  A recent experimental claim of the detection of analogue Hawking radiation in
an optical system [PRL 105 (2010) 203901] has led to some controversy [PRL 107
(2011) 149401, 149402]. While this experiment strongly suggests some form of
particle creation from the quantum vacuum (and hence it is per se very
interesting), it is also true that it seems difficult to completely explain all
features of the observations by adopting the perspective of a Hawking-like
mechanism for the radiation. For instance, the observed photons are emitted
parallel to the optical horizon, and the relevant optical horizon is itself
defined in an unusual manner by combining group and phase velocities. This
raises the question: Is this really Hawking radiation, or some other form of
quantum vacuum radiation? Naive estimates of the amount of quantum vacuum
radiation generated due to the rapidly changing refractive index --- sometimes
called the dynamical Casimir effect --- are not encouraging. However we feel
that naive estimates could be misleading depending on the quantitative
magnitude of two specific physical effects: "pulse steepening" and "pulse
cresting". Plausible bounds on the maximum size of these two effects results in
estimates much closer to the experimental observations, and we argue that the
dynamical Casimir effect is now worth additional investigation.
