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Issue A&A
Volume 502, Number 3, August II 2009
Page(s) 749 - 770
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200912128
Published online 15 June 2009

A&A 502, 749-770 (2009)
DOI: 10.1051/0004-6361/200912128

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

F. Aharonian1, 2, A. G. Akhperjanian3, G. Anton4, U. Barres de Almeida5, A. R. Bazer-Bachi6, Y. Becherini7, B. Behera 
8, W. Benbow 
1, K. Bernlöhr1, 9, C. Boisson 
10, A. Bochow1, V. Borrel 
6, E. Brion11, J. Brucker4, P. Brun11, R. Bühler1, T. Bulik12, I. Büsching13, T. Boutelier 
14, P. M. Chadwick5, A. Charbonnier15, R. C. G. Chaves1, A. Cheesebrough5, L.-M. Chounet16, A. C. Clapson1, G. Coignet 
17, L. Costamante1, 18, 19, M. Dalton9, M. K. Daniel5, I. D. Davids20, 13, B. Degrange16, C. Deil1, H. J. Dickinson5, A. Djannati-Ataï7, W. Domainko1, L. O'C. Drury2, F. Dubois17, G. Dubus14, J. Dyks12, M. Dyrda21, K. Egberts1, D. Emmanoulopoulos8, P. Espigat7, C. Farnier22, F. Feinstein22, A. Fiasson22, A. Förster1, G. Fontaine 
16, M. Füßling9, S. Gabici2, Y. A. Gallant22, L. Gérard7, B. Giebels16, J. F. Glicenstein11, B. Glück4, P. Goret11, D. Göhring 
4, D. Hauser8, M. Hauser8, S. Heinz4, G. Heinzelmann23, G. Henri14, G. Hermann1, J. A. Hinton24, A. Hoffmann25, W. Hofmann1, M. Holleran13, S. Hoppe1, D. Horns23, A. Jacholkowska15, O. C. de Jager13, C. Jahn4, I. Jung4, K. Katarzyński26, U. Katz4, S. Kaufmann8, E. Kendziorra25, M. Kerschhaggl9, D. Khangulyan1, B. Khélifi16, D. Keogh5, W. Kluźniak12, T. Kneiske23, Nu. Komin11, K. Kosack1, G. Lamanna 
17, J.-P. Lenain10, T. Lohse9, V. Marandon7, J. M. Martin10, O. Martineau-Huynh15, A. Marcowith 
22, D. Maurin 
15, T. J. L. McComb5, M. C. Medina10, R. Moderski12, L. A. G. Monard27, E. Moulin11, M. Naumann-Godo16, M. de Naurois15, D. Nedbal28, D. Nekrassov1, J. Niemiec21, S. J. Nolan5, S. Ohm1, J.-F. Olive6, E. de Oña Wilhelmi7, 18, K. J. Orford 
5, M. Ostrowski29, M. Panter1, M. Paz Arribas9, G. Pedaletti8, G. Pelletier14, P.-O. Petrucci14, S. Pita7, G. Pühlhofer8, M. Punch7, A. Quirrenbach 
8, B. C. Raubenheimer13, M. Raue1, 18, S. M. Rayner5, M. Renaud7, 1, F. Rieger1, 18, J. Ripken23, L. Rob 
28, S. Rosier-Lees17, G. Rowell30, B. Rudak12, C. B. Rulten5, J. Ruppel31, V. Sahakian3, A. Santangelo 
25, R. Schlickeiser31, F. M. Schöck4, R. Schröder31, U. Schwanke9, S. Schwarzburg25, S. Schwemmer8, A. Shalchi31, M. Sikora12, J. L. Skilton24, H. Sol10, D. Spangler5, Ł. Stawarz29, R. Steenkamp20, C. Stegmann4, G. Superina16, A. Szostek29, 14, P. H. Tam8, J.-P. Tavernet15, R. Terrier7, O. Tibolla1, 8, M. Tluczykont23, C. van Eldik1, G. Vasileiadis22, C. Venter13, L. Venter10, J. P. Vialle17, P. Vincent15, M. Vivier11, H. J. Völk1, F. Volpe1, 16, 18, S. J. Wagner8, M. Ward5, A. A. Zdziarski12, and A. Zech10

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 $\gamma$-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 $\gamma$-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 $\gamma$-ray bands is strongly correlated, with no evidence of lags. The spectra also evolve with similar patterns, and are always soft (photon index $\Gamma$ > 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 $\gamma$-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$\times$) 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 $(L_{\rm C}/L_{\rm S}\sim 10)$ – 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.


Key words: galaxies: active -- galaxies: BL Lacertae objects: individual: PKS 2155–304 -- gamma rays: observations -- X-rays: galaxies



© ESO 2009


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