New research suggests the Amaterasu particle, one of the most energetic cosmic rays detected, could be an ultraheavy atomic nucleus rather than a proton. The findings, from scientists at Penn State, were published in Physical Review Letters. They indicate such nuclei could retain extreme energy over vast distances in space.
The Amaterasu particle was detected in 2021 by the Telescope Array in Utah with an energy of about 240 exa-electron volts. Its arrival direction points to a cosmic void, leaving its origin unclear. Researchers modeled how different particles travel through intergalactic space and found that nuclei heavier than iron lose energy more slowly than protons. Our research showed that at energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, said Kohta Murase, the lead researcher from Penn State. Possible sources include massive star collapses into black holes or mergers of neutron stars. Future observatories could test for signatures of these ultraheavy particles in cosmic ray data.