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