Fast TeV variability in blazars: jets in a jet

The fast TeV variability of the blazars Mrk 501 and PKS 2155-304 implies a compact emitting region that moves with a bulk Lorentz factor of Γ_em ~ 100 towards the observer. The Lorentz factor is clearly in excess of the jet Lorentz factors Γ_j <~ 10 measured on sub-pc scales in these sources. We propose that the TeV emission originates from compact emitting regions that move relativistically within a jet of bulk Γ_j ~ 10. This can be physically realized in a Poynting flux-dominated jet. We show that if a large fraction of the luminosity of the jet is prone to magnetic dissipation through reconnection, then material outflowing from the reconnection regions can efficiently power the observed TeV flares through synchrotron-self-Compton emission. The model predicts simultaneous far-ultraviolet/soft X-ray flares.