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

TL;DR

This work tests whether the population of gamma-ray bright sources cataloged by Fermi-LAT can reproduce the Auger-observed dipole in UHECR arrival directions above 8 EeV. Using CRPropa3 with the 4FGL-DR4 catalog and two source-weighting schemes, the authors find that the resulting dipole is significantly larger than observed, even under optimistic magnetic-field strengths, implying that gamma-ray-bright sources alone cannot account for the UHECR population. The analysis shows that unresolved or gamma-ray dim UHECR sources must contribute substantially to match the measured dipole, with a high inferred source density required when adding isotropically distributed unresolved sources. The findings highlight the need for a gamma-ray dim population of accelerators and have implications for multi-messenger searches, including UHE neutrinos, under current GMF/EGMF assumptions.

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.

Figures (6)

• Figure 1: Skymap of the 4FGL-DR4 catalog after Galactic sources and unassociated sources within $|b|<10^\circ$ are removed. The colorbar indicates the gamma-ray energy flux in erg cm$^{- 2}$ s$^{- 1}$. The dimmest source in the catalog has energy flux $3.09 \times 10^{-13}$ erg cm$^{- 2}$ s$^{- 1}$.
• Figure 2: Redshift distribution of sources in 4FGL-DR4 after removing Galactic sources and low-latitude unassociated sources. Sources with known redshifts are indicated by the blue shaded region. Sources without distance information are assigned random redshifts following an SFR distribution SFR as indicated by the black dotted curve. The sum of the two are shown as the red solid curve.
• Figure 3: Spectrum at Earth of simulated UHECRs from Fermi-LAT sources. The energy range below 8 EeV is shaded in gray as only events with energy greater than 8 EeV are used in evaluating the dipole as in Auger_dipole_2017.
• Figure 4: Flux map corresponding to the case of uniform source weighting. The flux map is obtained by taking the ratio of the smoothed count density and exposure map following Auger_dipole_2017. The skymap is smoothed with a $45^\circ$ top-hat function using the healpy.sphtfnc.beam2bl function.
• Figure 5: Flux map corresponding to the case of gamma-ray flux weighting. The flux map is obtained by taking the ratio of the smoothed count density and exposure map following Auger_dipole_2017. The skymap is smoothed with a $45^\circ$ top-hat function using the healpy.sphtfnc.beam2bl function.