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Abstract:
Very-high-energy (VHE) γ-ray astronomy has opened a unique window into the Universe of the highest energies, allowing one to efficiently investigate the question of the origin of high-energy cosmic rays. The H.E.S.S. array of four imaging atmospheric Cerenkov (IACT) telescopes is utilised in this work to search for VHE γ-rays from selected candidate extragalactic objects that are expected to contain significant populations of cosmic rays: galaxy clusters Abell 496, Abell 85, Coma cluster, Abell 754, Centaurus cluster and Hydra A, starburst galaxies NGC 253 and M 83, from an ultraluminous infrared galaxy Arp 220 and from an active galactic nucleus (AGN), RGB J0152+017. The instrument is described, giving an overview of the IACT technique, the data acquisition, data-quality determination, calibration, performance, and the actual analysis that was used to obtain
the results.
In the starburst galaxy NGC 253, a hint of a VHE signal is found at a ∼3σ significance level. No significant VHE signal is found from the remaining non-AGN objects, and upper limits are presented. The upper limits of Abell 85 and Abell 496 are used to constrain the non-thermal to thermal energy ratio, which is ∼8% for Abell 85 and is thus challenging
theoretical estimates. The AGN, RGBJ0152+017, is in this work discovered in VHE γ rays and its first-time broad-band spectral energy distribution is presented. While no
new type of a VHE emitter is established, the presented theoretical expectations are very promising for the planned CTA observatory.
