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Exploring Nine Simultaneous Transients on April 12th, 1950 (2021)

Nine point sources appeared within half an hour on a region within $$\sim $$ 10 arcmin of a red-sensitive photographic plate taken in April 1950 as part of the historic Palomar Sky Survey. All nine sources are absent on both previous and later photographic images, and absent in modern surveys with CCD detectors which go several magnitudes deeper. We present deep CCD images with the 10.4-m Gran Telescopio Canarias, reaching brightness $$r \sim 26$$ mag, that reveal possible optical counterparts, although these counterparts could equally well be just chance projections. The incidence of transients in the investigated photographic plate is far higher than expected from known detection rates of optical counterparts to e.g. flaring dwarf stars, Fast Radio Bursts, Gamma Ray Bursts or microlensing events. One possible explanation is that the plates have been subjected to an unknown type of contamination producing mainly point sources with of varying intensities along with some mechanism of concentration within a radius of $$\sim $$ 10 arcmin on the plate. If contamination as an explanation can be fully excluded, another possibility is fast (t $$<0.5$$ s) solar reflections from objects near geosynchronous orbits. An alternative route to confirm the latter scenario is by looking for images from the First Palomar Sky Survey where multiple transients follow a line.

For example, it has been shown 15 that the detonation of an atomic bomb in Alamogordo, New Mexico in July 1945 left radioactive contaminants due to the isotope Ce-141, seen as fogged spots, on photographic sensitive x-ray film produced by the Eastman Kodak Company. Small local events in an observatory have been known give rise to anomalous detections, e.g. the potassium flare stars 16 that were spectra contaminated by French matches, the discovery of unusual radio bursts, that were caused by microwave ovens 17. If on top of this already extraordinary scenario, we see the GTC counterparts in an “off” state, some of the objects need to follow (artificial) trajectories mimicking stars, in order to compensate for the huge expected proper motion and thus moving at least tens of AU every year.

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