Author(s): Aharonian F., G. Akhperjanian A., R. Bazer-Bachi A., Chounet L.-M., Degrange B., Djannati-Ataï A., Dubus G., Espigat P., Feinstein F., Fontaine G., Gallant Y., Giebels B., Jacholkowska Agnieszka, Lemière A., Lemoine-Goumard M., Leroy N., Martineau-Huynh O., Marcowith A., De Naurois M., Ouchrif M., Pita S., Punch M., Raux J., Rolland L., Tavernet J.-P., Terrier R., Théoret C.G., Tluczykont M., Vasileiadis G., Vincent P.
(Article) Published:
Astronomy & Astrophysics, vol. 441 p.465-472 (2005)
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Ref HAL: in2p3-00024534_v1
Ref Arxiv: astro-ph/0507207
DOI: 10.1051/0004-6361:20053478
Ref. & Cit.: NASA ADS
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Abstract:
A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) gamma-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 (3.1 sigma) 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) +/- 0.3 (syst)) x 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 hrs 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.