VHE gamma -ray emission of PKS2155-304: spectral and temporal variability

Context. Observations of very high-energy gamma -rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma -rays of the well-known high-frequency-peaked BLLac object PKS2155-304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 plus or minus 0.09 sub(stat) plus or minus 0.86 sub(syst)) x 10 super(-11) cm super(-2) s super(-1) above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Gamma = 3.53 plus or minus 0.06 sub(stat) plus or minus 0.10 sub(syst). During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE gamma -ray emitting blazars. The variability amplitude characterized by the fractional rms F sub(var) is strongly energy-dependent and is alpha E super(0.10 plus or minus 0.01). The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of ~2. Conclusions. This unique data set shows evidence of a low-level gamma -ray emission state from PKS2155-304 that possibly has a different origin than the outbursts. The discovery of the light curve lognormal behavior might be an indicator of the origin of aperiodic variability in blazars.