Precession during merger: Strong polarization changes are observationally accessible features of strong-field gravity during binary black hole merger

The short gravitational wave signal from the merger of compact binaries encodes a surprising amount of information about the strong-field dynamics of merger into frequencies accessible to ground-based interferometers. In this paper we describe a previously unknown ``precession'' of the peak emission direction with time, both before and after the merger, about the total angular momentum direction. We demonstrate that the gravitational wave polarization encodes the orientation of this direction to the line of sight. We argue that the effects of polarization can be estimated nonparametrically, directly from the gravitational wave signal as seen along one line of sight, as a slowly varying feature on top of a rapidly varying carrier. After merger, our results can be interpreted as a coherent excitation of quasinormal modes of different angular orders, a superposition which naturally ``precesses'' and modulates the line-of-sight amplitude. Recent analytic calculations have arrived at a similar geometric interpretation. We suspect the line-of-sight polarization content will be a convenient observable with which to define new high-precision tests of general relativity using gravitational waves. Additionally, as the nonlinear merger process seeds the initial coherent perturbation, we speculate that the amplitude of this effect provides a new probe of the strong-field dynamics during merger. To demonstrate that the ubiquity of the effects we describe, we summarize the postmerger evolution of 104 generic precessing binary mergers. Finally, we provide estimates for the detectable impacts of precession on the waveforms from high-mass sources. These expressions may identify new precessing binary parameters whose waveforms are dissimilar from the existing sample.