Multipole moments on the common horizon in a binary-black-hole simulation

TL;DR

This paper constructs covariant multipole moments on the common horizon of a binary black hole system, revealing their strong correlation with gravitational waves and their description by quasinormal modes, even near merger.

Contribution

It introduces a covariant method to define multipole moments on the common horizon and demonstrates their relation to gravitational wave signals and quasinormal modes.

Findings

Multipole moments correlate strongly with gravitational waveforms.

Multipole moments are well described by quasinormal modes at late times.

Including overtones improves the quasinormal-mode fit for the mass multipole.

Abstract

We construct the covariantly defined multipole moments on the common horizon of an equal-mass, non-spinning, quasicircular binary-black-hole system. We see a strong correlation between these multipole moments and the gravitational waveform. We find that the multipole moments are well described by the fundamental quasinormal modes at sufficiently late times. For each multipole moment, at least two fundamental modes of different $\ell$ are detectable in the best model. These models provide faithful estimates of the true mass and spin of the remnant black hole. We also show that by including overtones, the $\ell=m=2$ mass multipole moment admits an excellent quasinormal-mode description at all times after the merger. This demonstrates the perhaps surprising power of perturbation theory near the merger.