AHEAD2020 WP9.8 Soft Proton Response Matrix

Low energy (< 300 keV) protons entering the field of view of the XMM-Newton telescope and scattering with the mirror surface are observed in the form of a sudden increase in the background level. Such flaring events, affecting about 30-40% of the XMM-Newton observing time, can hardly be disentangled from true X-ray events and cannot be rejected on board. All future high throughput gracing incidence X-ray telescopes operating outside the radiation belts (e.g. ATHENA) are potentially affected by soft proton induced contamination that must be foreseen and limited since the design phase. On the other side, a clear description of the interaction model would link the observed soft proton spectra by XMM-Newton to the ones hitting the telescope pupil, mapping the low energy particle environment along its orbit.

Thanks to the latest validation studies on the physics models describing the reflection process of protons at grazing angles, we build a proton response matrix for the XMM-Newton and ATHENA missions, describing the effective area and energy redistribution of protons entering the mirror aperture. The simulation pipeline comprises two independent simulation frameworks for the X-ray optics reflectivity, based on ray-tracing and Geant4, and a Geant4 simulation for the proton transmission efficiency caused by the combination of optical filters, on-chip electrodes and the detection depletion regions, requiring a detailed mass model of the focal plane assemblies.

The response matrix for protons will allow a better understanding of the proton radiation environment, with the aim of modeling the in-flight non X-ray background of current and future X-ray focusing telescopes. The XMM-Newton matrix will be used to analyze the mean energy spectra of the background flares, converting the mission into a “proton telescope”, while characterizing its particle background. The matrices for the ATHENA telescope will allow for a fast evaluation of the soft proton induced background for any input population, driving the design of shielding solutions.

The response matrix is formatted according to the NASA OGIP (Office of Guest Investigators Program) calibration database (caldb) format, and it consists of an RMF and ARF file in FITS (Flexible Image Transport System) format. X-ray data analysis tools available to the X-ray astronomy community such as Xspec and SPEX can be used to simulate or analyse the soft proton-induced background spectra.

The research leading to these results has received funding from the European Union’s Horizon 2020 Programme under the AHEAD2020 project (grant agreement n. 871158).