The Origin of the Unfocused XMM-Newton Background, Its Variability, and Lessons Learned for ATHENA

We analyzed the unexposed to the sky (outFOV) region of the MOS2 detector on board XMM-Newton covering 15 yr of data amounting to 255 Ms. We show convincing evidence that the origin of the unfocused background in XMM-Newton is due to energetic protons, electrons, and hard X-ray photons. Galactic cosmic rays are the main contributors as shown by the tight correlation (2.6% of total scatter) with the 1 GeV proton data of the SOHO EPHIN detector. Tight correlations are found with a proxy of the Chandra background rate, revealing the common source of background for detectors in similar orbits, and with the data of the EPIC Radiation Monitor, only when excluding Solar energetic particle events. The entrance to the outer electron belts is associated with a sudden increase in the outFOV MOS2 rate and a spectral change. These facts support the fact that MeV electrons can generate an unfocused background signal. The correlation between MOS2 outFOV data and the SOHO EPHIN data reveals a term constant in time and isotropic, similar to the one found in the study of the pn data. The most plausible origin of this component is hard unfocused X-ray photons of the cosmic X-ray background Compton scattering in the detector as supported by the strength of the signal in the two detectors with different thicknesses. Based on this physical understanding, a particle radiation monitor on board the Advanced Telescope for High Energy Astrophysics has been proposed and it is currently under study. It will be able to track different species with the necessary accuracy and precision to guarantee the challenging requirement of 2% reproducibility of the background.