Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-01-16
Astronomy and Astrophysics
Astrophysics
Minor changes, accepted for publication in MNRAS Letters
Scientific paper
10.1111/j.1745-3933.2008.00454.x
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.
Ghisellini Gabriele
Tavecchio Fabrizio
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