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dc.contributor.authorNicholas, B.-
dc.contributor.authorRowell, G.-
dc.identifier.citationAstronomy and Astrophysics: a European journal, 2010; 520(13):1-16-
dc.description.abstractContext. Observations of very high-energy γ-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) γ-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 of the Crab Nebula, and a power-law photon index of Γ = 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 γ-ray emitting blazars. The variability amplitude characterized by the fractional rms Fvar is strongly energy-dependent and is . 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 γ-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.-
dc.description.statementofresponsibilityA. Abramowski... B. Nicholas... G. Rowell... et al.-
dc.publisherE D P Sciences-
dc.rights© ESO, 2010-
dc.subjectgamma rays: general-
dc.subjectgalaxies: active-
dc.subjectgalaxies: jets-
dc.subjectBL Lacertae objects: individual: PKS 2155–304-
dc.titleVHE γ-ray emission of PKS 2155-304: spectral and temporal variability-
dc.title.alternativeVHE gamma-ray emission of PKS 2155-304: spectral and temporal variability-
dc.typeJournal article-
dc.contributor.organisationH.E.S.S. Collaboration-
dc.identifier.orcidRowell, G. [0000-0002-9516-1581]-
Appears in Collections:Aurora harvest
Chemistry and Physics publications
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