PSF-like Alpha-Particle Events in LSST Images

Abstract

Rare $\alpha$-particle-induced charge clusters appear in LSST images as compact, PSF-like sources with a median FWHM of $0.{}^{\prime \prime }95$ and median ellipticity consistent with zero, closely resembling unresolved astrophysical point sources. These events are detected in both dark and science exposures at a rate of approximately $10^{-12}{\mathrm{pixel}}^{-1}{\mathrm{s}}^{-1}$. Their collected charge and morphology are consistent with energy deposition from $\sim$5 MeV $\alpha$-particles in silicon CCDs, and their spatial distribution across the focal plane suggests a localized material origin, plausibly associated with trace radioactive contamination in the cryostat aluminum. Despite their deceptive appearance, we demonstrate that a simple broadness statistic based on fourth-order moments cleanly separates these events from stellar PSFs, enabling efficient rejection in coadded images and real-time alert streams. Such charge clusters do not impose an intrinsic bright-end contamination floor for Rubin transient searches, as genuine fast astrophysical events would exhibit characteristically different morphological signatures.

7 pages, 4 figures

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