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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.