Orateur
Summary
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 blazar 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. Prospects in this field opened by future large arrays of Cherenkov
telescopes are also discussed.
