fluctuations, and averaging over an ensemble of realizations. That is called the truncated Wigner approximation (TWA). In this paper we do not include the fluctuations, because our purpose here was to understand first the mean field behavior entailed by the experimental conditions.

Our study has revealed the importance, in configurations similar to those in the experiment, of two features that are not present in previous, idealized studies of sonic analog black holes: (i) the condensate density increases towards the center of the atom trap and, (ii) as a result of this inhomogeneity, and the shape of the trap potential, there is no single inertial frame in which the condensate is stationary. A key consequence of the latter feature is that the white hole horizon, defined in the locally relevant sense explained below, is not at rest with respect to the black hole horizon.

As a result of these features, growth of a standing wave and emission of Hawking radiation both occur purely hydrodynamically, in a way that appears to be similar to what was observed in the experiment. As described below, we have identified the mechanisms producing these simulated phenomena, and we find that the black hole laser mechanism plays 