In 2021, scientists detected a cosmic ray entering Earth’s atmosphere with as much kinetic energy as a fast-moving tennis ball, all contained in a single subatomic particle. Named the “Amaterasu particle” after the Japanese sun goddess, the event left researchers questioning its origin because its arrival direction pointed to a region of space with no known source capable of producing such energy.
Researchers at Penn State now suggest that the puzzle may stem more from assumptions about the particle’s composition rather than its missing source.
A recent study in Physical Review Letters suggests that the Amaterasu particle, along with other extremely energetic cosmic rays, may actually be atomic nuclei heavier than iron. If correct, the idea could change how researchers search for the sources of cosmic rays.
A Particle of Unknown Origins
Ultrahigh-energy cosmic rays are some of the rarest and most violent phenomena in nature. These particles slam into Earth’s atmosphere with more than 100 exa-electron volts of energy, which is about 10 million times more than what the Large Hadron Collider can produce. The Amaterasu particle, found by the Telescope Array in Utah in May 2021 and reported in Science in 2023, had about 244 exa-electron volts of energy. This makes it one of the most energetic natural events ever recorded.
The mystery extended not only to the particle’s energy, but also to its origin. It came from the direction of the Local Void, which is an almost empty area of space next to the Milky Way, where there is no known source that could create such a powerful particle.
“The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years, since the first example was reported,” said Kohta Murase, professor of physics and astronomy at Penn State’s Eberly College of Science and the study’s lead researcher.
The Heavy Nucleus Hypothesis
The Penn State team ran computational simulations to study how particles of different masses travel through intergalactic space. They found that atomic nuclei heavier than iron, known as ultraheavy nuclei, lose energy much more slowly than protons or lighter nuclei as they move through the radiation fields of deep space.
Most research on cosmic rays has assumed that the most extreme events involve protons, which are the simplest and lightest atomic nuclei. Ultraheavy nuclei lose energy much more slowly during their journey, allowing them to reach Earth carrying far more energy than lighter particles traveling similar distances. That makes them strong candidates for the most powerful cosmic ray events we’ve seen, including the Amaterasu particle.
“When we detect individual cosmic-ray particles such as the Amaterasu particle here on Earth, we can often use their energies, arrival directions and expected magnetic deflections to infer their possible cosmic sources,” Murase said.
New Theoretical Limits
The hypothesis that these particles are ultraheavy nuclei also changes the range of possible sources. If the particles are indeed heavier nuclei rather than protons, their production would require some of the most extreme environments in the universe. The researchers identify collapsars, which are massive stars collapsing directly into black holes, and binary neutron-star mergers as likely candidates. Both phenomena are also associated with gamma-ray bursts, which are among the most energetic explosions known to science.
The team’s calculations set new theoretical limits on how much ultraheavy nuclei may contribute to the overall population of ultrahigh-energy cosmic rays. Murase also noted that these sources could help explain a possible asymmetry in the ultrahigh-energy cosmic-ray spectrum between the northern and southern hemispheres.
“The most promising sites for producing and accelerating such ultraheavy nuclei are massive star deaths involving explosive collapse into black holes or strongly magnetized neutron stars, as well as binary neutron-star mergers known to be powerful gravitational-wave emitters,” Murase said.
Testing the Idea
The researchers stress that they are not saying all ultrahigh-energy cosmic rays are ultraheavy nuclei, but some of the most extreme ones might be. This could change what scientists look for in future studies. New observatories and upgrades, like AugerPrime in Argentina and the planned Global Cosmic Ray Observatory, could test these ideas. If ultraheavy nuclei are important, future observations should find evidence that protons alone cannot explain.
More than six decades after scientists first detected ultrahigh-energy cosmic rays, researchers may finally have a new clue about what the most extreme particles in the universe are made of.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.