Impact of a high-energy gamma ray
The origin of cosmic rays has been a major topic of research since their discovery by Victor Hess in 1912. When very energetic primary gamma rays hit the upper layers of the atmosphere, the particle shower they produce gives off a cone of so-called Cherenkov light. When these sparks of light are detected by large telescopes on Earth, the exact position of the cone can be identified and the gamma's flight path reconstructed. As these neutral particles travel in straight lines, the direction of their sources in outer space can be deduced.
HESS/LPNHE