  We investigate the formation dynamics of sonic horizons in a Bose gas
confined in a (quasi) one-dimensional trap. This system is one of the most
promising realizations of the analogue gravity paradigm and has already been
successfully studied experimentally. Taking advantage of the exact solution of
the one-dimensional, hard-core, Bose model (Tonks-Girardeau gas) we show that,
by switching on a step potential, either a sonic (black-hole-like) horizon or a
black/white hole pair may form, according to the initial velocity of the fluid.
Our simulations never suggest the formation of an isolated white-hole horizon,
although a stable stationary solution of the dynamical equations with those
properties is analytically found. Moreover, we show that the semiclassical
dynamics, based on the Gross-Pitaevskii equation, conforms to the exact
solution only in the case of fully subsonic flows while a stationary solution
exhibiting a supersonic transition is never reached dynamically.
