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Isotropy of Cosmic Rays beyond 10^{20}  eV Favors Their Heavy Mass Composition.

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We estimated the composition of ultrahigh energy cosmic rays (UHECRs) using sky distribution data. Results suggest a heavy composition at very high energies, potentially lighter at lower energies depending on extragalactic magnetic fields.

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Area of Science:

  • Astrophysics
  • Particle Physics

Background:

  • Ultrahigh energy cosmic rays (UHECRs) are particles with energies exceeding 10^18 eV.
  • Understanding UHECR composition is crucial for identifying their sources and propagation mechanisms.

Purpose of the Study:

  • To estimate the injected mass composition of UHECRs at energies above 10 EeV.
  • To investigate the influence of extragalactic magnetic fields (EGMFs) on UHECR composition estimations.

Main Methods:

  • Analyzing the energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector.
  • Comparing UHECR event distribution with the Large Scale Structure of the local Universe.
  • Modeling scenarios with varying strengths of extragalactic and galactic magnetic fields.

Main Results:

  • A relatively heavy injected composition is inferred at approximately 10 EeV, becoming lighter up to 100 EeV.
  • The UHECR composition at energies above 100 EeV is found to be very heavy, even with strong EGMFs.
  • A lighter composition at lower energies is possible if a strong EGMF is present; galactic magnetic field uncertainties have a subdominant effect.

Conclusions:

  • The injected UHECR composition shows significant energy dependence.
  • Extragalactic magnetic fields play a crucial role in interpreting UHECR composition, especially at lower energies.
  • The observed UHECRs likely originate from a mix of sources with varying elemental compositions.