Microscopy assessment of two multiple-transient candidates

Abstract

We present a microscopy-based assessment of two representative cases of multiple-transient candidates identified on historical photographic plates: the nine-transient event of 12 April 1950 (XE325) and the triple-transient event of 19 July 1952 (XE186). These events consist of spatially clustered, point-like sources detected within single long-exposure plates, lacking the streaks expected from moving objects and therefore suggestive of short-duration flashes. We analyse microscopy images of the original glass-plate material together with an independent copy plate and archival metadata, and we register these images against the published survey digitizations so that individual catalogued sources can be located. In both cases the candidates appear slightly sharper and more compact than typical stellar point spread functions, qualitatively consistent with sub-second to few-second emission during exposures of $\sim 50$~min, and they are reproducible across different representations of the same field, which disfavours a digitization-artifact origin. However, microscopy of individual point-like features cannot by itself separate a short-duration flash from an intrinsic emulsion defect, because the two scenarios produce qualitatively similar signatures at the single-source level. We also show that the multi-observatory parallax test, while powerful in principle, is not feasible for these two specific events with presently available material, owing to the absence of a simultaneous second-site plate reaching comparable depth. We conclude that resolving the nature of these events requires population-level statistics and, where future observations allow, purpose-built multi-station imaging. This work provides a proof-of-concept framework for combining microscopy with archival analysis in the study of historical transient phenomena.