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Simultaneous multiwavelength observations of the second exceptional gamma-ray flare of PKS2155-304 in July 2006
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2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 502, no 3, p. 749-770Article, review/survey (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2009. Vol. 502, no 3, p. 749-770
Keywords [en]
galaxies: active, galaxies: BL Lacertae objects: individual: PKS 2155-304, gamma rays: observations, X-rays: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
URN: urn:nbn:se:lnu:diva-30262DOI: 10.1051/0004-6361/200912128ISI: 000268944000004OAI: oai:DiVA.org:lnu-30262DiVA, id: diva2:706000
Available from: 2014-03-18 Created: 2013-11-08 Last updated: 2017-12-05Bibliographically approved

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Becherini, Yvonne

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