The project centred on the development of the SIXTE (Simulation of X‑ray Telescope Events) software package, a comprehensive end‑to‑end simulation tool for the Athena X‑ray observatory. SIXTE models every stage of the observation chain, from the intrinsic properties of astrophysical sources—spectral shape, spatial intensity distribution, and temporal variability—to the interaction of X‑rays with the telescope optics, the response of the two focal‑plane instruments (the Wide Field Imager, WFI, and the X‑ray Integral Field Unit, X‑IFU), and the subsequent data processing on board and at the ground segment. By providing a realistic synthetic data set that closely resembles what will be measured during the mission, the software enables scientists to validate instrument designs, optimise observing strategies, and develop analysis pipelines before launch.

Key scientific results include the generation of realistic spectra and images for a variety of source classes. For example, simulations of a gamma‑ray burst at redshift z = 2 with a 50 ks exposure produced X‑IFU spectra that reveal absorption features at 0.53 eV, corresponding to interstellar medium lines, and distinct absorption edges at z = 0.4388 and z = 0.0382. Sensitivity studies incorporating both particle background and stray light from off‑axis sources demonstrate that Athena’s effective area and background levels meet the mission’s science requirements. The team also refined algorithms for calculating the effective area, improving the accuracy of sensitivity estimates. A dedicated ray‑tracing module was implemented to quantify stray‑light contributions, showing that single‑reflection photons from outside the field of view can raise the background and must be accounted for in sensitivity calculations.

In addition to the simulation framework, the project produced detector‑response matrices and ancillary files essential for system‑level simulations. These files describe the energy redistribution, point‑spread function, and time‑resolution characteristics of the WFI and X‑IFU detectors, enabling end‑to‑end simulations to reproduce the full instrumental response. The software has been adopted by a broad international community, including institutions such as NASA Goddard, NIST, and several European universities, underscoring its utility beyond the Athena mission.

The collaboration involved a large consortium of European and international partners. The core team at Friedrich‑Alexander‑Universität Erlangen‑Nürnberg (FAU) worked closely with the Max‑Planck‑Institute for Extraterrestrial Physics, the Max‑Planck‑Semiconductor Laboratory, the University of Tübingen, SRON in the Netherlands, IRAP and CNES in Toulouse, the Universities of Santander and Alicante, ESA‑ESTEC in Noordwijk, the University of Leicester, and the Harvard‑Smithsonian Center for Astrophysics. Additional contributors included NASA Goddard, NIST, the University of Cork, Dublin University, University College Dublin, and INAF in Bologna. The consortium’s roles ranged from providing scientific requirements and instrument specifications to validating simulation outputs and contributing to the development of analysis tools.

The project timeline was driven by ESA’s review process. In early 2022, ESA confirmed that the instrument designs satisfied scientific goals, but later that year the agency revised the project approach due to cost concerns, leading to the cancellation of Phase A and the initiation of a new mission concept. By summer 2023, the revised concept was nearly complete, and ESA had verified that the mission now fell within the approved budget. Throughout this period, the FAU team maintained active participation in the scientific preparation of Athena, ensuring that the simulation tools and detector models remained aligned with evolving mission parameters. The combined technical achievements and collaborative framework position the Athena mission to deliver high‑fidelity X‑ray observations that will advance our understanding of the high‑energy universe.