TIBET ASgamma: Dawn of sub-PeV gamma-ray astronomy

Abstract

The Tibet air shower (AS) array and underground water-Cherenkov-type muon detector (MD) array have been successfully operating since 2014 at an altitude of 4,300 m in Tibet, China. This hybrid experiment is useful not only for the gamma-ray observation but also for the energy spectrum measurement in the knee energy region of the cosmic-ray energy spectrum. The energy and arrival directions of air showers are determined by the AS array, while the number of muons contained in air showers is counted with the MD array. In terms of gamma-ray observation, the MD array enables us to suppress more than 99.9% of background cosmic rays against gamma-ray signals above 100 TeV. Our gamma-ray and cosmic-ray data from the AS+MD array is well reproduced by our detailed air shower simulations. Here we report on the latest results from the Tibet ASγ experiment, especially focusing on the world’s first detection of sub-PeV gamma rays from the Crab Nebula and the world’s first detection of sub-PeV diffuse gamma-ray emissions from the Galactic plane.