Journal
ASTRONOMY & ASTROPHYSICS
Volume 502, Issue 3, Pages 749-770Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/200912128
Keywords
galaxies: active; galaxies: BL Lacertae objects: individual: PKS 2155-304; gamma rays: observations; X-rays: galaxies
Categories
Funding
- German Ministry for Education and Research (BMBF)
- Max Planck Society
- French Ministry for Research
- CNRS
- UK Science and Technology Facilities Council (STFC)
- Polish Ministry of Science and Higher Education
- South African Department of Science and Technology
- National Research Foundation
- University of Namibia
- National Aeronautics and Space Administration
- Science and Technology Facilities Council [PP/E001319/1, ST/G003084/1, ST/F002963/1] Funding Source: researchfish
- STFC [PP/E001319/1, ST/G003084/1, ST/F002963/1] Funding Source: UKRI
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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 similar to 11 times the Crab flux (>400 GeV), with rise/decay timescales of similar to 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 G > 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 similar to 400-600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (similar to 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 (L-C/L-S similar to 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.
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