Cosmic mystery: For years, astrophysicists have puzzled over the origin of the most energetic cosmic rays, because some of these ultra-high-energy particles seem to come from empty space. Now there could be an explanation: The cosmic outlier particles may consist of heavy atomic nuclei instead of protons and light elements, as previously thought. This would explain why they lose little energy during their flight, as researchers report. The sources of these puzzle particles could then be found more easily.
Earth is constantly bombarded by streams of energetic particles from space, but the sources of this bombardment are largely unknown. It seems clear that this cosmic radiation occurs when matter is accelerated greatly, for example by black holes or in cosmic explosions. However, in recent years some cosmic particles have been detected whose extreme energy of more than 100 exaelectron volts (EeV) is difficult to explain.
Where do the extreme particles come from?
One of these mystery particles is the “Amaterasu” particle, discovered in 2021, which hit the Earth’s atmosphere at 240 exaelectron volts – seemingly from nowhere. “When we detect radiation particles like Amaterasu, we can often draw conclusions about their origin based on their energy, direction of flight and magnetic deflections,” explains co-author Kohta Murase from Pennsylvania State University.
But for Amaterasu, these analyzes revealed something unexpected: the origin of this extreme particle seemed to lie in the local void, a largely empty region in the vicinity of our galaxy. This raised the question of what sources this and similar ultra-high-energy cosmic rays (UHECR) particles come from. “The origins and acceleration mechanisms of ultra-energy cosmic rays are one of the greatest mysteries in our field,” says Murase.
Heavy atomic nuclei instead of protons
Now there could be a solution to this mystery: Murase, lead author Theodore Zhang from Kyoto University and their team have investigated whether the type and mass of the cosmic particles could explain their unusually high energy. Normally, cosmic rays consist largely of protons and the nuclei of other light elements. Because these charged particles are strongly deflected and slowed down during their flight by the magnetic influences of the interstellar medium, they quickly lose energy – this also limits their range.
However, this changes when the ultra-energy particles consist of heavy atomic nuclei – nuclei of elements heavier than iron, such as selenium, tellurium or platinum. They can arise from events such as neutron star collisions, gamma-ray bursts or particularly strong core collapse supernovae, as Zhang and his colleagues explain.
More energy and longer range
“Such ultra-heavy atomic nuclei have the advantage that they can be accelerated to energies of more than 100 exaelectron volts, far more than conventional low or medium mass nuclei,” write the astrophysicists. Even more important, however, is that because of their higher mass, the ultraheavy particles lose less energy during their flight and later disintegrate. This could explain why they hit Earth with unusually high energies.
Zhang and his team have now calculated in more detail for the first time what this scenario actually means for a particle like Amaterasu and how such ultra-heavy atomic nuclei fit into the spectrum of high-energy cosmic rays. To do this, they simulated cosmic rays with and without ultraheavy nuclei and compared this with data from the Pierre Auger Observatory in Argentina and the Telescope Array in Utah.
New clues to the origin of Amaterasu
The result: “If ultraheavy atomic nuclei are included as an additional population of ultra-high-energy cosmic rays, the results fit better with the spectrum and composition of the cosmic rays measured by the Telescope Array,” report the astrophysicists. The heavy particles could, above all, explain the previously puzzling excess in the highest energy range.
The new scenario could also solve the origin puzzle of the mysterious Amaterasu particle. “If one assumes lighter particles up to the mass of iron, the origin of the Amaterasu particle would lie in the local void,” explains the team. “But if this particle is an ultra-heavy atomic nucleus, then its source could lie beyond the local void or even near the supergalactic plane.” In this plane there are several galaxy clusters from which the particle could have come.
Does this solve the mystery?
According to astrophysicists, heavy atomic nuclei could explain at least part of the ultra-high-energy cosmic radiation. “We are not saying that all cosmic rays consist of such ultra-heavy atomic nuclei,” emphasizes Murase. “But if some of the most energetic events detected so far are based on such particles, then this could also help in the search for their sources.”
Future measurements with new detectors, including an expansion of the Auger Observatory in Argentina and the planned Global Cosmic Ray Observatory in the USA, could reveal whether their scenario is correct. These are specifically designed to capture and analyze ultra-high-energy cosmic rays.
Source: B. Theodore Zhang (Kyoto University) et al., Physical Review Letters, 2026; doi: 10.1103/221m-gvs3