High-precision time study of gamma-ray bursts during thunderstorms

Strong gamma-ray bursts initiated by lightning discharges and thunderclouds have been observed since the 1980s. However, the process of these emissions is under debate. Observed gamma-ray bursts are classified into two types according to their duration. One is a short-duration burst, called a terrestrial gamma-ray flash (TGF), and the other is a gamma-ray glow, which has a longer duration. The observation of a TGF is challenging because of its extremely brief duration (less than a second). As a result, the physics behind TGF is relatively poorly understood compared to classical long-duration bursts, the gamma-ray glows. To study short gamma-ray bursts, we developed a new detector system with time precision of 10 ns and focused on low-energy (E < 100 keV) measurement. We began our observations in November 2019 in a mountain area over 300 m above sea level and approximately 25 km from the Japan Sea. In this paper, we report the observations leading to the detection of the three types of interesting gamma-ray bursts, some of which were associated with lightning discharges. These results suggest the importance of measurements with high-time resolution and low-energy photons for understanding gamma-ray bursts related to thunderclouds. Moreover, we also report our first observation of gamma-ray imaging of a gamma-ray glow detected in the 2021-2022 season.