The Amaterasu particle is one of the most energetic cosmic rays ever detected. It was recorded by the Telescope Array in Utah in 2021 with an estimated energy of about 240 exa-electron volts, placing it in the same extreme category as the famous “Oh-My-God particle” detected in 1991.
The problem is that its apparent arrival direction pointed toward a cosmic void, a region where there is no obvious astrophysical source powerful enough to accelerate a particle to such an enormous energy. That made it difficult to explain both where it came from and what kind of particle it actually was.
New research suggests that the answer may be that some of the highest-energy cosmic rays are not protons, and not even ordinary heavier nuclei such as iron, but ultraheavy atomic nuclei, meaning nuclei heavier than iron. This matters because different particles lose energy in different ways as they travel through intergalactic space.
Protons and lighter nuclei interact with background radiation fields and tend to lose energy more quickly over cosmic distances.
According to the simulations, ultraheavy nuclei can retain their energy more efficiently at the extreme energies involved, especially below roughly 300 EeV, making it more plausible that they could travel from distant sources and still reach Earth with energies like that of the Amaterasu particle.
This does not mean that every ultrahigh-energy cosmic ray is ultraheavy. The proposal is more careful than that: if some of the most extreme events are ultraheavy nuclei, then their propagation through space, their magnetic deflection and their possible sources would need to be interpreted differently.
Ultraheavy nuclei carry more electric charge than protons, so magnetic fields can bend their trajectories more strongly. That could help explain why the arrival direction of a particle like Amaterasu does not point neatly back to an obvious source. Its true origin may be elsewhere, with its path distorted during the journey.
The most plausible production sites would be some of the most violent environments in the Universe: collapsing massive stars that form black holes, strongly magnetized neutron stars, and mergers of neutron stars. These are environments already associated with extreme particle acceleration, gravitational waves and, in some cases, gamma-ray bursts.
The study also suggests that if ultraheavy nuclei really contribute significantly at the highest energies, future observatories should detect signs that the composition of the most energetic cosmic rays is heavier than iron. That would be a direct observational test of the idea.
The result is important because it gives a physically plausible way to reduce the mystery around particles like Amaterasu. Instead of requiring a nearby, obvious and extremely powerful source in the direction from which the particle appeared to arrive, the ultraheavy-nucleus scenario allows for a more complex picture: a particle born in a violent cosmic explosion, retaining more of its energy than expected during its journey, and arriving at Earth from a direction that may have been substantially altered by magnetic fields.
It is not a final solution, but it gives researchers a concrete and testable framework for one of the oldest problems in high-energy astrophysics: where the most energetic particles in the Universe come from.
👉
share.google/rniyptKpv6YwQ2V…