Results of the First ESTHER Summer Campaign: Detection of an Intense Positron Burst During a Summer Thunderstorm on Mount Etna

We report the results achieved by the Experiment to Study Thunderstorm High-Energy Radiation (ESTHER), a small ground-based project devoted to the investigation of high-energy radiation in thunderstorms, installed on Mt. Etna (Italy), during the first observational campaign of summer 2024. The experimental setup was installed at high altitude, at the Citelli Refuge (1741 m a.s.l.) and at the Etnean Observatory (2818 m a.s.l.), and acquired data for more than 4 months, experiencing 22 days of thunderstorms and recording correlated variations in the gamma-ray background. The most interesting result encountered during these first data takes is the detection of a 6.3 min high-energy event that occurred during an intense thunderstorm, which was recorded at the first installation site, on 22 July 2024. The gamma-ray detection system revealed a high-energy emission consisting of several episodes: an initial weak gamma-ray glowing, a following shallow prolonged emission, and a final intense burst. The last two episodes exhibited a remarkable 511 keV emission, with the last burst releasing more than 12% of its total counts within 511±25 keV and exhibiting a count rate in that energy range five times higher than that typically encountered in the environmental background. We interpret this emission as the possible result of positron annihilation occurring inside the parent thundercloud. Several lightning discharges took place nearby the installation site, with the closest one occurring at less than 500 m from the detectors, just before the onset of the final burst dominated by positron annihilation.