Rapid Variability of Gamma-Ray Blazars: A Model for MKN 421
  The extremely rapid burst of TeV photons from Mkn 421 (15 May 1996) can be reconciled with the standard properties of a relativistic gamma-ray emitting jet (bulk Lorentz factor $\approx$ 10; size $\10^{17}$ cm) if one assumes that the electrons are accelerated in conical shocks with both opening and viewing angles $\approx 1/\Gamma$. If the injection time and the cooling time are much less than the photon crossing time, an emission ring moves along the jet and leads to the appearance of a very rapid flare, in satisfactory agreement with the observations. 
M.Salvati, M.Spada, F.Pacini