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