Search for nanosecond-fast optical transients with TAIGA-HiSCORE array.

The wide-angle Cherenkov array TAIGA-HiSCORE (FOV $\sim$0.6 ster), was originally buit as part of the TAIGA installation for high-energy gamma-ray astronomy and cosmic ray physics. It was proven that the array can also be used to search for nanosecond events in the optical range. This report discusses the method of searching for optical transients using the HiSCORE array and demonstrates its performance on a real example of detecting signals from a man-made Earth satellite. Search for short flares in the HiSCORE data of winter season 2018-2019 is carried out. One candidate for double optical repeater has been detected, but the estimated probability of a random chance of such a transient by background EAS is 78\%. An upper limit on the probability of optical spikes with flux density of more than $10^{-4} \mathrm{erg/sec/cm}^2$ and a duration more than 5 ns is established as $\sim 2 \times 10^{-3}$ events/sr/hour.