A team of physicists has proposed a novel explanation for one of astronomy's enduring puzzles: the origin of the Amaterasu particle, an extraordinarily energetic cosmic ray detected in 2021. Rather than being a proton as previously assumed, the particle may actually be an ultraheavy atomic nucleus heavier than iron, according to new research.
The Amaterasu particle arrived at Earth with roughly 244 exajoules of energy, making it one of the most powerful cosmic rays ever recorded. Such extreme particles present a fundamental problem. Photons in the cosmic microwave background should theoretically strip away energy from any proton traveling across vast intergalactic distances, yet Amaterasu retained nearly all its power upon arrival.
The new hypothesis solves this problem elegantly. Ultraheavy nuclei with high atomic numbers interact differently with background radiation than protons do. Their greater mass and charge allow them to preserve their energy more effectively during the journey across space. This makes it plausible for such particles to reach Earth from distant sources without losing the tremendous energy they carried when first created.
This reinterpretation carries implications beyond solving a single cosmic mystery. It suggests that the most energetic cosmic rays originating from extreme astrophysical events like supernovae or active galactic nuclei may predominantly consist of heavy elements rather than hydrogen nuclei. Understanding the composition of ultra-high-energy cosmic rays provides clues about the violent processes that accelerate them to such extraordinary velocities.
The research addresses a long-standing challenge in cosmic ray physics. Astronomers have struggled to explain how protons could retain sufficient energy to reach Earth from sources billions of light-years away. If the Amaterasu particle and similar extreme events involve heavy nuclei, physicists gain a more coherent model for how these cosmic messengers traverse the universe.
Future observations will test this hypothesis