Scientists have identified an exceptionally powerful cosmic ray colliding with Earth, known as the Amaterasu particle, with an energy surpassing 240 exa-electron volts (EeV). This energy level is millions of times greater than particles generated by the Large Hadron Collider, the most potent accelerator ever constructed, but falls just short of the 1991 “Oh-My-God” particle at 320 EeV.
Discovered during a routine data check at the Telescope Array Project in Utah on May 27, 2021, by astronomer Toshihiro Fujii from Osaka Metropolitan University in Japan, the signals indicated a super-energetic impact on the detectors. Despite the expectation that high-energy particles like Amaterasu, which don’t deflect off magnetic fields like low-energy cosmic rays, should be traceable, researchers were unable to determine its source.
Calculations by the scientists indicated that the cosmic ray originated from a void-like region with few galaxies. Attempts to correlate it with known galaxies or objects in the vicinity proved futile, leaving the researchers with three potential explanations. First, the particle could originate from an unidentified source. Second, it might have been magnetically deflected beyond current models’ predictions. Lastly, scientists might need to revise their incomplete understanding of high-energy particle physics.
John Matthews, co-author of a related paper, highlighted the challenge in locating such high-energy events, stating, “If you take the two highest-energy events — the one that we just found, the ‘Oh-My-God’ particle — those don’t even seem to point to anything. It should be something relatively close. Astronomers with visible telescopes can’t see anything really big and really violent.”
The Telescope Array, operational since 2008, comprises 507 large surface detectors covering 700 square kilometers. While it has observed over 30 ultra-high-energy cosmic rays, none matched the magnitude of the Amaterasu particle.
