Signature of horizon dynamics in binary black hole gravitational waveforms

Gravitational waves from merging binary black holes carry the signature of the strong field dynamics of the newly forming common horizon. This signature presents itself in the amplitudes and phases of various spherical harmonic modes as deviations from the point particle description provided by post-Newtonian theory. Understanding the nature of these departures will aid in (a) formulating better models of the emitted waveforms in the strong field regime of the dynamics, and (b) relating the waveforms observed at infinity to the common horizon dynamics. We use a combination of numerical relativity simulations and post-Newtonian theory to search for the modes of radiation whose amplitude is most affected by the strong field phase. We find that modes with large amplitudes or spherical harmonic indices l = m are least modified from their dominant post-Newtonian behavior, while the weaker l ≠ m modes are affected to the greatest extent. The addition of spins to the binary components only affects current-multipole modes with l + m = odd at the order of interest and seems to stabilize some mode amplitudes against deviations from post-Newtonian theory.