  Stimulated Hawking radiation in an analogue black hole in a Bose-Einstein
condensate was reported seven years ago, and it was claimed that the
stimulation was of the black hole lasing variety. The study was based on
observation of rapidly-growing negative-energy waves. We find that the Hawking
particles are directly observable in the experimental plots, which confirms the
stimulated Hawking radiation. We further verify this result with new
measurements. Also, the observed Hawking particles provide a sensitive,
background-free probe of the underlying mechanism of the stimulation. The
experiment inspired the prediction of the Bogoliubov-Cherenkov-Landau (BCL)
mechanism of stimulated Hawking radiation. By computing the Bogoliubov
coefficient for Hawking radiation, we find that the stimulation was of the BCL
type, rather than black-hole lasing. We further confirm the results with
numerical simulations of both black hole lasing and BCL stimulation.
