H.E.S.S. observations of PKS 2155-304

Abstract

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been\ndetected with high significance (~45 sigma) at energies greater than 160 GeV,\nusing the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in\nNamibia. A strong signal is found in each of the data sets corresponding to the\ndark periods of July and October, 2002, and June-September, 2003. The observed\nflux of VHE gamma rays shows variability on time scales of months, days, and\nhours. The monthly-averaged integral flux above 300 GeV varies between 10% and\n60% of the flux observed from the Crab Nebula. Energy spectra are measured for\nthese individual periods of data taking and are characterized by a steep power\nlaw with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square\nper degree of freedom is found when either a power law with an exponential\ncutoff energy or a broken power law are fit to the time-averaged energy\nspectrum. However, the significance of the improvement is marginal (~2 sigma).\nThe suggested presence of features in the energy spectrum may be intrinsic to\nthe emission from the blazar, or an indication of absorption of TeV gamma rays\nby the extragalactic infrared background light.\n