Using the XMM-Newton small window mode to investigate systematic uncertainties in the particle background of X-ray charge-coupled device detectors

The level and uncertainty of the particle-induced background in charge-coupled device (CCD) detectors play a crucial role in future X-ray instruments, such as the Wide Field Imager (WFI) onboard Athena. To mitigate the background systematic uncertainties, which will limit the Athena science goals, we aim to understand the relationship between the energetic charged particles interacting in the detector and satellite, and the instrumental science background to an unprecedented level. In addition, we characterized the temporal variability of the instrumental background from minutes to years. These particles produce easily identified “cosmic-ray tracks” along with less easily identified signals produced by secondary particles, e.g., X-rays generated by particle interactions with the instrument and indistinguishable from genuine sky X-rays. We utilized the small window mode of the PN camera onboard XMM-Newton to understand the time, spatial, and energy dependence of the various background components, particularly the particle-induced background. Although the distribution of particle events follows expected detector readout patterns, we found a particle track length distribution inconsistent with the simple, isotropic model. We also found that the detector mode-specific readout results in a shifted Cu fluorescent line. We illustrated that on long timescales the variability of the particle background correlates well with the solar cycle. This 20-year light curve can be reproduced by a particle detector onboard Chandra, the high-resolution camera anti-coincidence shield. We concluded that the self-anti-coincidence method of removing X-ray–like events near detected particle tracks in the same frame can be optimized with the inclusion of additional information, such as the energy of the X-ray. The results presented here are relevant for any future pixelated X-ray imaging detector and could allow the WFI and similar instruments to probe to truly faint X-ray surface brightness.