Bold claim: the universe still guards its ultimate high-energy secrets, and Amaterasu might be closer to home than we thought. But here’s where it gets controversial: the most energetic cosmic ray ever detected may not originate from the farthest, most dramatic corners of the cosmos after all.
Scientists are investigating the source of one of the most energetic particles ever observed striking Earth. The Amaterasu particle, named after the Japanese sun goddess, was first detected in 2021 and carries energy vastly surpassing what the world’s largest particle accelerator, the Large Hadron Collider, can produce—about 40 million times higher. Amaterasu is a type of cosmic ray, a charged particle racing through space at nearly the speed of light. It ranks as the second most energetic cosmic ray detected, behind the so-called “Oh-My-God” particle found in 1991. These ultra-high-energy particles are incredibly rare, which is precisely why scientists are eager to uncover their origins. The prevailing thinking points to violent galactic processes, such as the remnants of supernovae and the bustling regions surrounding accreting supermassive black holes.
A key advance comes from Francesca Capel and Nadine Bourriche of the Max Planck Institute for Physics. They propose that Amaterasu’s birth may not be confined to a sparsely populated region of space like the Local Void. Instead, they suggest the particle could have arisen in relatively nearby, dynamic environments—potentially in a nearby star-forming galaxy such as M82.
Their conclusions arise from a data-driven methodology that maps how Amaterasu could traverse the cosmos, shaped by galactic magnetic fields. The researchers employed a three-dimensional statistical technique known as Approximate Bayesian Computation to test different source scenarios.
“In this framework, we compare realistic physics-based simulations with real observational data to infer the most probable origin locations,” Bourriche explained.
The analysis yielded a set of probability maps that point to origin candidates beyond the Local Void. Although these results refine our view of where ultra-high-energy cosmic rays might originate, they also carry broader implications. Pinpointing the likely birthplaces of Amaterasu could help identify which cosmic events act as factories for these extreme particles.
“Studying ultra-high-energy cosmic rays teaches us how the universe can accelerate matter to extraordinary energies and reveals environments where matter behaves under extreme conditions,” Capel noted. “Our aim is to develop advanced statistical tools that maximize the information stored in available data and deepen our understanding of potential particle sources.”
The researchers published their work on January 28 in The Astrophysical Journal.
Commentary from science writer Rob Leas—based in the U.K.—highlights his ongoing work in science communication, with bylines in Physics World, New Scientist, Astronomy Magazine, and more. He notes the importance of clear reporting as we refine our grasp of cosmic accelerators. You can follow Rob on X (formerly Twitter) at @sciencef1rst.
Would you agree that these local, star-forming galaxies deserve more attention as high-energy cosmic-ray factories, or should we still reserve the largest-distant-engine explanations for Amaterasu’s origins? Share your thoughts in the comments.
