On Gamma Rays as Predictors of Ultra-high-energy Cosmic-Ray Flux in Active Galactic Nuclei

Abstract

Active galactic nuclei (AGN) are among the main candidates for ultra-high-energy cosmic-ray (UHECR) sources. However, while some theoretical and phenomenological works favor AGNs as the main sources, recent works have shown that using the very-high-energy γ-ray flux as a proxy for the UHECR flux leads to a bad agreement with data. In this context, the energy spectrum and composition data are hardly fitted. At the same time, the arrival directions map is badly described and a spurious dipole direction is produced. In this work, we propose a possible solution to these contradictions. Using the observed γ-ray flux as a proxy may carry the implicit assumption of beamed UHECR emission and, consequently, its beam will remain collimated up to its detection on Earth. We show that assuming an isotropic UHECR emission and correcting the γ-ray emission proxy by Doppler boosting can overcome the problem. The combined fit of the spectrum and composition is improved, with a change of reduced χ2 from 4.6 to 3.1. In particular, the tension between the observed and modeled dipole directions can be reduced from 5.9 (2.1)σ away from the data to 3.5 (1.1)σ for E > 8 EeV (E > 32 EeV). We also show that this effect is particularly important when including AGNs of different classes in the same analysis, such as radio galaxies and blazars.