A high-rate foreground of sub-second flares from geosynchronous satellites

The Weizmann Fast Astronomical Survey Telescope is a 55 cm optical survey telescope with a high-cadence (25 Hz) monitoring of the sky over a wide field of view (≈7 deg²). The high frame rate allows detection of sub-second transients over multiple images. We present a sample of ~0.1-0.3 s duration flares detected in an untargeted survey for such transients. We show that most, if not all of them, are glints of sunlight reflected off geosynchronous and graveyard orbit satellites. The flares we detect have a typical magnitude of 9-11, which translates to ~14-16th magnitude if diluted by a 30 s exposure time. We estimate the rate of events brighter than ~11 mag to be of the order of 30-40 events per day per deg², for declinations between -20° and +10°, not including the declination corresponding to the geostationary belt directly above the equator, where the rate can be higher. We show that such glints are common in large area surveys (e.g. Zwicky Transient Facility and Legacy Survey of Space and Time), and that some of them have a point-like appearance, confounding searches for fast transients such as fast radio burst counterparts and gamma-ray bursts (GRBs). By observing in the direction of the Earth's shadow, we are able to put an upper limit on the rate of fast astrophysical transients of 0.052 deg^-2 day^-1 (95 per cent confidence limit) for events brighter than 11 mag. We also suggest that the single image, high declination flare observed in coincidence with the GN-z11 galaxy and assumed to be a GRB, is also consistent with such a satellite glint.