  The space-time focusing of a (continuous) succession of localized X-shaped
pulses is obtained by suitably integrating over their speed, i.e., over their
axicon angle, thus generalizing a previous (discrete) approach. First, new
Superluminal wave pulses are constructed, and then tailored in such a wave to
get them temporally focused at a chosen spatial point, where the wavefield can
reach for a short time very high intensities. Results of this kind may find
applications in many fields, besides electromagnetism and optics, including
acoustics, gravitation, and elementary particle physics.
  PACS nos.: 41.20.Jb; 03.50.De; 03.30.+p; 84.40.Az; 42.82.Et; 83.50.Vr;
62.30.+d; 43.60.+d; 91.30.Fn; 04.30.Nk; 42.25.Bs; 46.40.Cd; 52.35.Lv.
  Keywords: Localized solutions to Maxwell equations; Superluminal waves;
Bessel beams; Limited-diffraction pulses; Finite-energy waves; Electromagnetic
wavelets; X-shaped waves; Electromagnetism; Microwaves; Optics; Special
relativity; Localized acoustic waves; Seismic waves; Mechanical waves;
Elementary particle physics; Gravitational waves
