VHE Gamma-ray sources at the resolution limit of H.E.S.S

Humboldt Universität, Dissertation, 2015; Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 131 pp. (2015). doi:10.18452/17517 = Humboldt Universität, Dissertation, 2015 Very-high-energy (VHE)γ-ray astronomy deals with the ground-based detection of pho-tons with energies of a few tens of GeV to∼100 TeV by employing the Imaging Air Cherenkov Technique (IACT). This method uses the atmosphere as a detector for VHE γ-rays, exploiting that photons in that energy range produce particle showers by inter-acting with atmospheric particles. The particle showers, in turn, emit Cherenkov light, which is collected by telescopes with large mirror areas to image the particle shower. Theproperties of theγ-rays, specifically their energy and direction, can be deduced from the shower images. However, the interactions in the atmosphere are statistical processes, imposing a natural limit on the direction reconstruction with IACT experiments. In addition, the direction reconstruction is limited by the detection efficiency of Cherenkov photons of the telescope. The quality of the direction reconstruction depends on the energy of the primary particle, the telescope properties, observational conditions and reconstruction algorithm. The precision of the direction reconstruction of single photons is characterized as the angular resolution. In this work, the angular resolution of H.E.S.S. , an IACT experiment located in Namibia, is studied in detail. H.E.S.S. consists of five telescopes, four of which were built for the energy range above a few hundreds of GeV and started operating in 2004. For this sub-array, the systematic errors on the angular resolution and their dependence on observation parameters are estimated from known point sources in H.E.S.S. data and from Monte-Carlo simulations. A mismatch between H.E.S.S. data andMonte-Carlo simulations is quantified and the simulated angular resolution is corrected forit. With the correction, two phenomena that require a profound knowledge of the angular resolution are assessed. First, the size of the the Crab Nebula at VHE is investigated. The Crab Nebula is one of the best-studied objects beyond our solar system and was the first source detected inVHE γ-rays. Models predict a size of the emission region of 0.009◦ to ∼ 0.03◦ in the energy range observed with H.E.S.S. In this work, a size of $(0.01±0.002_{(stat)}±0.015_{(syst)})$ deg isfound, i.e. the source is not significantly extended. Including a detailed accounting of the systematic errors, a conservative upper limit on the size of the VHE γ-ray emission region of the Crab is given by 0.034◦ at a 95% confidence level. Second, extended emission around Active Galactiv Nuclei(AGN) is searched for. AGN aredistant objects with a supermassive black hole as their central engine. Due to interactions with the cosmic microwave background and the extragalactic background light, the particles emitted by AGN are expected to form cascades. The effect of the cascades on the observable size of the VHE emission region of AGN depends on the magnetic field.Two scenarios for different magnetic field strengths are probed with H.E.S.S. data, pair halo emission and beam-broadened cascade emission. The most constraining upper lim-its on the pair halo scenario are found, compared to published figures. Employing the beam-broadened cascade scenario, extra-galactic magnetic field strengths in the range of (0.1−10)·10$^{−15}$ G are excluded at a 99% confidence level. All of the studies are conducted for two models of the extragalactic background light, in analogy to H.E.S.S. Collaboration et al. (2014b) which was published in the scope of this thesis. Finally, an outlook on the angular resolution of the future IACT experiment CTA and its ability to measure the phenomena mentioned here is given. Published by Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät