Searching for Fast Astronomical Transients in Archival Photographic Plates

Fast astronomical transients were observed by the VASCO Project (Villarroel et al 2020) in photographic sky surveys conducted in the 1950s. Those searches analyzed the Palomar Observatory Sky Survey (POSS-I and POSS-II) digitized plates. In this article, we present a preliminary report on a similar but independent search using archival plates taken at the Hamburg Observatory with the Gro{\ss}er Schmidtspiegel 1.2-m Schmidt camera, also from the mid-1950s. These plates were digitized by the APPLAUSE Archive, which provides both images and tables of detected objects. By analyzing pairs of plates taken in rapid sequence (about 30 minutes apart) of the same sky regions, we find evidence of transients similar to those previously reported by the VASCO Project for POSS plates. While the analysis is ongoing, one notable result is that our findings independently confirm that these transients exhibit systematically narrow full width at half maximum (FWHM) compared to stellar point spread functions. This provides further support for their interpretation as sub-second optical flashes, consistent with reflections from flat, rotating objects in orbit around Earth.