Preparing for Athena WFI — Using XMM-Newton SWM data to understand the particle background

The Wide Field Imager (WFI) flying on Athena will usher in the next era of studying the hot and energetic Universe. Among Athena’s ambitious science programs are observations of faint, diffuse sources that are limited by statistical and systematic uncertainty in 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. The SWM operates at a similar frame time to the Athena WFI, without onboard rejection of minimum ionizing particle (MIP) events, and facilitates a unique study of otherwise unavailable events. We present results from a study of background-reduction techniques that exploit the spatial correlation between cosmic-ray particle tracks and secondary events with the goal not only to verify results based on tailored Geant4 simulations, but also reach a deeper understanding of the various background components and measure the actual properties of the particle background signal. To maximize the science output we consider the full energy band, which requires careful treatment at the soft end due to electronic noise. Important preliminary findings comprise the distribution of particle track properties, which are compared to simple simulations. 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.