Scientists have long puzzled over the origin of the most energetic particles ever detected, and a recent study offers a compelling new theory. The Amaterasu particle, detected in 2021, is one of the most powerful cosmic-ray events ever observed, yet its source remains a mystery. Researchers now propose that ultraheavy atomic nuclei, heavier than iron, could be the key to unlocking this enigma.
Ultrahigh-energy cosmic rays, like the Amaterasu particle, are incredibly rare and incredibly powerful. They can exceed 100 exa-electron volts, making them millions of times more energetic than particles at the Large Hadron Collider. These particles are so powerful that they can only be accelerated by some of the most extreme sources in the universe, such as colliding neutron stars or massive star collapses.
The new study, led by scientists at Penn State, suggests that ultraheavy nuclei may lose energy more slowly than protons or lighter nuclei while traveling through intergalactic space. This means they could survive the journey to Earth, carrying extreme amounts of energy. The research, conducted with collaborators from the Yukawa Institute for Theoretical Physics in Japan and Virginia Tech, provides valuable insights into the types of cosmic objects that could launch such particles.
One of the most intriguing aspects of the Amaterasu particle is its estimated arrival direction, which traces back to a cosmic void. This void presents a challenge, as it's difficult to imagine a source capable of producing such powerful cosmic rays. However, the study's findings suggest that ultraheavy nuclei could be the answer. By simulating how particles of different sizes gain or lose energy in intergalactic space, the researchers determined that ultraheavy nuclei are better able to survive the journey to Earth at extreme energies.
The team's calculations also set new limits on the contribution of ultraheavy nuclei to the population of observed ultrahigh-energy cosmic rays. They identified massive star deaths involving black holes or strongly magnetized neutron stars, as well as binary neutron-star mergers, as potential sources. These violent cosmic phenomena can also power gamma-ray bursts, which are among the most energetic explosions in the universe. The study suggests that a significant contribution from these sources could help explain the difference observed between the northern and southern skies in the ultrahigh-energy cosmic-ray spectrum.
Future observatories, such as the proposed AugerPrime in Argentina and the Global Cosmic Ray Observatory, may be able to test these ideas. Additional theoretical work on cosmic explosions involving black holes and strongly magnetized neutron stars could also reveal where ultrahigh-energy cosmic rays are born. While the mystery of the Amaterasu particle remains, this study offers a fascinating new perspective on the origins of these extraordinary particles.
