Observations of selected AGN with HESS

Abstract

A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (HESS), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) γ-ray emitters range from z = 0.00183 to z = 0.333. Significant detections were already reported for some of these objects, such as PKS 2155-304 and Markarian 421. Marginal evidence (3.1σ) for a signal is found from large-zenith-angle observations of Markarian 501, corresponding to an integral flux of I(>1.65 TeV) = (1.5±0.6 stat±0.3syst) × 10-12 cm -2 s-1 or ∼15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ∼1 to ∼8 h live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects. © ESO 2005.