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

¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
² Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany e-mail: francesca.volpe@mpi-hd.mpg.de; volpe@llr.in2p3.fr
⁴ Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
⁵ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁶ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁷ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁸ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁹ Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis Diderot, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France (UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris)) e-mail: santiago.pita@apc.univ-paris7.fr
¹⁰ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹¹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹² LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹³ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252 Paris Cedex 5, France
¹⁴ CEA Saclay, DSM/IRFU, 91191 Gif-Sur-Yvette Cedex, France
¹⁵ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁶ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹⁷ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁸ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁹ Laboratoire d'Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
²⁰ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²¹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
²² University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
²³ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁴ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²⁵ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
²⁶ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²⁷ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
²⁸ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁹ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³⁰ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³¹ School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
³² Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³³ Oskar Klein Centre, Department of Physics, Royal Institute of Technology (KTH), Albanova, 10691 Stockholm, Sweden
³⁴ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 22 March 2010
Accepted: 14 May 2010

Abstract

Context. 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 ± 0.09_stat ± 0.86_syst) × 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 ± 0.06_stat ± 0.10_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 γ-ray emitting blazars. The variability amplitude characterized by the fractional rms F_var 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.