Rapid Variability of Gamma-Ray Blazars: A Model for Markarian 421

Abstract
The extremely rapid burst of TeV photons from Markarian 421 (1996 May 15) can be reconciled with the standard properties of a relativistic γ-ray-emitting jet (bulk Lorentz factor Γ~10; size ~1017 cm) if one assumes that the electrons are accelerated in conical shocks with both opening and viewing angles ≈ 1/Γ. 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.
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