
The distribution of cosmic rays by energy suggests these particles come in two varieties. The most extreme ultra-high-energy cosmic rays (UHECRs) are believed to come from remote galaxies (see the May 2021 issue of Sky & Telescope to learn more about these harbingers). But the majority of cosmic rays, with energies below 4 PeV, are thought to originate in the Milky Way. However, the true nature of the PeVatron particle accelerators has remained unknown, largely because the paths of cosmic rays are bent by galactic magnetic fields, so they do not "point back" to their origin.
A large team of Chinese and Japanese scientists known as the Tibet ASγ Collaboration has now detected a few dozen very-high-energy (VHE) gamma rays from the Milky Way that aren't associated with known sources. These gamma rays, collected between 2014 and 2017, are thought to be produced when cosmic rays slam into atomic nuclei in the interstellar medium. Theory says they carry about 10% of the original cosmic-ray energy. The most energetic one detected by the Tibet ASγ team packs a punch of 0.957 PeV - an all-time record.












Comment: Research is also revealing that conventional agriculture destroys these symbiotic relationships: