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Observations of very high energy gamma-rays from blazars provide informationabout acceleration mechanisms occurring in their innermost regions. Studies ofvariability in these objects allow a better understanding of the mechanisms atplay. To investigate the spectral and temporal variability of VHE (>100 GeV)gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range offlux states. Data collected from 2005 to 2007 are analyzed. Spectra are derivedon time scales ranging from 3 years to 4 minutes. Light curve variability isstudied through doubling timescales and structure functions, and is comparedwith red noise process simulations. The source is found to be in a low statefrom 2005 to 2007, except for a set of exceptional flares which occurred inJuly 2006. The quiescent state of the source is characterized by an associatedmean flux level of 4.32 +/-0.09 x 10^-11 cm^-2 s^-1 above 200 GeV, orapproximately 15% of the Crab Nebula, and a power law photon index of 3.53+/-0.06. During the flares of July 2006, doubling timescales of ~2 min arefound. The spectral index variation is examined over two orders of magnitude influx, yielding different behaviour at low and high fluxes,which is a newphenomenon in VHE gamma-ray emitting blazars. The variability amplitudecharacterized by the fractional r.m.s. is strongly energy-dependent and isproportional to E^(0.19 +/- 0.01). The light curve r.m.s. correlates with theflux. This is the signature of a multiplicative process which can be accountedfor as a red noise with a Fourier index of ~2. This unique data set showsevidence for a low level gamma-ray emission state from PKS 2155-304, whichpossibly has a different origin than the outbursts. The discovery of the lightcurve lognormal behaviour might be an indicator ..

VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability