Probing the ringdown perturbation in binary black hole coalescences with an improved quasi-normal mode extraction algorithm

Understand

Using gravitational waves to probe the geometry of the ringing remnant black hole formed in a binary black hole coalescence is a well-established way to test Einstein's theory of general relativity.

• However, doing so requires knowledge of when the predictions of black hole perturbation theory, i.e., quasi-normal modes (QNMs), are a valid description of the emitted gravitational wave as well as what the amplitudes of these excitations are.
• In this work, we develop an algorithm to systematically extract QNMs from the ringdown of black hole merger simulations.
• Our algorithm improves upon previous ones in three ways: it fits over the two-sphere, enabling a complete model of the strain; it performs a reverse-search in time for QNMs using a more robust nonlinear least squares routine called \texttt{VarPro}; and it checks the variance of QNM amplitudes, which we refer to as ``stability'', over an interval matching the natural time scale of each QNM.