Simultaneous multiwavelength observations of the second exceptional γ-ray flare of PKS 2155–304 in July 2006

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany e-mail: luigi.costamante@stanford.edu; Rolf.Buehler@mpi-hd.mpg.de
² Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
³ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁴ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁵ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁶ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁷ Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13; UMR 7164, CNRS, Université Paris VII, CEA, Observatoire de Paris, France
⁸ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁰ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹¹ IRFU/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette, Cedex, France
¹² Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹³ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹⁴ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁵ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252 Paris Cedex 5, France
¹⁶ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁷ Laboratoire d'Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue, BP 110, 74941 Annecy-le-Vieux Cedex, France
¹⁸ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁹ Stanford University, W.W. Hansen Experimental Physics Laboratory & Kavli Institute for Particle Astrophysics and Cosmology, Stanford, CA 94305-4085, USA
²⁰ University of Namibia, Private Bag 13301, Windhoek, Namibia
²¹ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²² Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
²³ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
²⁴ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
²⁵ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁶ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
²⁷ Bronberg Observatory, CBA Pretoria, PO Box 11426, Tiegerpoort 0056, South Africa
²⁸ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00
²⁹ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³⁰ School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
³¹ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

Received: 21 March 2009
Accepted: 17 April 2009

Abstract

Aims. The X-ray–TeV connection and the evolution of the emitting particle population is studied in high-energy peaked BL Lac objects, by obtaining spectral information in both bands on sub-hour timescales.

Methods. Simultaneous observations with HESS, Chandra and the Bronberg optical observatory were performed on the BL Lac object PKS 2155–304 in the night of July 29–30 2006, when the source underwent a major γ-ray outburst during its high-activity state of Summer 2006. This event took place about 44 h after the other major outburst of the night of July 27–28, which is known for its ultrafast variability. An unprecedented 6 to 8 h of simultaneous, uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2 min timescales, respectively.

Results. The source exhibited one major flare along the night, at high energies. The γ-ray flux reached a maximum of ~11 times the Crab flux (>400 GeV), with rise/decay timescales of ~1 h, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE γ-ray bands is strongly correlated, with no evidence of lags. The spectra also evolve with similar patterns, and are always soft (photon index Γ > 2), indicating no strong shift of the peaks in the spectral energy distribution towards higher energies. Only at the flare maximum is there evidence that the γ-ray peak is inside the observed passband, at ~400–600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (~22) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in a high-energy peaked BL Lac object a large Compton dominance – rapidly evolving – and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.