On The Heavy Chemical Composition of the Ultra-High Energy Cosmic Rays

Abstract

The Pierre Auger Observatory's (PAO) shower profile measurements can be used to constrain the chemical composition of the ultra-high energy cosmic ray (UHECR) spectrum. In particular, the PAO's measurements of the average depth of shower maximum and the fluctuations of the depth of shower maximum indicate that the cosmic ray spectrum is dominated by a fairly narrow distribution (in charge) of heavy or intermediate mass nuclei at the highest measured energies (E > 10^{19} eV), and contains mostly lighter nuclei or protons at lower energies (E ~ 10^{18} eV). In this article, we study the propagation of UHECR nuclei with the goal of using these measurements, along with those of the shape of the spectrum, to constrain the chemical composition of the particles accelerated by the sources of the UHECRs. We find that with modest intergalactic magnetic fields, 0.3 nG in strength with 1 Mpc coherent lengths, good fits to the combined PAO data can be found for the case in which the sources accelerate primarily intermediate mass nuclei (such as nitrogen or silicon). Without intergalactic magnetic fields, we do not find any composition scenarios that can accommodate the PAO data. For a spectrum dominated by heavy or intermediate mass nuclei, the Galactic (and intergalactic) magnetic fields are expected to erase any significant angular correlation between the sources and arrival directions of UHECRs.

10 pages, 8 figues

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• Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory
• Open Questions in Cosmic-Ray Research at Ultrahigh Energies
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• Extragalactic propagation of ultrahigh energy cosmic-rays
• Probing the origin of ultra-high-energy cosmic rays with neutrinos in the EeV energy range using the Pierre Auger Observatory
• A new view on Auger data and cosmogenic neutrinos in light of different nuclear disintegration and air-shower models
• Indications of Negative Evolution for the Sources of the Highest Energy Cosmic Rays
• Resistive Magnetic Field Generation at Cosmic Dawn
• The need for a local source of UHE CR nuclei
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• Ultrahigh Energy Cosmic Ray Nuclei from Extragalactic Pulsars and the effect of their Galactic counterparts
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• Heavy Nuclei Synthesized in Gamma-Ray Burst Outflows as the Source of UHECRs
• UHECR Composition Models
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• Very-High-Energy Gamma-Ray Signal from Nuclear Photodisintegration as a Probe of Extragalactic Sources of Ultrahigh-Energy Nuclei
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• Ultra-High Energy Cosmic Rays Detected by Auger and AGASA:Corrections for Galactic Magnetic Field Deflections, Source Populations, and Arguments for Multiple-Components
• IceCube expectations for two high-energy neutrino production models at active galactic nuclei
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• Cosmic Ray Acceleration in Active Galactic Nuclei - On Centaurus A as a possible UHECR Source
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