  We have numerically investigated close scattering processes of two black
holes (BHs). Our careful analysis shows for the first time a non-merging
ringdown gravitational wave coming from dynamical tidal deformations of
individual BHs during their close encounter. The ringdown wave frequencies turn
out to agree well with the quasi-normal ones of a single BH in perturbation
theory, despite its distinctive physical context from the merging case. Our
study shows a new type of gravitational waveform and opens up a new exploration
of strong gravitational interactions using BH encounters.
