Abstract
The Wide Field Imager (WFI) is one of two focal plane instruments of the Advanced Telescope for High-Energy Astrophysics (Athena), ESA’s next large x-ray observatory, planned for launch in the early 2030s. The current baseline halo orbit is around L2, and the second Lagrangian point of the Sun-Earth system L1 is under consideration. For both potential halo orbits, the radiation environment, solar and cosmic protons, electrons, and He-ions will affect the performance of the instruments. A further critical contribution to the instrument background arises from the unfocused cosmic hard x-ray background. It is important to understand and estimate the expected instrumental background and to investigate measures, such as design modifications or analysis methods, which could improve the expected background level to achieve the challenging scientific requirement (<5 × 10 − 3 counts / cm2 / keV / s at 2 to 7 keV). Previous WFI background simulations done in Geant4 have been improved by taking into account new information about the proton flux at L2. In addition, the simulation model of the WFI instrument and its surroundings employed in Geant4 simulations has been refined to follow the technological development of the WFI camera.