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arxiv: 2606.01295 · v1 · pith:KYH4SH54new · submitted 2026-05-31 · 🌌 astro-ph.IM · physics.ins-det

PSF-like Alpha-Particle Events in LSST Images

Guillem Megias Homar , Craig S. Lage , Pierre-Fran\c{c}ois L\'eget , Steven M. Kahn , Christopher W. Stubbs , S. R. Kulkarni , Ian S. Sullivan , James F. Bosch
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Eli S. Rykoff
This is my paper

Pith reviewed 2026-06-28 16:17 UTC · model grok-4.3

classification 🌌 astro-ph.IM physics.ins-det
keywords LSSTalpha particlesCCD charge clusterspoint spread functiontransient detectionfourth-order momentsRubin Observatory
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The pith

A broadness statistic from fourth-order moments separates alpha-particle charge clusters from stellar PSFs in LSST images.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that alpha-particle events produce compact charge clusters in LSST CCD images that closely match the appearance of unresolved point sources, with median FWHM near 0.95 arcseconds and near-zero ellipticity. These clusters occur at a low rate of about 10 to the minus 12 per pixel per second and match the expected signature of 5 MeV alpha particles from trace radioactive material. The central demonstration is that a simple statistic based on fourth-order moments measures their excess broadness and cleanly rejects them from both coadded images and real-time alerts. This separation implies that the clusters create no intrinsic bright-end floor of false transients for Rubin searches, because real fast astrophysical events carry different morphological traits.

Core claim

Rare α-particle-induced charge clusters appear in LSST images as compact, PSF-like sources with a median FWHM of 0.95 arcsec and median ellipticity consistent with zero. These events are detected in both dark and science exposures at a rate of approximately 10^{-12} pixel^{-1} s^{-1}. Their collected charge and morphology are consistent with energy deposition from ~5 MeV α-particles in silicon CCDs. 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

What carries the argument

The broadness statistic computed from fourth-order moments of the charge distribution, which quantifies excess spread to flag alpha-induced clusters versus true stellar PSFs.

If this is right

  • The statistic allows rejection of the clusters in both stacked images and live alert pipelines without removing real sources.
  • No bright-end contamination floor arises for transient searches from these events.
  • The rate and spatial pattern point to a localized origin in cryostat materials rather than a uniform focal-plane effect.
  • The same fourth-order approach can be applied directly to future LSST data releases for automated cleaning.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar moment-based filters could be tested on other silicon-based imagers to check for comparable contamination.
  • If the alpha rate varies with temperature or voltage, that dependence could be used to confirm the particle origin experimentally.
  • Adding the statistic as a real-time veto might reduce the volume of alerts that require human review.

Load-bearing premise

Genuine fast astrophysical events would exhibit characteristically different morphological signatures from the alpha-particle clusters.

What would settle it

Detection of a confirmed fast astrophysical transient whose fourth-order moment broadness value falls inside the same range as the alpha-particle clusters.

Figures

Figures reproduced from arXiv: 2606.01295 by Christopher W. Stubbs, Craig S. Lage, Eli S. Rykoff, Guillem Megias Homar, Ian S. Sullivan, James F. Bosch, Pierre-Fran\c{c}ois L\'eget, S. R. Kulkarni, Steven M. Kahn.

Figure 1
Figure 1. Figure 1: (Left, Center) Example of a PSF-like α-cluster detected in only one of two consecutive 30-s exposures obtained on 2025-05-29 (sequences 264 and 265; detector 63) at RA = 327.163◦ , Dec = −13.209◦ . Pixel values are shown in units of electron–hole pairs. (Right) One-dimensional profile across the source, comparing the α-cluster to a median PSF star, normalized to the same peak flux. The profiles closely mat… view at source ↗
Figure 2
Figure 2. Figure 2: (Left) Sample of twenty-five α-clusters identified in dark exposures, illustrating the range of observed morphologies. While many clusters appear nearly circular, others exhibit mild trailing. (Right) Spatial distribution of all detected clusters across the focal plane, combining science and dark samples. Although the total number of clusters is limited, clusters are more frequently observed toward the edg… view at source ↗
Figure 3
Figure 3. Figure 3: Energy spectra of the identified α-clusters in dark and science exposures. The y-axis gives the event rate in each energy bin, expressed in units of pixel−1 s −1 and computed from the number of counts per bin. The science sample shows a broader peak, likely reflecting the larger flux uncertainty caused by sky-background contributions. the entrance surface. The greater frequency of occur￾rence toward the ed… view at source ↗
Figure 4
Figure 4. Figure 4: Second-order moments, FWHM, and broadness of the detected α-clusters in science images. Black histograms show the α-cluster population, while red histograms show median PSF stars on the corresponding detectors for comparison. Here Ixx and Iyy are the adaptive second central moments along the detector axes, with T = Ixx + Iyy, and the ellipticity components are defined as e1 = (Ixx − Iyy)/T and e2 = 2Ixy/T.… view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The paper reports the detection of rare α-particle-induced charge clusters in LSST images that appear as compact, PSF-like sources (median FWHM 0.95″, ellipticity consistent with zero) at a rate of ~10^{-12} pixel^{-1} s^{-1}. These events are shown to be consistent with ~5 MeV α-particles from trace radioactive contamination in the cryostat. The central claim is that a simple broadness statistic derived from fourth-order moments cleanly separates these clusters from stellar PSFs, enabling rejection in coadds and alert streams, and that they therefore impose no intrinsic bright-end contamination floor for Rubin transient searches because genuine fast astrophysical transients would exhibit different morphological signatures.

Significance. If the fourth-order moment separation from stellar PSFs is robustly validated and the assumption regarding transient morphologies holds, the result would be significant for LSST data processing pipelines and transient alert systems. It supplies an empirical rate measurement and a practical rejection statistic that could be implemented in real-time streams, directly addressing a potential source of false positives at the bright end of transient searches. The work is grounded in empirical measurements rather than parameter fitting.

major comments (1)
  1. [Abstract] Abstract (final sentence): The assertion that 'genuine fast astrophysical events would exhibit characteristically different morphological signatures' is presented without any comparison, distribution, or application of the fourth-order moment statistic to actual or simulated transients. This untested assumption is load-bearing for the conclusion that the clusters 'do not impose an intrinsic bright-end contamination floor.'
minor comments (1)
  1. [Abstract] Abstract: No sample sizes, validation metrics (e.g., separation efficiency, false-positive rates), error bars, or explicit exclusion criteria are supplied for the claimed clean separation, which limits immediate evaluation of the statistic's performance.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thoughtful review and for highlighting this important point regarding the abstract. We address the major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract (final sentence): The assertion that 'genuine fast astrophysical events would exhibit characteristically different morphological signatures' is presented without any comparison, distribution, or application of the fourth-order moment statistic to actual or simulated transients. This untested assumption is load-bearing for the conclusion that the clusters 'do not impose an intrinsic bright-end contamination floor.'

    Authors: We agree that the manuscript does not include any direct application or distribution of the fourth-order moment statistic to actual or simulated astrophysical transients, and that the statement in the abstract therefore rests on an untested assumption. The work focuses on empirical separation from stellar PSFs and the physical origin of the clusters; the implication for transients follows from the expectation that genuine fast events (e.g., trailed asteroids or variable/extended sources) would not produce identical static, compact morphologies. To address the referee's concern, we will revise the abstract to remove the load-bearing claim about transients and instead limit the conclusion to the demonstrated separation from stellar PSFs and the resulting practical utility for rejection in coadds and alert streams. revision: yes

Circularity Check

0 steps flagged

No circularity; empirical statistic and external assumption stand independently

full rationale

The paper reports direct measurements of alpha-particle charge clusters in LSST data, their rate, morphology, and spatial distribution, then states that a fourth-order moment broadness statistic separates them from stellar PSFs. This separation is presented as demonstrated by observation rather than by any equation that reduces to a fitted input by construction. The decisive claim that genuine fast astrophysical transients would show different signatures is an external assumption, not a derivation internal to the paper's equations or self-citations. No self-definitional loops, fitted parameters renamed as predictions, or load-bearing self-citation chains appear; the work is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Analysis assumes standard silicon CCD response to alpha particles and that fourth-order moments capture the relevant morphological difference; no free parameters or new entities are introduced.

axioms (1)
  • domain assumption Collected charge and morphology of clusters are consistent with ~5 MeV alpha particles in silicon
    Invoked to identify the physical origin of the events.

pith-pipeline@v0.9.1-grok · 5758 in / 1058 out tokens · 47541 ms · 2026-06-28T16:17:31.011357+00:00 · methodology

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Reference graph

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