Discovery of Very High Energy gamma - ray emission from the extreme BL Lac object H2356-309 with H.E.S.S.

S. Pita (1) , W. Benbow , L. Costamante , A. Djannati-Ataï (1) , D. Horns , M. Ouchrif (2) , M. Tluczykont (3)

1 APC (UMR_7164) - AstroParticule et Cosmologie
2 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
3 LLR - Laboratoire Leprince-Ringuet

The understanding of acceleration mechanisms in active galactic nuclei (AGN) jets and the measurement of the extragalactic-background-light (EBL) density are closely linked and require the detection of a large sample of very-high-energy (VHE) emitting extragalactic objects at varying redshifts. We report here on the discovery with the H.E.S.S. (High Energy Stereoscopic System) atmospheric-Cherenkov telescopes of the VHE Gamma-ray emission from H2356 - 309, an extreme BL Lac object located at a redshift of 0.165. The observations of this object, which was previously proposed as a southern-hemisphere VHE candidate source, were performed between June and December 2004. The total exposure is 38.9 hours live time, after data quality selection, which yields the detection of a signal at the level of 9.0$\sigma$ (standard deviations) .

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

S. Pita, W. Benbow, L. Costamante, A. Djannati-Ataï, D. Horns, et al.. Discovery of Very High Energy gamma - ray emission from the extreme BL Lac object H2356-309 with H.E.S.S.. 29th International Cosmic Ray Conference (ICRC 2005), Aug 2005, Pune, India. International Union of Pure and Applied Physics, pp.287-290, 2005. ⟨in2p3-00025646⟩