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

Abstract : The High Energy Stereoscopic System (H.E.S.S.) is one of the three major Imaging Atmospheric Cherenkov Telescope (IACT) arrays currently in operation and is located in the Khomas Highlands of Namibia at an altitude of 1800 m above sea level. As such, it is designed to indirectly detect photons at the highest energies currently observable, called gamma-rays, originating from the most violent cosmic phenomena such as supernovae, pulsars, accreting black holes and gamma-ray bursts. H.E.S.S. is constituted of five telescopes, four of them having a segmented hexagonal mirror with a diameter of 12 m and one with an almost rectangular shape with a diameter of 28 meter. As these multiple telescopes 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 (UAV) 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. This will be important for future arrays constituted of multiple telescope types and expected to have an unprecedented sensitivity and accuracy such as the future Cherenkov Telescope Array (CTA). This thesis discusses the inter-calibration results of a first successful campaign and shows that we are indeed able to inter-calibrate the H.E.S.S. telescopes. In addition, it shows that it is possible to use the same UAV observation data 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 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 emission produced by the interactions of these particles with the interstellar gas and the radiation fields. This diffuse emission constitutes the main observational signal detected by gamma-ray satellites sensitive to the lower end of the gamma-ray spectrum (GeV energies). The study of this diffuse emission is particularly interesting since it allows to characterise all the processes involved in its production and so provides among other an additional handle on the cosmic ray spectrum, the gas distribution inside galaxies and the population of accelerated electrons near potentially undiscovered sources, but also since it constitutes a background for all source detections. I use a novel background subtraction method, initially developed for the detection of diffuse emission in the Milky Way, based on a run-by-run Monte Carlo simulation including all the relevant observation parameters and conditions "run-wise" simulation) of the expected background. I then subtract this fully simulated background from the observed data after having determined its normalisation with a maximum likelihood fit. Finally, I fit the residual emission to the data with a maximum likelihood (keeping the known gamma-ray sources under a mask). 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. Alternatively, this component might be due to the collective effect of unresolved sources.
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Jacques Muller. Airborne Inter-Calibration of H.E.S.S. Telescopes and Diffuse Very-High-Energy Gamma-Ray Emission in the Large Magellanic Cloud. Astrophysics [astro-ph]. Institut Polytechnique de Paris, 2021. English. ⟨NNT : 2021IPPAX125⟩. ⟨tel-03541344⟩

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