The high-frequency peaked BL Lac PKS 2155 - 304 at redshift z = 0.117 has been detected with high significance ( \sim 45 \sigma ) at energies greater than 160 GeV , using the H.E.S.S .
stereoscopic array of imaging air-Cherenkov telescopes in Namibia .
A strong signal is found in each of the data sets corresponding to the dark periods of July and October , 2002 , and June-September , 2003 .
The observed flux of VHE gamma rays shows variability on time scales of months , days , and hours .
The monthly-averaged integral flux above 300 GeV varies between 10 % and 60 % of the flux observed from the Crab Nebula .
Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of \Gamma = 3.32 \pm 0.06 .
An improved \chi ^ { 2 } per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum .
However , the significance of the improvement is marginal ( \sim 2 \sigma ) .
The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar , or an indication of absorption of TeV gamma rays by the extragalactic infrared background light .