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

F. Aharonian, A. G. Akhperjanian, G. Anton, U. Barres De Almeida, A. R. Bazer-Bachi, Y. Becherini, B. Behera, W. Benbow, K. Bernlöhr, C. Boisson, A. Bochow, V. Borrei, E. Brion, J. Brucker, P. Brun, R. Bühler, T. Bulik, I. Büsching, T. Boutelier, P. M. ChadwickA. Charbonnier, R. C G Chaves, A. Cheesebrough, L. M. Chounet, A. C. Clapson, G. Coignet, L. Costamante, M. Dalton, M. K. Daniel, I. D. Davids, B. Degrange, C. Deil, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O C Drury, F. Dubois, G. Dubus, J. Dyks, M. Dyrda, K. Egberts, D. Emmanoulopoulos, P. Espigat, C. Farnier, F. Feinstein, A. Fiasson, A. Förster, G. Fontaine, M. Füßling, S. Gabici, Y. A. Gallant, L. Gérard, B. Giebels, J. F. Glicenstein, B. Glück, P. Goret, D. Göhring, D. Hauser, M. Hauser, S. Heinz, G. Heinzelmann, G. Henri, G. Hermann, J. A. Hinton, A. Hoffmann, W. Hofmann, M. Holleran, S. Hoppe, D. Horns, A. Jacholkowska, O. C. De Jager, C. Jahn, I. Jung, K. Katarzyński, U. Katz, S. Kaufmann, E. Kendziorra, M. Kerschhaggl, D. Khangulyan, B. Khélifi, D. Keogh, W. Klužniak, T. Kneiske, Nu Komin, K. Kosack, G. Lamanna, J. P. Lenain, T. Lohse, V. Marandon, J. M. Martin, O. Martineau-Huynh, A. Marcowith, D. Maurin, T. J L McComb, M. C. Medina, R. Moderski, L. A G Monard, E. Moulin, M. Naumann-Godo, M. De Naurois, D. Nedbal, D. Nekrassov, J. Niemiec, S. J. Nolan, S. Ohm, J. F. Olive, E. De Oña Wilhelmi, K. J. Orford, M. Ostrowski, M. Panter, M. Paz Arribas, G. Pedaletti, G. Pelletier, P. O. Petrucci, S. Pita, G. Piihlhofer, M. Punch, A. Quirrenbach, B. C. Raubenheimer, M. Raue, S. M. Rayner, M. Renaud, F. Rieger, J. Ripken, L. Rob, S. Rosier-Lees, G. Rowell, B. Rudak, C. B. Rulten, J. Ruppel, V. Sahakian, A. Santangelo, R. Schlickeiser, F. M. Schöck, R. Schröder, U. Schwanke, S. Schwarzburg, S. Schwemmer, A. Shalchi, M. Sikora, J. L. Skilton, H. Sol, D. Spangler, Stawarz, R. Steenkamp, C. Stegmann, G. Superina, A. Szostek, P. H. Tam, J. P. Tavernet, R. Terrier, O. Tibolla, M. Tluczykont, C. Van Eldik, G. Vasileiadis, C. Venter, L. Venter, J. P. Vialle, P. Vincent, M. Vivier, H. J. Völk, F. Volpe, S. J. Wagner, M. Ward, A. A. Zdziarski, A. Zech

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113 Scopus citations


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 2155304 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 T > 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 (∼22x) 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 (LcJLa ∼ 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.

Original languageEnglish (US)
Pages (from-to)749-770
Number of pages22
JournalAstronomy and Astrophysics
Issue number3
StatePublished - Aug 2009


  • Galaxies: BL lacertae objects: individual: PKS 2155-304
  • Galaxies: active
  • Gamma rays: observations
  • X-rays: galaxies


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