  A model of steady-state X-shaped wave generation by a superluminal
(supersonic) pointlike source infinitely moving along a straight line is
extended to a more realistic causal scenario of a source pulse launched at time
zero and propagating rectilinearly at constant superluminal speed. In the case
of infinitely short (delta) pulse, the new model yields an analytical solution,
corresponding to the propagation-invariant X-shaped wave clipped by a
droplet-shaped support, which perpetually expands along the propagation and
transversal directions, thus tending the droplet-shaped wave to the X-shaped
one.
