Short-term VHE variability in blazars: PKS 2155-304

Context. The γ-ray blazar PKS 2155-304 has attracted considerable attention because of its extreme TeV variability characteristics during an exceptional flaring period in 2006. Among the observed key findings are (i) a minimum variability timescale as short as ~200 s and (ii) highly variable TeV emission, which in the frequency interval [ 10-4 Hz, 10-2 Hz] can be described by a log-normal distribution and suggests an underlying multiplicative (and not additive) process. Aims. Simultaneously accounting for these findings appears difficult within conventional approaches. Following earlier suggestions for the TeV blazar Mkn 501, we explore a possible scenario where PKS 2155-304 is supposed to harbor a supermassive binary black hole system and where the observed TeV variability is dominated by emission from the less massive black hole. Methods. We analyze the constraints on the very high energy (VHE) source imposed by the observed variability characteristics and the integrated VHE luminosity output, and discuss its implications for a binary black hole system. Results. We show that for a secondary mass of mBH ~ 107 $M_{\odot}$, fluctuations in the disk accretion rate that feed the jet could account for the observed red-noise type variability process down to frequencies of ~10-2 Hz. Jet curvature induced by orbital motion, on the other hand, could further relax constraints on the intrinsic jet speeds. Conclusions. Because a binary system can lead to different (yet not independent) periodicities in different energy bands, a longterm (quasi-) periodicity analysis could offer important insights into the real nature of the central engine of PKS 2155-304