Using the XMM-Newton Small Window Mode to investigate systematic uncertainties in the particle background of X-ray CCD detectors

Future X-ray observatories with pixelated CCD detectors, such as Athena/WFI, will be critically limited by systematic uncertainties, especially the level and knowledge of the background produced by high-energy cosmic ray particles. These particles produce easily identified “cosmic-ray tracks” along with less easily identified signals produced by secondary photons, i.e. X-rays generated by particle interactions with the instrument. Such secondaries produce identical signals to the X-rays focused by the optics and cannot be filtered without also eliminating these precious photons. As part of a larger effort to estimate the level of unrejected background, we utilize the Small Window Mode (SWM) of the EPIC PN camera onboard XMM-Newton to understand the time, spatial and energy dependence of the various background components, in particular the particle induced background. We illustrate that on long timescales the variability of the particle background correlates well with the solar cycle. This 20-year lightcurve, including its small variations, can be reproduced by a particle detector onboard Chandra, the HRC shield. To understand the variability on shorter timescales, we compare power spectra derived from the two datasets, which both indicate no significant time variability below timescales of one hour. Finally, we conclude 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, and the spatial alignment of the particle track. The results presented here are relevant for any future silicon-based, pixelated X-ray imaging detector, and could allow the WFI and similar instruments to probe to truly faint X-ray surface brightness.