Black hole binary inspiral: Analysis of the plunge

Binary black hole coalescence has its peak of gravitational wave generation during the “plunge,” the transition from quasicircular early motion to late quasinormal ringing. Although advances in numerical relativity have provided plunge waveforms, there is still no intuitive or phenomenological understanding of plungecomparable to that of the early and late stages. Here we make progress in developing such understanding by focusing on the excitation of quasinormal ringing (QNR) during the plunge. We rely on insights of the linear mathematics of the particle perturbation model for the extreme mass limit. Our analysis, based on the Fourier domain Green function, and a simple initial model, point to the crucial role played by the kinematics near the “light ring” (the circular photon orbit) in determining the excitation of QNR. That insight is then shown to successfully explain Schwarzschild QNR found with evolution codes. Lastly, a phenomenological explanation is given for the underlying importance of the light ring.