Probing particle acceleration in stellar binary systems using gamma-ray observations

Only a few gamma-ray sources have been established as proton accelerators over the last decades, among them two extraordinary binary systems, the massive colliding wind binary Eta Carinae (eta Car) and the recurrent nova RS Ophiuchi (RS Oph). In this thesis, the nature of acceleration processes up to TeV energies are probed in these systems using very-high energy (VHE; ≥ 100 GeV) gamma-ray data from the High Energy Stereoscopic System (H.E.S.S.) in conjunction with data from the Fermi Large Area Telescope (LAT). To obtain reliable results from Imaging Atmospheric Cherenkov Telescopes like H.E.S.S., an accurate match between simulations and actual observations is crucial. Thus, in the first part of this thesis the successful validation of the simulations of the full 5-telescope H.E.S.S. array is presented. Based on this, the scientific verification of the monoscopic analysis was achieved using data from the large 28 m-telescope recently upgraded with a FlashCam prototype. The resulting spectrum of the Crab Nebula, the standard candle in the VHE regime, is found to be in good agreement with previous measurements by H.E.S.S. and other instruments. Using this verified analysis configuration, the nova RS Oph was successfully detected during its 2021 outburst, making it the first nova with confirmed VHE emission. A detailed light curve was derived from the highly statistically significant gamma-ray signal observed with the full H.E.S.S. array. The combined properties of the H.E.S.S. measurements with simultaneous Fermi-LAT data clearly favor a common origin of the whole gamma-ray emission, implying efficient acceleration of hadrons at the external shock caused by the eruption. eta Car has been firmly established as a source of gamma-rays by Fermi-LAT and H.E.S.S. over the last decade. With its highly eccentric orbit lasting 5.5 years, the periastron passage of the two stars is extremely close, making it a particularly interesting phase range. The 2020 periastron passage was the first such event to be extensively monitored by H.E.S.S. In this thesis, the detection of a VHE signal from eta Car during the 2020 periastron is presented, making use of a novel time-based image cleaning technique for the monoscopic analysis. In combination with simultaneous Fermi-LAT data, its spectral properties are characterized and together with previous and follow-up observations, for the first time, a VHE light curve spanning a full orbit is derived. At least some fraction of the accelerated particles traced by the gamma-ray emission likely escape from eta Car, potentially interacting with target material on different spatial scales. With the detection of Fermi-LAT excess emission associated to molecular clouds in the Carina Nebula, this hypothesis is tested. Whereas the cosmic ray density profile is indicative of an origin of the interacting cosmic rays from eta Car, a larger escaping flux than predicted by models or a contribution from other cosmic ray sources is needed to match the measured flux