Towards an Understanding of Galactic Ultra-high Energy γ-ray Emission

Just recently, we are starting to explore the γ-ray sky at energies above 100 TeV. A detailed understanding of the emission at these extreme energies is therefore of paramount importance. γ rays at these energies are produced by highly energetic particles, the so-called cosmic rays. This thesis investigates three different aspects related to the ultra-high energy emission. The first aspect concerns the leptonic or hadronic origin of sources at these energies. While the suppression of inverse Compton emission at these energies disfavours a leptonic origin, it is shown that in environments with sufficiently high radiation energy densities or low magnetic fields, leptonic ultra-high energy emitters are possible. Furthermore, the viability of such leptonic emitters is confirmed by modelling newly detected sources in this energy regime. The second aspect concerns hadronic emission, specifically the effects of different compositions of the hadronic particles producing γ rays at these energies. These effects are thoroughly investigated in this thesis. It is shown that the presence of heavier cosmic ray species decreases the resulting emission and shifts spectral features to lower energies. The influence of different compositions on the diffuse Galactic emission at ultra-high energies is investigated. For this use case, the composition can have an important influence on the resulting γ-ray and neutrino production. The models are compared to current data. Although current measurements do not allow to constrain the composition of the Galactic cosmic rays, future observations will be able to do so. For the third aspect, the particle acceleration and γ-ray emission by colliding stellar winds is investigated. This is done in the case of the colliding wind binary η Carinae. The developed time-dependent model is able to explain the resulting flux and variability properties of the emission detected from η Carinae. The γ-ray emission is likely produced by the collisions of accelerated hadronic particles and not by electrons. The variability in the X-ray emission is explained by the inhibition of electron injection and heating during the closest approach of the two stars