In May 2021, a single particle hit the atmosphere over Utah carrying as much kinetic energy as a fast-moving tennis ball. The Telescope Array detected it and it was later reported in Science in 2023, at about 244 exa-electron volts. Researchers named it the Amaterasu particle after the Japanese sun goddess.
It came from the direction of the Local Void, an almost empty region of space beside the Milky Way with no known source that could produce such a thing. Physicists have puzzled over that for years.
A Penn State team now argues the puzzle may lie in the assumption about what the particle was made of. Their study in Physical Review Letters suggests Amaterasu, and other extremely energetic cosmic rays, may be atomic nuclei heavier than iron.
Why heavy nuclei fit
Most cosmic-ray research has assumed the most extreme events involve protons, the simplest and lightest nuclei. The team ran computational simulations of how particles of different masses cross intergalactic space. Ultraheavy nuclei lost energy much more slowly than protons or lighter nuclei as they moved through the radiation fields of deep space.
That means such a nucleus could arrive on Earth with far more energy than a lighter particle covering the same distance. Kohta Murase, professor of physics and astronomy at Penn State’s Eberly College of Science and lead researcher, said the team is not claiming too much. “We are not saying that all ultrahigh-energy cosmic rays are ultraheavy nuclei,” he said. “But if some of the highest-energy events are ultraheavy nuclei, that would impact how we search for their sources.”
Ultrahigh-energy cosmic rays exceed 100 exa-electron volts, which is about 10 million times more energy than the Large Hadron Collider can produce. Murase said detecting single particles lets researchers use “their energies, arrival directions and expected magnetic deflections to infer their possible cosmic sources.”
Where they might come from
If the particles are heavy nuclei, their sources must be extreme. The researchers point to collapsars, massive stars that collapse straight into black holes, and to binary neutron-star mergers. Both are associated with gamma-ray bursts.
“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.
The team’s calculations also set new theoretical limits on how much ultraheavy nuclei may contribute to the overall population of these cosmic rays. Murase added that such sources could help explain a possible asymmetry between the northern and southern hemispheres in the spectrum.
Testing the idea
The study rests on simulations, not a direct identification of Amaterasu’s makeup. New observatories and upgrades could test it. Those include AugerPrime in Argentina and the planned Global Cosmic Ray Observatory. If ultraheavy nuclei matter, Murase said, future data should show a composition heavier than iron.
The first example of an ultrahigh-energy cosmic ray was reported more than 60 years ago, and their origin has been one of the field’s biggest mysteries since.
Sources
- The Amaterasu Particle May Not Be a Proton at All, New Research Suggests
- Scientists think they solved the mystery of the Amaterasu particle
Sources
- The Amaterasu Particle May Not Be a Proton at All, New… – The Debrief
- Scientists think they solved the mystery of the Amaterasu particle
