There are plenty of mysteries in the cosmos, and researchers from the University of Utah have found a new clue in one from more than 30 years ago. In 1991, scientists detected a cosmic ray so powerful that it shouldn’t exist. Now, another particle in the same extreme high-energy range has been detected. This is a potentially important piece of the puzzle, but no one is sure what it means yet.
Cosmic rays are nuclei and subatomic particles that move near the speed of light, giving them tremendous energy. They can come from nearby sources like the sun or tumultuous far-away objects we can’t even detect. The 1991 event, later dubbed the Oh-My-God particle, shocked physicists because no known object in our galaxy could produce something with so much energy.
Ultra-high-energy cosmic rays are rare—we can say this unequivocally after 16 years of Telescope Array operation. The international collaboration has over 300 square miles of Utah desert covered in cosmic ray detectors. It has recorded 30 ultra-high-energy events since then, but none have been in the same range as the Oh-My-God particle until now.
The new detection from May 27, 2021 got close to the record set by the Oh-My-God particle with 2.4 x 10^20 eV. That’s the same energy as a brick dropped from waist height. But remember, this is a single subatomic particle vastly smaller than a brick. When cosmic rays hit the atmosphere, they shatter oxygen and nitrogen nuclei in the upper atmosphere, and it’s the resulting shower of secondary particles that light up cosmic ray detectors. The event triggered 23 detectors in the northwest region of the Telescope Array, covering about 18.5 square miles.
Credit: Osaka Metropolitan University
Like the Oh-My-God particle, no known object in the Milky Way galaxy could produce the new detection. It’s so powerful that it should be unaffected by magnetic fields. Thus, you can trace its path to find out where it began its journey. However, study co-author John Matthews explains that search was fruitless. “In the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there’s nothing high energy enough to have produced it,” says Matthews. The particle arrived at Earth from the direction of the Local Void, a region of space bordering the Milky Way that appears largely empty.
The study, published in Science, gives this new detection a name: the Amaterasu particle, a reference to the sun goddess in Japanese mythology. The researchers note that the Amaterasu particle and the Oh-My-God particle came from different areas of the sky and were detected using different observational techniques. So, they’re confident these extreme high-energy events are real—they’re just very rare. It will take more such detections to understand where these highly energetic particles are generated, but hopefully, it doesn’t take 30 years to detect another one.
