Search for Astrophysical Nanosecond Optical Transients with TAIGA-HiSCORE Array

TL;DR

This paper explores the use of the TAIGA-HiSCORE array, originally designed for gamma-ray and cosmic ray detection, to search for nanosecond optical transients, demonstrating its capabilities and setting upper limits on transient frequency.

Contribution

It introduces a method for detecting optical transients with the HiSCORE array and provides the first search results, including an upper bound on transient occurrence rates.

Findings

Detected one candidate transient, but with a high probability of being background.

Established an upper limit of approximately 2 x 10^{-3} events per sr per hour for certain optical spikes.

Demonstrated the array's potential for astrophysical transient searches.

Abstract

A wide-angle Cerenkov array TAIGA-HiSCORE (FOV $\sim$0.6 sr), was originally created as a part of TAIGA installation for high-energy gamma-ray astronomy and cosmic ray physics. Array now consist on nearly 100 optical stations on the area of 1 km$^2$. Due to high accuracy and stability ($\sim$1 ns) of time synchronization of the optical stations the accuracy of EAS arrival direction reconstruction is reached 0.1$^\mathrm{o}$. It was proven that the array can also be used to search for nanosecond events of the optical range. The 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 an artificial Earth satellite. The search for this short flares in the HiSCORE data of the winter season 2018--2019 is carried out. One candidate for double repeater has been detected, but the estimated probability of random simulation of such a transient by background EAS events is not less than 10%, which does not allow us to say that the detected candidate corresponds to a real astrophysical transient. An upper bound on the frequency of optical spikes with flux density of more than $10^{-4} \mathrm{erg/s/cm}^2$ and a duration of more than 5\,ns is established as $\sim 2 \times 10^{-3}$ events/sr/hour.