Ultrahigh-energy cosmic messengers may carry ultraheavy secrets

There may be an ultraheavy explanation for the mystery surrounding the origins of the highest-energy particles ever observed. Ultrahigh-energy cosmic rays are particles from space that strike Earth with energies far beyond those reachable by human-made particle accelerators. One of the most extreme events ever recorded is the “Amaterasu particle,” detected by the Telescope Array in Utah in 2021 and named after the sun goddess in Japanese mythology. Its reported energy places it among the highest-energy cosmic-ray events ever observed, comparable to the “Oh-My-God particle” detected in 1991, yet its origin—and even its identity—remain uncertain.

Now, new research led by Penn State scientists and published in the journal Physical Review Letters suggests that some of the highest-energy cosmic rays may consist of atomic nuclei heavier than iron. Atomic nuclei are the tiny central cores of atoms, made of protons and neutrons. They contain nearly all of an atom’s mass, while occupying only an extremely small fraction of its volume. The team’s calculations show that these ultraheavy nuclei can lose energy more slowly than just protons or lighter nuclei as they travel through intergalactic space, allowing them to reach Earth at extreme energies.

The findings, made in collaboration with researchers at the Yukawa Institute for Theoretical Physics in Japan, Virginia Tech and other institutions, could help narrow down the cosmic sources capable of accelerating these particles.

“Ultrahigh-energy cosmic rays can only be accelerated by some of the most powerful sources in the universe,” said Kohta Murase, professor of physics and of astronomy and astrophysics in the Penn State Eberly College of Science and the leader of the research team. “When we detect individual cosmic-ray particles such as the Amaterasu particle here on Earth, we can often use their energies, arrival directions and expected magnetic deflections to infer their possible cosmic sources.”

The inferred direction of the Amaterasu particle, however, pointed back to a cosmic void in space, with no obvious source of ultrahigh-energy cosmic rays.

“The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years, since the first example was reported,” Murase said.

With energies above 100 exa-electron volts (100 quintillion electron volts), these particles are about seven orders of magnitude (10 million times) more energetic than particles accelerated in the Large Hadron Collider, the world’s largest and most powerful particle accelerator. The Amaterasu particle’s reported energy was about 240 exa-electron volts—roughly the kinetic energy of a fast-moving tennis ball but carried by a single cosmic-ray particle—making it one of the most energetic cosmic rays ever detected.

“These highest-energy cosmic rays are thought to come from extreme astrophysical sources, like two neutron stars colliding or a massive star collapsing,” Murase said. “For many cosmic-ray events taken together, their energy distribution, arrival-direction pattern and statistically inferred composition provide important clues about where these particles come from and how they are accelerated.”

To try to understand the types of particles that could reach Earth at these extreme energies, the team performed detailed computational simulations of how the energies of different-sized particles would change as they travel through intergalactic space.

“Our research showed that at energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, making them better able to survive cosmic distances and reach Earth at extreme energies,” Murase said. “We are not saying that all ultrahigh-energy cosmic rays are ultraheavy nuclei. But if some of the highest-energy events are ultraheavy nuclei, that would impact how we search for their sources.”

The calculations by the research team also placed new constraints on how such ultraheavy nuclei contribute to the overall population of observed ultrahigh-energy cosmic rays.

“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.

“These violent cosmic phenomena can also power gamma-ray bursts that are among the most energetic explosions in the universe. A contribution from these sources could also help explain a possible difference seen between the northern and southern skies in the ultrahigh-energy cosmic-ray spectrum. If ultraheavy nuclei contribute significantly at the highest energies, future data should indicate a composition heavier than iron.”

Next-generation observatories, such as the proposed AugerPrime in Argentina and the proposed Global Cosmic Ray Observatory, could test these signatures, Murase said, noting that further theoretical studies of cosmic explosions involving black holes and strongly magnetized neutron stars could help clarify the origin of ultrahigh-energy cosmic rays.