Ultra-high-energy Cosmic Ray Sources can be Gamma-ray Dim

Abstract

Ultra-high-energy cosmic rays, accelerated hadrons that can exceed energies of $10^{20}$ eV, are the highest-energy particles ever observed. While the sources producing UHECRs are still unknown, the Pierre Auger Observatory has detected a large-scale dipole anisotropy in the arrival directions of cosmic rays above 8 EeV. In this work, we explore whether resolved gamma-ray sources can reproduce the Auger dipole. We use various Fermi Large Area Telescope catalogs as sources of cosmic rays in CRPropa simulations. We find that in all cases, the simulated dipole has an amplitude significantly larger than that measured by Auger, even when considering large extragalactic magnetic field strengths and optimistic source weighting schemes. Our result implies that the resolved gamma-ray sources are insufficient to account for the population of sources producing the highest-energy cosmic rays, and there must exist a population of UHECR sources that lack gamma-ray emission or are unresolved by the current-generation gamma-ray telescopes.

accepted for publication in ApJ-Letters

References in corpus (10)

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• Extracting the gamma-ray source-count distribution below the Fermi-LAT detection limit with deep learning

Cited by in corpus (4)

• Cosmic Clues from Amaterasu: Blazar-Driven Ultrahigh-Energy Cosmic Rays?
• Combined Fit of Spectrum and Composition for FR0 Radio-galaxy-emitted Ultra-high-energy Cosmic Rays with Resulting Secondary Photons and Neutrinos
• A Hierarchical Shock Model of Ultra-High-Energy Cosmic Rays
• On gamma rays as predictors of UHECR flux in AGNs