Kavli Affiliate: Steven Allen
| First 5 Authors: Eric D. Miller, Catherine E. Grant, Marshall W. Bautz, Silvano Molendi, Ralph Kraft
| Summary:
The Wide Field Imager (WFI) flying on Athena will usher in the next era of
studying the hot and energetic Universe. WFI observations of faint, diffuse
sources will be limited by uncertainty in the background produced by
high-energy particles. These particles produce easily identified "cosmic-ray
tracks" along with signals from secondary photons and electrons generated by
particle interactions with the instrument. The signal from these secondaries is
identical 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
understand the WFI background, 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, sort this into simulated WFI frames,
and process those frames in a similar way to the expected flight and ground
software to produce a 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.
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