  We develop a simple yet powerful technique to study Bogoliubov-Cherenkov
radiation by producing a pulsed atom laser from a strongly confined
Bose-Einstein condensate. Such radiation results when the atom laser pulse
falls past a Bose-Einstein condensate at high-hypersonic speeds, modifying the
spatial profile to display a characteristic twin jet structure and a
complicated interference pattern. The experimental observations are in
excellent agreement with mean-field numerical simulations and an analytic
theory. Due to the highly hypersonic regime reached in our experiment, this
system offers a highly controllable platform for future studies of
condensed-matter analogs of quantum electrodynamics at ultrarelativistic
speeds.
