Since 2002 the VHE ( > 100 GeV ) \gamma -ray flux of the high-frequency peaked BL Lac PKS 2155 - 304 has been monitored with the High Energy Stereoscopic System ( HESS ) .
An extreme \gamma -ray outburst was detected in the early hours of July 28 , 2006 ( MJD 53944 ) .
The average flux above 200 GeV observed during this outburst is \sim 7 times the flux observed from the Crab Nebula above the same threshold .
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 .