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

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, 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 $\sim$11 times the Crab flux (>400 GeV), with rise/decay timescales of $\sim$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 $\sim$400-600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations ($\sim$$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 emissio