a later time for studies that depend on the validity of black hole perturbation theory.