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

TL;DR

This paper discusses rapid TeV flux variability in the blazar PKS 2155-304, suggesting highly relativistic jet regions and implications for jet structure and unification models.

Contribution

It provides a detailed analysis of rapid variability, proposing that small, fast-moving active regions within larger jets explain the observations.

Findings

TeV flux changed by a factor of 2 in 3-5 minutes

Jet has a bulk Lorentz factor around 50

Jet is matter dominated and viewed at angles less than 1 degree

Abstract

Recent Cherenkov observations of BL Lac objects showed that the TeV flux of PKS 2155-304 changed by a factor 2 in just 3-5 minutes. This fast variability can be accounted for if the emitting region is moving with a bulk Lorentz factor Gamma~50 and a similar relativistic Doppler factor. If this Gamma 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 degree 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.