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

TL;DR

This study investigates whether known gamma-ray sources can explain the observed anisotropy of ultra-high-energy cosmic rays, concluding they cannot, implying the existence of gamma-ray dim or unresolved sources.

Contribution

It demonstrates that resolved gamma-ray sources are insufficient to account for UHECR anisotropy, suggesting hidden or unresolved sources are responsible.

Findings

Simulated dipole amplitudes exceed observed values

Resolved gamma-ray sources cannot explain UHECR anisotropy

Unresolved or gamma-ray dim sources likely produce UHECRs

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.