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

TL;DR

This paper reports on simultaneous multiwavelength observations of PKS 2155-304 during its second major gamma-ray flare in 2006, revealing rapid variability, strong correlation between gamma-ray and X-ray emissions, and challenging existing emission models.

Contribution

It presents unprecedented continuous coverage and detailed spectral analysis of a major gamma-ray flare, highlighting rapid variability and a large Compton dominance in a high-frequency peaked BL Lac object.

Findings

Gamma-ray flux reached 11 times the Crab flux with hour-scale variability.

Strong correlation between VHE gamma-ray and X-ray fluxes without lags.

Discovery of a large Compton dominance and a cubic flux relation during decay.

Abstract

Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44 hours after the July 28 outburst, known for its ultrafast variability. An unprecedented 6 to 8 hours of uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2-minute timescales, respectively. The gamma-ray flux reached a maximum of 11x the Crab flux (>400 GeV), with rise/decay timescales of ~1 hour, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE bands is strongly correlated, both in flux and spectrum, with no evidence of lags. 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 an HBL a large Compton dominance (L_C/L_S ~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.