η Carinae detection and characterisation of the first colliding-wind binary in very-high-energy γ-rays with H.E.S.S

The exceptional binary star η Carinae has been fascinating scientists and the people in the Southern hemisphere alike for hundreds of years. It survived an enormous outbreak, comparable to a supernova energy-wise, and for a short period became the brightest star of the night sky. From observations from the radio regime to X-rays the system's characteristics and its emission in photon energies up to ~50 keV are well studied today. The binary is composed of two massive stars of ~30 and ~100 solar masses. Either star drives a strong stellar wind that continuously carries away a fraction of its mass. The collision of these winds leads to a shock on each side of the encounter. In the windwind- collision region plasma gets heated when it is overrun by the shocks. Part of the emission seen in X-rays can be attributed to this plasma. Above ~50 keV the emission is no longer of thermal origin: the required plasma temperature exceeds the available mechanical energy input of the stellar winds. In contrast to its observational history in thermal energies observational evidence of η Carinae's non-thermal emission has only recently built up. In high-energy γ-rays η Carinae is the only binary of its kind that has been detected unambiguously. Its energy spectrum reaches up to ~ hundred GeV, a regime where satellite-based γ-ray experiments run out of statistics. Ground-based γ-ray experiments have the advantage of large photon collection areas. H.E.S.S. is the only γ-ray experiment located in the Southern hemisphere and thus able to observe η Carinae in this energy range. H.E.S.S. measures γ-rays via electromagnetic showers of particles that very-high-energy γ-rays initiate in the atmosphere. The main challenge in observations of η Carinae with H.E.S.S. is the UV emission of the Carina nebula that leads to a background that is up to 10 times stronger than usual for H.E.S.S. This thesis presents the first detection of a colliding-wind binary in very-high-energy γ-rays and documents the studies that led to it. The differential γ-ray energy spectrum of η Carinae is measured up to 700 GeV. A hadronic and leptonic origin of the γ-ray emission is discussed and based on the comparison of cooling times a hadronic scenario is favoured.