Tracing source imprints on the gravitational direct wave
The direct wave component of black hole (BH) ringdown, revealed through quasinormal-mode filters, carries a non-quasinormal-mode imprint of the plunging perturber's dynamics. However, the connection between the source trajectory and the observed direct wave remains incompletely understood. We first show that the full waveform filtered by the frequency-dependent black hole response gives a substantially more localized kernel than the standard rational filter approach. We then show that analytically continuing the Teukolsky source variables to complex coordinates allows for a semianalytic direct wave which closely reproduces numerical integration throughout the entire plunge. Our generalized steepest-descent method clarifies how coherent emission shapes the imprint of source dynamics on the observed direct wave beyond the light-cone picture.