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. In this work we have used 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 of the evolution. These modes are identified to carry the signature of the strong field regime due to significant deviations of the numerical data from the leading order post-Newtonian predictions. We find that modes with large amplitudes or with spherical harmonic indices $\ell=m$ are least modified from their dominant post-Newtonian behavior, while the weaker $\ell\neq m$ modes are modified to the greatest extent. The addition of spins to the binary components only affects the current-multipole modes with $\ell + m= \text{odd}$ at the order of interest and does seem to stabilize some of these modes, the $(\ell, m)=(3,2)$ mode being the exception. This mode is the most promising candidate to observe the signature of strong field dynamics as it shows the deviations from post-Newtonian behavior equally for binaries with non-spinning and aligned spinning black holes.