arXiv subjects

Stephen Bruehl

Publications and source records attributed to Stephen Bruehl.

4 recordsLinked to original sources

arXiv2026

Earth-Projected Clustering of Historical Optical Transients in the Palomar Observatory Sky Survey-I (POSS-I)

Palomar Observatory Sky Survey-I (POSS-I) images obtained before the launch of Sputnik contain star-like point sources consistent with sub-second flashes that are absent from all subsequent observations. The nature and origin of these transients remain poorly understood. Based on models suggesting they are in geosynchronous orbit, we estimated the latitude/longitude over which POSS-I transients appeared. From a dataset of 107,875 POSS-I transients, we selected a subset with high probability of representing real objects based on a machine learning model. Two independent approaches were used to test whether transients non-randomly grouped over specific locations (hotspots). Based on comparing transient numbers in discrete 5{\deg} x 5{\deg} Earth-projected regions to numbers randomly expected in these same regions, 16 statistically-significant hotspots were identified. Separately, two-step cluster analysis identified 6 highly-distinct hotspots (silhouette value = 0.70). The two methods identified several similar hotspots not attributable to POSS-I survey biases: 1) Pacific ocean west of southern Mexico/Central America, 2) southern Gulf of Mexico, 3) southwestern US (Sedona, AZ, White Sands, NM regions). Transients accounting for reported transient-nuclear testing associations displayed location specificity, with exclusively Pacific transient locations during Pacific nuclear testing and primarily southwestern US locations during Nevada testing. Findings hint at intriguing POSS-I transient characteristics.

astro-ph.EP↗

arXiv2026

Modelling Palomar Transients: Constraints from Reflection Geometry and Orbital Altitude

Recent searches of digitised photographic plates from the Palomar Observatory have uncovered tens of thousands of short-lived transients, each visible in only a single exposure. Their morphologies indicate sub-second flashes, consistent with specular reflections from highly reflective objects in near-Earth orbits. In this paper, we present a modelling study using geometric shadow modelling, Monte Carlo simulations, and photometric constraints to infer their physical properties. We examine the observed groupings and alignments, which appear to be consistent with sudden changes in attitude. Assuming a spherical-shell model, the angular profile of the measured transient deficit around the antisolar point is consistent with a population at characteristic altitudes of $\sim 20,000$--25,000 km. A complementary estimate, based on the altitude dependence of the global Earth-shadow deficit, yields a broader characteristic range of $\sim$ 20,000--35,000 km above Earth's surface, extending into the geosynchronous orbital (GSO) region. Under simplified geometric and photometric assumptions and assuming $\sim20{,}000$ to $35{,}786$ km above the Earth's surface, the inferred sizes of the specularly reflecting facets range from centimetre scales up to $\sim3$ m, with characteristic flash durations of $\sim320$ ms and slow rotation rates. We explore both natural and non-natural toy models consistent with the data, and provide constraints to guide future observational searches.

astro-ph.EP↗

arXiv2026

A Response to paper Critical Evaluation of Studies Alleging Evidence for Technosignatures in the POSS1-E Photographic Plates by Watters et al. (2026)

We respond to the critique by Watters et al. (2026) of the statistical analyses in Villarroel et al. (2025) and Bruehl & Villarroel (2025). We argue that the critique conflates object-level validation with ensemble-level statistical inference and relies on a reduced, heterogeneously filtered subset originally constructed for a different scientific purpose. We further question whether the aggressively filtered subset used in Watters et al. (2026) demonstrates a meaningful improvement in sample purity, given the twenty-fold reduction in sample size. Our simple, visual check does not suggest that it does. The subset further lacks complete temporal information and is seriously statistically underpowered for testing the reported Earth-shadow deficit. We emphasise that the horizontal separation metric used for plate assignment and time reconstruction as in Watters et al. (2026) depends on the inclusion of the cos(Dec) factor to ensure geometric consistency. Any omission would alter plate assignment and inferred observation times. Moreover, the analyses presented in Watters et al. (2026) do not include uncertainty estimates or error propagation, limiting the interpretability of the claimed null results. We conclude that the principal findings reported in Villarroel et al. (2025) and Bruehl & Villarroel (2025) are not invalidated by the analyses presented in Watters et al. (2026).

astro-ph.IM↗

arXiv2026

Machine Learning Supports Existence of Previously Unrecognized Transient Astronomical Phenomena in Historical Observatory Images

Transient, star-like point sources that appear and vanish over short timescales are described in astronomical images prior to launch of Sputnik. We have reported that transient numbers diminish significantly in Earth's shadow (shadow deficit) and are more likely within (plus/minus) one day of nuclear testing (nuclear window). These findings remain debated with some arguing that transients identified via existing automated pipelines are simply plate defects. Therefore, we use machine learning (ML) to enhance transient identification accuracy and validate the phenomenon. The model was trained against 250 transient image pairs taken 30 minutes apart that were classified as real versus plate defect by expert visual review; the model demonstrated good discrimination (out-of-fold AUC$=$0.81; sensitivity$=$0.71, specificity$=$0.71). After deployment in a dataset of 107,875 previously-identified transients, the model assigned each a probability of being real. After controlling for ML-identified artifacts, transient counts were significantly elevated for dates within a nuclear window (p$=$.024); transients with the highest probability of being real were more likely to occur within a nuclear window (p$<$.0001). The shadow deficit was significant (p$<$.0001) and largest in the highest probability transients relative to lower probability transients (p$=$.003). Results strongly support existence of an unrecognized population of transient objects in historical astronomical plates warranting further study.

astro-ph.IM↗