A Penn State led team argues some ultra high energy cosmic rays may be atomic nuclei heavier than iron, which lose energy more slowly than protons on the trip to Earth and would therefore demand a smaller set of cosmic launch sites.
For five years, the most energetic cosmic ray detected in the Northern Hemisphere has carried an open question: what kind of object, sitting in apparent cosmic emptiness, could have flung a single subatomic particle to Earth at nearly the speed of light? A new Physical Review Letters paper from a Penn State-led team now offers a specific answer, and it depends less on the sky than on what the particle actually was.
The Amaterasu particle arrived at the Telescope Array in Utah in 2021, registering roughly 240 exa-electron volts, an energy about ten million times beyond what the Large Hadron Collider can produce. The ScienceDaily release on the Amaterasu particle describes the event as one of the most energetic cosmic rays ever recorded, comparable to the 1991 "Oh-My-God" particle. What made Amaterasu unusual was not just its energy but the empty sky behind it. Backtracking its arrival direction pointed into a cosmic void with no known galaxy cluster, no obvious neutron star, and no active galactic nucleus in range. Something in that apparent emptiness had to be capable of launching a single subatomic particle at relativistic speed, and the field has been arguing about what that something was ever since.
Kohta Murase, an astrophysicist at Penn State, and his collaborators, including B. Theodore Zhang, then at Penn State and now at Kyoto University's Yukawa Institute for Theoretical Physics, argue the answer may be composition rather than location. In their model, the Amaterasu particle was not a proton but an atomic nucleus heavier than iron, a class of "ultraheavy" nuclei that interact differently with the cosmic microwave background and intergalactic magnetic fields than lighter particles do. The relevant physics is that heavier nuclei, in this regime, lose energy more slowly than protons or intermediate-mass nuclei while crossing intergalactic space, so they can arrive at Earth still carrying the energy of their original launch. A proton making the same trip would have bled off far more energy along the way.
That asymmetry is what makes the hypothesis testable. If some fraction of the highest-energy cosmic rays are ultraheavy nuclei, the composition of the cosmic ray spectrum at the very top of the energy scale should shift toward heavier elements, and that shift should appear more strongly for events whose arrival directions point through the longer stretches of intergalactic space. The team is careful about the claim. As Murase put it, "we are not saying all UHECRs are ultraheavy nuclei"; the paper sets new upper limits on how much of the ultra-high-energy flux could come from these heavier species and predicts that future data should show composition heavier than iron at the highest energies, as reported in the ScienceDaily summary of the Murase et al. PRL paper.
If the model holds, the candidate accelerators narrow rather than expand. Ultraheavy nuclei need launch sites that can synthesize and eject elements beyond iron in the periodic table, places where the periodic table itself is being built. The paper points to a short list: massive stars collapsing into black holes, strongly magnetized neutron stars (magnetars), and binary neutron-star mergers, the same kinds of events that power long gamma-ray bursts. That has a knock-on consequence. Active galactic nuclei and other traditional ultrahigh-energy cosmic ray candidates, which have dominated the field's models for decades, would face tighter constraints, because the conditions needed to strip a heavy nucleus of all its electrons and still launch it intact are not generic.
The hypothesis is built on a single high-profile event and a model rather than a measurement, and the field treats it as such. AugerPrime in Argentina and a proposed Global Cosmic Ray Observatory are designed to gather the statistics that one paper cannot, and a north/south asymmetry in the cosmic ray spectrum, with the Southern Hemisphere's events clustering differently from the Northern Hemisphere's, is one of the downstream patterns the team expects future data to test. If the composition of the highest-energy events does skew heavier than iron over the next decade of observations, Amaterasu's 2021 detection will look less like a mystery and more like the first clear example of a new class of cosmic messenger. If it does not, the void behind it stays a void.