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

VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability,10.1051/0004-6361/201014484,Astronomy & Astrophysics,A. Abramowski,F. Aharon

VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability  
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A. Abramowski, F. Aharonian, A. G. Akhperjanian, G. Anton, U. Barres de Almeida, A. R. Bazer-Bachi, Y. Behera Becherini B, W. Benbow, K. Bernlöhr, A. Bochow, C. Boisson, J. Bolmont
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 BL Lac object PKS 2155-304 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 ± 0.09stat ± 0.86syst) × 10-11 cm-2 s-1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Gamma = 3.53 ± 0.06stat ± 0.10syst. 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 Fvar is strongly energy-dependent and is ∝ E0.19±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 PKS 2155-304 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.
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