Blazars, a subclass of active galactic nuclei (AGN), are known to be bright -ray sources, frequently exhibiting active (flaring) periods. The blazar PKS~2155-304 is a high synchrotron-peaked BL Lac object located at redshift z=0. 116. On 2006 July 28, an extremely remarkable outburst of VHE -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 variability timescale of this extraordinary flare was as short as approximately 200~s. In order to guarantee the transparency of the emission region for TeV photons, the fast variability demands an extremely high Doppler factor ₃>50 of the jet within the classical one-zone model, leading to the so-called ``Doppler factor crisis''. Here we demonstrate that the stochastic dissipation model, which is a multi-blob scenario for blazars, can self-consistently explain the giant TeV flares of PKS~2155-304 and the low-state emission before and after the flares, in terms of both multi-wavelength spectral and variability characteristics. The required Doppler factor in this model can be as low as 20, which is a reasonable and typical value for blazar jets. The obtained model parameters may shed some light on the physical properties of the relativistic jet.
Tan et al. (Wed,) studied this question.