The emission of blazars in very - high - energy gamma - rays viewed as a random stationary process : the case of PKS 2155 - 304

With the advent of the new generation of Cherenkov telescopes such as H.E.S.S., MAGIC, CANGAROO III and VERITAS, about 20 active galactic nuclei belonging to the blazar category have been clearly detected at very high energies. Spectacular flares were observed in a few of them, such as Mkn 421, Mkn 501 and PKS 2155-304, during which gamma-ray fluxes could be sampled with time intervals of a few minutes, an important asset in the quest of a smallest time-scale in the underlying phenomena. In the very-high-energy domain, the experimental situation is however more tricky than in variability studies carried out by X-ray satellites: the time resolution critically depends on the flux itself, due to both photon statistics and background contamination, and the continuous time series are never longer than a few hours. Nevertheless, a description of the observed phenomena by a random stationary process characterized by a simple power density spectrum — a power law of frequency — can be investigated. Such a study requires simulations of very long time series in which the experimental effects are taken into account. Using H.E.S.S. observations of PKS 2155-304 in July 2006, it is shown that different observables, namely excess variances measured over different durations as well as Kolmogorov structure functions, can be consistently accounted for by a simple log-normal process.