Move over, CERN. Unknown sources in the Milky Way dubbed "PeVatrons" accelerate protons to energies of a few peta-electronvolts - dozens of times higher than the yield of the Large Hadron Collider.
Now, new data from a high-altitude experiment in Tibet confirm that such very-high-energy cosmic rays are indeed produced in our own galaxy.
© APS; Background (atomic hydrogen distribution): HEASARC / LAMBDA / NASA / GFSC
Ultra high-energy diffuse gamma rays (yellow points) are distributed along the Milky Way Galaxy. The gray shaded area indicates the area outside the detectors' field of view.
"The results paint a much fuller picture of the PeVatron population in the Milky Way," says Pat Harding (Los Alamos National Laboratory), who was not involved in the study.
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.
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