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

### Abstract

To appear in proceedings of 29th International Cosmic Ray Conference (ICRC 2005)International audienceThe 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)

Topics: [PHYS.ASTR.CO] Physics [physics]/Astrophysics [astro-ph]/Cosmology and Extra-Galactic Astrophysics [astro-ph.CO], [SDU.ASTR] Sciences of the Universe [physics]/Astrophysics [astro-ph]
Publisher: International Union of Pure and Applied Physics
Year: 2005
OAI identifier: oai:HAL:in2p3-00025646v1
Provided by: Hal-Diderot