Unveiling the origin of XMM-Newton soft proton flares: I. Design and validation of a response matrix for proton spectral analysis

TL;DR

This paper develops and validates a detailed simulation model for soft proton flares affecting XMM-Newton, linking observed background increases to the Earth's magnetospheric proton environment, and providing tools for future mission background prediction.

Contribution

It introduces a validated Geant4-based response matrix for proton spectral analysis, connecting soft proton flares to the Earth's magnetosphere and aiding future mission background modeling.

Findings

The simulation accurately reproduces observed soft proton spectra.

The model links soft proton flares to Earth's magnetospheric environment.

Response files for the Athena mission are provided.

Abstract

Low-energy (<300 keV) protons entering the field of view of XMM-Newton are observed in the form of a sudden increase in the background level, the so-called soft proton flares, affecting up to 40% of the mission observing time. In-flight XMM-Newton's observations of soft protons represent a unique laboratory to validate and improve our understanding of their interaction with the mirror, optical filters, and X-ray instruments. At the same time, such models would link the observed background flares to the primary proton population encountered by the telescope, converting XMM-Newton into a monitor for soft protons. We built a Geant4 simulation of XMM-Newton, including a verified mass model of the X-ray mirror, the focal plane assembly, and the EPIC MOS and pn-CCDs. We encoded the energy redistribution and proton transmission efficiency into a redistribution matrix file (RMF) and an auxiliary response file (ARF). For the validation, three averaged soft proton spectra, one for each filter configuration, were extracted from a collection of 13 years of MOS observations of the focused non X-ray background and analysed with Xspec. The best-fit model is in agreement with the power-law distribution predicted from independent measurements for the XMM-Newton orbit, spent mostly in the magnetosheath and nearby regions. For the first time we are able to link detected soft proton flares with the proton radiation environment in the Earth's magnetosphere, while proving the validity of the simulation chain in predicting the background of future missions. Benefiting from this work and contributions from the Athena instrument consortia, we also present the response files for the Athena mission and updated estimates for its focused charged background.