A multiwavelength view of the flaring state of PKS 2155-304 in 2006^⋆

¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
² Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ National Academy of Sciences of the Republic of Armenia, Yerevan
⁶ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁷ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁸ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
⁹ CEA Saclay, DSM/IRFU, 91191 Gif-Sur-Yvette Cedex, France
¹⁰ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
¹¹ Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis Diderot, 10, rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13 ; UMR 7164: CNRS, Université Paris VII, CEA, Observatoire de Paris, France
¹² Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹³ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁴ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹⁵ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁶ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France
e-mail: andreas.zech@obspm.fr; catherine.boisson@obspm.fr
¹⁷ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252 Paris Cedex 5, France
¹⁸ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁹ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁰ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
²¹ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
²² Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²³ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
²⁴ UJF-Grenoble 1, CNRS-INSU, Institut de Plantologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble, France
²⁵ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
²⁶ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²⁷ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
²⁸ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁹ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³⁰ School of Chemistry & Physics, University of Adelaide, 5005 Adelaide, Australia
³¹ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³² School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
³³ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG  
³⁴ Oskar Klein Centre, Department of Physics, Royal Institute of Technology (KTH), Albanova, 10691 Stockholm, Sweden
³⁵ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, Station de radioastronomie de Nançay, 5 pl. Jules Janssen, 92195 Meudon, France
³⁶ Universitat de Valencia, Poligono la Coma s/n, Paterna, 46980 Valencia, Spain
³⁷ College of Engineering, Mathematical & Physical Sciences, School of Physics, UCD Science Centre, Belfield, Dublin 4, Irlande
³⁸ Bronberg Observatory, CBA Pretoria, PO Box 11426, Tiegerpoort 0056, South Africa
³⁹ Hartebeesthoek Radio Astronomy Observatory (HartRAO), PO Box 443, 1740 Krugersdorp, South Africa
⁴⁰ CSIRO Australia Telescope National Facility, Locked Bag 194, 2390 Narrabri NSW, Australia
⁴¹ GEPI, Observatoire de Paris, CNRS, Station de radioastronomie de Nançay, 5 pl. Jules Janssen, 92195 Meudon, France

Received: 17 June 2011
Accepted: 16 January 2012

Abstract

Context. Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July and August 2006, the period when two exceptional flares at very high energies (VHE, E ≳ 100 GeV) occurred, provide a detailed picture of the evolution of its emission. The complete data set from this campaign is presented, including observations in VHE γ-rays (H.E.S.S.), X-rays (RXTE, Chandra, Swift XRT), optical (Swift UVOT, Bronberg, Watcher, ROTSE), and in the radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to 2008 are compared to the available VHE data from this period, to put the 2006 campaign into the context of the long-term evolution of the source.

Aims. The data set offers a close view of the evolution of the source on different time scales and yields new insights into the properties of the emission process. The predictions of synchrotron self-Compton (SSC) scenarios are compared to the MWL data, with the aim of describing the dominant features in the data down to the hour time scale.

Methods. The spectral variability in the X-ray and VHE bands is explored and correlations between the integral fluxes at different wavelengths are evaluated. SSC modelling is used to interpret the general trends of the varying spectral energy distribution.

Results. The X-ray and VHE γ-ray emission are correlated during the observed high state of the source, but show no direct connection with longer wavelengths. The long-term flux evolution in the optical and radio bands is found to be correlated and shows that the source reaches a high state at long wavelengths after the occurrence of the VHE flares. Spectral hardening is seen in the Swift XRT data.

Conclusions. The nightly averaged high-energy spectra of the non-flaring nights can be reproduced by a stationary one-zone SSC model, with only small variations in the parameters. The spectral and flux evolution in the high-energy band during the night of the second VHE flare is modelled with multi-zone SSC models, which can provide relatively simple interpretations for the hour time-scale evolution of the high-energy emission, even for such a complex data set. For the first time in this type of source, a clear indication is found for a relation between high activity at high energies and a long-term increase in the low frequency fluxes.