A high-energy particle fell from space and hit the Earth’s surface, but its origin and exact nature are unknown. This may sound like science fiction, but it’s a real scientific discovery, according to research led by Associate Professor Toshihiro Fujii of Osaka Metropolitan University’s Faculty of Science and Institute for Theoretical and Experimental Physics.

Cosmic rays are charged, energetic particles that come from galactic and extragalactic sources in space. Extremely high-energy cosmic rays are exceptionally rare; they can have over 1018 electronvolts of energy or an exa-electronvolt (EeV), which is about a million times more powerful than the most powerful particle accelerators ever built by humans.

Since 2008, Professor Fujii and an international team of scientists have been operating the Telescope Array experiment to track these rays in space. This specialized cosmic ray detector consists of 507 surface detector stations covering a huge detection area of 700 square kilometers in Utah. On May 27, 2021, the researchers detected a particle with an energy level of 244 EeV.

When Fujii first discovered this ultra-high-energy cosmic ray, he thought it must be an error because its unprecedented energy level had not been seen in over 30 years, he said.

This energy level matches the most energetic cosmic ray ever observed, nicknamed the “Oh-My-God particle”, which was estimated to have an energy of 320 EeV when detected in 1991.

From many candidates, Professor Fujii and his colleagues chose to name the particle “Amaterasu”, after the Shinto sun goddess who legend says played a key role in Japan’s creation.

Particle Amaterasu may be as mysterious as the Japanese goddess herself. Where did it come from? What was it exactly? These questions remain unanswered. They hope Amaterasu will help reveal the origins of cosmic rays.

No promising astronomical object has been identified in the direction particle Amaterasu came from, suggesting unknown astronomical phenomena and new physical origins beyond the Standard Model, Fujii reflects.

In the future, they are committed to continuing the Telescope Array experiment, and embarking through their upgraded TAx4 experiment with quadruple sensitivities, and next-generation observatories, on more detailed investigation of this extremely energetic particle’s source.


Osaka Metropolitan University | Telescope Array, An extremely energetic cosmic ray observed by a surface detector array, Science (2023). DOI:10.1126/science.abo5095

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