  Electrically charged particles, moving faster than the speed of light in a
medium, emit Cherenkov radiation. Theory predicts electric and magnetic dipoles
to radiate as well, with a puzzling behavior for magnetic dipoles pointing in
transversal direction [I. M. Frank, Izv. Akad. Nauk SSSR, Ser. Fiz. 6, 3
(1942)]. A discontinuous Cherenkov spectrum should appear at threshold, where
the particle velocity matches the phase velocity of light. Here we deduce
theoretically that light bullets [Y. Silberberg, Opt. Lett. 15, 1282 (1990)]
emit an analogous radiation with exactly the same spectral discontinuity for
point-like sources. For extended sources the discontinuity turns into a
spectral peak at threshold. We argue that this Cherenkov radiation has been
experimentally observed in the first attempt to measure Hawking radiation in
optics [F. Belgiorno et al., Phys. Rev. Lett. 105, 203901 (2010)] thus giving
experimental evidence for a puzzle in Cherenkov radiation instead.
