Impact of a circumstellar medium on particle acceleration in supernova remnants

The very-high-energy gamma-ray emission observed from a number of Supernova remnants (SNRs) indicates particle acceleration to high energies at the shock of the remnants and a potentially significant contribution to Galactic cosmic rays. It is, however, difficult to determine whether protons (through hadronic interactions and subsequent pion decay) or electrons (through inverse Compton scattering on ambient photon fields) are responsible for this emission. For a successful diagnostic, a good understanding of the spatial and energy distribution of the underlying particle population is crucial. Most SNRs are created in core-collapse explosions and expand into the wind bubble of their progenitor stars. This circumstellar medium features a complex spatial distribution of gas and magnetic field which naturally strongly affects the resulting particle population. In this work, we use the RATPaC code that is designed for the time- and spatially dependent treatment of particle acceleration at SNR shocks to study the impact of a circumstellar medium on the resulting particle spectrum and subsequent non-thermal emission. We find that the complex structure of the circumstellar environment and magnetic field strongly affects the evolution of the shock, its jump conditions and particle confinement at the shock that is reflected in the particle spectrum and spatial distribution. Moreover, the interaction of the forward shock with various discontinuities in the medium triggers formation of reflected shocks that subsequently interact with both reverse and forward shocks speeding them up and boosting particle acceleration. We also discuss whether characteristic signatures in the spectrum and morphology of non-thermal emission that arise from the interaction of the SNR shock with the circumstellar medium can be detected by current and future generation experiments.