Rapid variability in TeV blazars: the case of PKS 2155—304

Recent Cherenkov observations of BL Lacertae (BL Lac) objects showed that the TeV (teraelectron volt) flux of PKS 2155−304 changed by a factor of 2 in just 3–5 min. This fast variability can be accounted for if the emitting region is moving with a bulk Lorentz factor Γ∼ 50 and a similar relativistic Doppler factor. If this Γ is adopted, several models can fit the data, but, irrespective of the chosen model, the jet is matter dominated. The Doppler factor requires viewing angles of the order of 1° or less: if the entire jet is as narrow as this, then we have problems with current unification schemes. This suggests that there are small active regions, inside a larger jet, moving faster than the rest of the plasma, occasionally pointing at us. Coordinated X-ray/TeV variability can discriminate between the different scenarios.