FWHM of genuine optical flashes on POSS1 plates

Determine whether genuine short-duration optical flashes recorded on Palomar Observatory Sky Survey first-epoch (POSS1) plates exhibit smaller full width at half maximum (FWHM) than stars, so that FWHM can be used as a reliable discriminant between true optical flashes and cataloged stellar sources.

Background

Hambly and Blair (2024) reported that a class of statistically distinctive, morphologically spurious objects on a POSS1-E plate exhibit smaller FWHM than stars, and trained a classifier to separate stars, galaxies, and spurious detections.

Villarroel et al. (2025a) countered that smaller FWHM could be expected for short-lived flashes at high-altitude orbits during long exposures, but the authors note key questions remain unanswered about whether optical flashes indeed show smaller FWHM on POSS1 plates.

References

The following questions remain unanswered after N. Hambly & A. Blair (2024): (i) whether optical flashes exhibit smaller FWHM on POSS1 plates; (ii) whether any of the 8,000 features selected on plate E0070 by N. Hambly & A. Blair (2024) represent an optical flash; and (iii) whether some categories of plate artifacts exhibit a smaller FWHM when compared to stars, on average.

— Critical Evaluation of Studies Alleging Evidence for Technosignatures in the POSS1-E Photographic Plates  (2601.21946 - Watters et al., 29 Jan 2026) in Section 2