Airborne Inter-Calibration of H.E.S.S. Telescopes and Diffuse Very-High-Energy Gamma-Ray Emission in the Large Magellanic Cloud

The High Energy Stereoscopic System (H.E.S.S.) in Namibia is one of the three major Imaging Atmospheric Cherenkov Telescope (IACT) arrays currently in operation. As such, it is designed to indirectly detect photons at the highest energies currently observable, called gamma-rays. As the five telescopes of H.E.S.S. work together as an array, it is not only necessary to calibrate them individually, but also to inter-calibrate them. The first part of this thesis proposes a novel method to determine the relative optical efficiencies of the telescopes of the H.E.S.S. array. It is based on an unmanned aerial vehicle positioned inside the field of view of all the telescopes on which a pulsed light source illuminating them is mounted. The method permits, unlike the one used so far, to monitor the wavelength dependent evolution of the efficiencies. Based on the results of a first campaign, this thesis shows that we are indeed able to inter-calibrate the H.E.S.S. telescopes and, in addition, to verify the time stamping and pointing corrections of H.E.S.S., which are necessary as the pointing is imperfect due to the deformation of the ground and the telescope structure due to its weight. The second part of this thesis is about the search for interstellar diffuse gamma-ray emission produced in the Large Magellanic Cloud. The LMC is a dwarf galaxy which, as the Milky Way, does not only contain stars, but also cosmic rays (mostly electrons and protons) and interstellar matter. Beside the astrophysical sources, one also expects diffuse interstellar emission produced by the interactions of these particles with the interstellar gas and the radiation fields. I use a novel background subtraction method based on a run-by-run Monte Carlo simulation ('run-wise' simulation) of the expected background which is subtracted from the observed data. I fit the residual emission to the data with a maximum likelihood. This leads to the emergence of an extended emission component near N 157B, which could be attributed to diffuse emission, and would so constitute its first detection with an IACT. (author)