Multipole Moments of the Common Horizons in a Binary-black-hole Simulation

The final state of a binary-black-hole system is a Kerr black hole. However, the black hole immediately after the merger is highly distorted. To analyze the evolution of this distortion, we construct a set of spatially gauge-invariant horizon multipole moments using the spectral Einstein code. We find that these moments are well described by the fundamental tones of spin-weight-2 quasinormal modes at late times. We see that each moment is comprised of several quasinormal modes instead of only one mode. We also explore the possible quasinormal description of these moments at early times near the merger.