Penn State scientists are proposing an unexpected component for the most energetic particles in the universe: atomic nuclei heavier than iron. Their work, published in Physical Review Letters, addresses the mystery surrounding ultrahigh-energy cosmic rays like the Amaterasu particle detected in 2021, an event comparable in energy to the Oh-My-God particle detected in 1991 but with an uncertain origin.
The team’s calculations demonstrate that these ultraheavy nuclei lose energy more slowly during intergalactic travel, potentially explaining how they reach Earth with extreme energies, as Kohta Murase, professor of physics and astronomy and astrophysics at Penn State, explains: “Ultrahigh-energy cosmic rays can only be accelerated by some of the most powerful sources in the universe.”
Ultraheavy Nuclei Explain Extreme Energies of Cosmic Rays
Calculations performed by a Penn State-led team suggest that ultraheavy nuclei, those with atomic masses exceeding iron, could be responsible for sustaining extreme energies during their intergalactic journey to Earth. This finding challenges the prevailing assumption that protons or lighter nuclei dominate the flux of these ultrahigh-energy particles, offering a potential solution to the long-standing mystery of their origins.
These calculations demonstrate that ultraheavy nuclei experience a slower rate of energy loss compared to their lighter counterparts as they traverse vast cosmic distances. This reduced energy dissipation allows these heavier nuclei to retain sufficient energy to be detected on Earth, even after traveling millions of light-years.
The team’s simulations incorporated detailed models of energy loss mechanisms, accounting for interactions with the cosmic microwave background and extragalactic magnetic fields. The implications extend to the identification of potential cosmic accelerators. The team posits that the most likely sources for these ultraheavy nuclei are cataclysmic events involving the deaths of massive stars or the mergers of neutron stars.
“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 stated. These events, characterized by immense energy release, could impart the necessary acceleration to propel ultraheavy nuclei to the observed energies. These energetic phenomena often coincide with gamma-ray bursts, providing a potential link between these different astrophysical observations.
The research also addresses a puzzling discrepancy observed in the distribution of ultrahigh-energy cosmic rays. Data suggests a possible difference in the cosmic-ray spectrum between the northern and southern skies, a phenomenon that has long baffled scientists. A significant contribution from ultraheavy nuclei could potentially explain this asymmetry.
“If ultraheavy nuclei contribute significantly at the highest energies, future data should indicate a composition heavier than iron,” Murase noted, suggesting a specific observational signature to confirm their hypothesis. The team’s work places constraints on the overall contribution of ultraheavy nuclei to the observed cosmic-ray population, guiding future searches for their sources.
While acknowledging that not all ultrahigh-energy cosmic rays are necessarily ultraheavy nuclei, the team emphasizes the importance of considering this possibility when interpreting observational data. “But if some of the highest-energy events are ultraheavy nuclei, that would impact how we search for their sources.” This shift in perspective could refine the search strategies employed by current and future cosmic-ray observatories. Instruments, such as the proposed AugerPrime in Argentina and the Global Cosmic Ray Observatory, are expected to provide crucial data to test these predictions.
These observatories are designed to measure the composition and energy of cosmic rays with precision, potentially revealing the telltale signatures of ultraheavy nuclei. “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.
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. Further theoretical studies focusing on the dynamics of cosmic explosions involving black holes and strongly magnetized neutron stars will also be essential to refine our understanding of these enigmatic particles and their origins.
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.
Kohta Murase, professor of physics and of astronomy and astrophysics in the Penn State Eberly College of Science
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