Astronomers used a telescope array experiment in Utah to discover a high-energy cosmic particle called "Amaterasu" with an unprecedented energy of 244EeV. This rare cosmic ray is comparable to the historical "Oh-My-God" particle, but its origin and composition have raised questions.

A high-energy particle fell from space to the Earth's surface - it's unclear where it came from or even what it is. This may sound like something out of a science fiction novel, but it is actually a scientific reality, as evidenced by research led by Toshihiro Fujii, associate professor at Osaka Metropolitan University Graduate School of Science and Nanbu Yoichiro Institute for Theoretical and Experimental Physics.

Scientists will detect an unknown extremely high-energy particle from space named after a Japanese myth.

Illustration of high-energy cosmic ray "Amaterasu particles" observed by the Telescope Array Experiment's surface detector array. Source: Osaka Metropolitan University/L-INSIGHT, Kyoto University/Takeshige Ryunosuke

Cosmic rays are high-energy charged particles that come from the Milky Way and beyond. Extremely energetic cosmic rays are so rare that they can exceed 1,018 electron volts, or 1 angstrom electron volt (EeV), which is about a million times more powerful than the most powerful accelerator ever built.

Artist's astronomical map of ultra-high-energy cosmic rays, contrasted with weaker cosmic rays affected by electromagnetic fields, to illustrate extremely energetic phenomena. Source: Osaka Metropolitan University/Kyoto University/Takeshige Ryunosuke

In order to track these cosmic rays from space, Professor Fujii and an international team of scientists began conducting telescope array experiments in 2008. This specialized cosmic ray detector consists of 507 scintillator surface stations covering a vast detection area of ​​700 square kilometers in Utah, USA. On May 27, 2021, researchers detected a particle with an energy as high as 244EeV.

Professor Fujii said: "When I first discovered this ultra-high energy cosmic ray, I thought there must be a mistake because it showed an energy level unprecedented in the past 30 years. This energy is equivalent to the highest energy cosmic ray ever observed, known as the 'Oh-My-God' particle, which was estimated to have an energy of 320 EeV when it was detected in 1991."

Animations of recorded signals and events of high-energy particles called Amaterasu particles. Source: Osaka Metropolitan University

Among the many candidate particle names, Professor Fujii and his colleagues finally chose "Amaterasu". "Amaterasu" is the name of the sun goddess in Shinto belief who played an important role in the birth of Japan.

Amaterasu particles may be as mysterious as the Japanese goddess herself. Where does it come from? What exactly is it? These problems still exist. It is hoped that Amaterasu particles will pave the way to uncovering the origins of cosmic rays.

Professor Fujii murmured: "No promising celestial body has been found that matches the arrival direction of cosmic rays, which indicates that there may be unknown astronomical phenomena and new physical origins beyond the standard model. In the future, we will continue to run telescope array experiments and conduct more detailed investigations into the source of this extremely high-energy particle through our ongoing upgraded experiment with four times the sensitivity (called TAx4) and the next-generation observatory."