2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5852We have developed a toy model for internal shocks which has been used to generate a large number of synthetic GRBs in order to find in the parameter space the conditions which can lead to the formation of X-ray flashes. The key condition appears to be a small contrast of the Lorentz factor in the relativistic wind emitted by the central engine.4 pages, 4 figures, to appear in "Gamma-ray bursts in the afterglow era", 3rd workshop, Rome 17-20 Sept. 2002Astrophysicstext