A Dense Foreground for Fast Optical Transients with Evryscope

Optical flashes from orbital debris form a vast and deep foreground that is morphologically similar to short-duration astrophysical transients in synoptic sky surveys. Understanding the properties of the debris-generated flash population is a key step in the search for fast optical transients, including the potential optical counterparts to fast radio bursts (FRBs). The Evryscopes are a north-south pair of multiplexed all-sky telescopes, hosted at Cerro Tololo Inter-American Observatory, Chile in the south and Mount Laguna Observatory, California in the north. Each Evryscope is composed of up to twenty-seven 61 mm telescope units on a single, hemispherical enclosure, which together image 16,512 square degrees at two minute cadence with 13 arcsecond resolution. Images from the systems are reduced in real-time with the Evryscope Fast Transient Engine (EFTE), which uses a custom image subtraction algorithm and an automated vetting system to produce a low-latency event stream. This event stream is dominated by single-epoch flashes that effectively mask any population of astrophysical transients occurring on timescales shorter than a single exposure. I will present event rates and magnitude distributions for the observed population of fast optical flashes that constrain the expected foreground for searches for sub-cadence duration transients in synoptic surveys like Evryscope.