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Blazars, a subclass of active galactic nuclei (AGN), are known to be bright$\gamma$-ray sources, frequently exhibiting active (flaring) periods. Theblazar PKS~2155-304 is a high synchrotron-peaked BL Lac object located atredshift $z=0.116$. On 2006 July 28, an extremely remarkable outburst of VHE$\gamma$-ray emission from this blazar was reported by the H.E.S.S. experiment,with an average flux more than 10 times the low-state level. The variabilitytimescale of this extraordinary flare was as short as approximately 200~s. Inorder to guarantee the transparency of the emission region for TeV photons, thefast variability demands an extremely high Doppler factor $\delta_{\rm D}>50$of the jet within the classical one-zone model, leading to the so-called``Doppler factor crisis''. Here we demonstrate that the stochastic dissipationmodel, which is a multi-blob scenario for blazars, can self-consistentlyexplain the giant TeV flares of PKS~2155-304 and the low-state emission beforeand after the flares, in terms of both multi-wavelength spectral andvariability characteristics. The required Doppler factor in this model can beas low as 20, which is a reasonable and typical value for blazar jets. Theobtained model parameters may shed some light on the physical properties of therelativistic jet.

Revisiting the TeV flare of PKS 2155-304 in 2006