One of the biggest mysteries in astrophysics may be getting closer to an answer.

A new study published in Physical Review Letters suggests that the famous “Amaterasu particle” — one of the most energetic cosmic rays ever detected — may not have been a proton at all. Instead, it could have been an atomic nucleus heavier than iron.

Discovered in 2021 by the Telescope Array in Utah, the Amaterasu particle carried an astonishing 240 exa-electron volts (EeV) of energy. That’s roughly the same kinetic energy as a fast-moving tennis ball — compressed into a single atomic nucleus.

What puzzled scientists most was its apparent origin. The particle seemed to arrive from a vast cosmic void, a region of space with no obvious object capable of accelerating particles to such extreme energies.

Using detailed simulations, researchers found that ultraheavy nuclei may survive intergalactic journeys far better than protons. While lighter particles lose energy through interactions with background radiation, nuclei heavier than iron can retain much more of their original energy over cosmic distances.

If correct, the finding could help explain how particles like Amaterasu reach Earth from seemingly impossible locations.

Possible sources include:
• Collapsing massive stars
• Neutron star mergers
• Gamma-ray bursts

Future instruments such as AugerPrime and the proposed Global Cosmic Ray Observatory may reveal whether these ultraheavy nuclei are truly responsible for some of the most extreme particles ever observed.

Every ultrahigh-energy cosmic ray is a messenger from one of the universe’s most violent events. Understanding what these particles are made of may help us uncover where they come from — and how nature accelerates matter to energies far beyond anything humans can create.

What do you think is the most likely source of particles this extreme?

📄 Original paper · ScienceDaily summary

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