Mitigating the effects of particle background on the Athena Wide Field Imager

Abstract

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 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 or Xrays 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 and mitigate its effects, we here present results from a study of background-reduction techniques that exploit the spatial correlation between cosmic-ray particle tracks and secondary events. We use Geant4 simulations to generate a realistic particle background signal, sort this into simulated WFI frames, and process those frames in a similar way to the expected flight and ground software to produce a realistic WFI observation containing only particle background. The technique under study, Self Anti-Coincidence or SAC, then selectively filters regions of the detector around particle tracks, turning the WFI into its own anti-coincidence detector. We show that SAC is effective at improving the systematic uncertainty for observations of faint, diffuse sources, but at the cost of statistical uncertainty due to a reduction in signal. If sufficient pixel pulse-height information is telemetered to the ground for each frame, then this technique can be applied selectively based on the science goals, providing flexibility without affecting the data quality for other science. 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.

Effectiveness of a dual solenoid magnetic shield at reducing x-ray-like background in silicon-based x-ray detectors

“…Normalization 17 – 19 to the CREME96 spectrum was performed using the equation: F=dNdE×πfCREME96AGNpAd,where F is the output flux density in units of cts/cm2/keV/s and dNdE is the count rate per keV as binned from the simulation outputs. fCREME96 is the integral flux of protons in CREME96 in units of protons/cm2/s/sr, here equal to 0.3729 for the simulated incoming energy range of between 1 and 100,000 MeV.…”

Reducing the background in x-ray imaging detectors via machine learning

“…In addition to the cosmic ray signals, the simulated frames include astrophysical X-ray signals, that is the 'genuine' X-ray photons that reach the detector via the telescope optics from celestial sources. Each frame contains a random number of cosmic ray and astrophysical X-ray events, with the numbers of events drawn from Poisson distributions corresponding to the anticipated mean event rates (one cosmic ray 9 and one X-ray event per frame, corresponding to a relatively low surface brightness source). X-rays are simulated with a uniform distribution of positions across the detector, applicable to observations of extended, low surface brightness X-ray sources, such as the intracluster medium, or ICM, of a galaxy cluster, or the cosmic X-ray background, and with a log-uniform distribution in energy between 0.3 and 10 keV.…”

“…If these secondary events can be associated with a primary proton track, this component of the background can be reduced. A recently-developed procedure, known as self-anticoincidence, 9 achieves this by masking areas of the detector around extended proton tracks. However, this comes at the cost of reducing the effective area of the detector, and can only remove secondary, valid events, when they appear close to a primary proton track in the same frame, which is not always present.…”

Reducing the Athena WFI charged particle background: Results from Geant4 simulations

“…In Ref. 6, we analyze the effects of SAC on different background reduction metrics, and explore the background improvement possible with enhanced, SAC-enabled post-processing algorithms.…”

“…This proceedings paper is excerpted from a much more comprehensive work that is in preparation. 6 In Section 2, we describe the Geant4 simulation output and in Section 3, how this output was converted into simulated WFI frames and event lists. Section 4 describes some characteristics of the simulated background signal and Section 5 provides a summary of these results.…”