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Abstract

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.