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The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.116 is awell-known VHE (>100 GeV) gamma-ray emitter. Since 2002 its VHE flux has beenmonitored using the H.E.S.S. stereoscopic array of imagingatmospheric-Cherenkov telescopes in Namibia. During the July 2006 dark period,the average VHE flux was measured to be more than ten times typical valuesobserved from the object. This article focuses solely on an extreme gamma-rayoutburst detected in the early hours of July 28, 2006 (MJD 53944). The averageflux observed during this outburst is I(>200 GeV) = (1.72$\pm$$0.05_{\rmstat}$$\pm$$0.34_{\rm syst}$) $\times$ 10$^{-9}$ cm$^{-2}$ s$^{-1}$,corresponding to ~7 times the flux, I(>200 GeV), observed from the Crab Nebula.Peak fluxes are measured with one-minute time scale resolution at more thantwice this average value. Variability is seen up to ~600 s in the Fourier powerspectrum, and well-resolved bursts varying on time scales of ~200 seconds areobserved. There are no strong indications for spectral variability within thedata. Assuming the emission region has a size comparable to the Schwarzschildradius of a ~10$^9 M_\odot$ black hole, Doppler factors greater than 100 arerequired to accommodate the observed variability time scales.

An Exceptional VHE Gamma-Ray Flare of PKS 2155-304