Observation of gamma ray bursts at ground level under the thunderclouds

TL;DR

This study reports ground-level gamma-ray burst observations associated with thunderclouds, suggesting electron acceleration via runaway avalanche, with neutron detection supporting high-energy particle interactions during thunderstorms.

Contribution

First ground-based detection of thundercloud-related gamma-ray bursts using a plastic scintillator detector with neutron observation capabilities.

Findings

Gamma-ray bursts detected at ground level during thunderstorms.

Electrons likely accelerated by relativistic runaway electron avalanches.

Neutron events observed coinciding with gamma-ray bursts.

Abstract

We observed three $\gamma$-ray bursts related to thunderclouds in winter using the prototype of anti-neutrino detector PANDA made of 360-kg plastic scintillator deployed at Ohi Power Station at the coastal area of the Japan Sea. The maximum rate of the events which deposited the energy higher than $3\,$MeV was $(5.5 \pm 0.1) \times 10^2 {\rm /s}$. Monte Carlo simulation showed that electrons with approximately monochromatic energy falling downwards from altitudes of order $100\,$m roughly produced the observed total energy spectra of the bursts. It is supposed that secondary cosmic-ray electrons, which act as seed, were accelerated in electric field of thunderclouds and multiplied by relativistic runaway electron avalanche. We actually found that the $\gamma$-rays of the bursts entered into the detector from the direction close to the zenith. The direction stayed constant during the burst within the detector resolution. In addition, taking advantage of the delayed coincidence detection of the detector, we found neutron events in one of the bursts at the maximum rate of $\sim 14\pm5\,{\rm /s}$.