Modeling individual nearby radio galaxies as ultra-high-energy cosmic-ray accelerators

Abstract

Nearby radio galaxies are among the most promising candidates for the acceleration of ultra-high-energy cosmic rays (UHECRs). In this work, we develop a physically motivated, source-resolved framework to quantify the contribution of the three nearest FR-I radio galaxies$-$Centaurus A, Virgo A, and Fornax A to the UHECR flux measured by the Pierre Auger Observatory. Acceleration spectra derived from detailed jet-acceleration models are combined with numerical simulations of extragalactic propagation, while the more distant radio-galaxy population is treated as a continuous background. By fitting exclusively the measured UHECR energy spectrum, we determine the relative contribution of each source and constrain the fraction of jet power converted into UHECR luminosity. We find that a small number of nearby radio galaxies can account for the highest-energy UHECR flux with acceleration efficiencies of order $10^{-3}-10^{-2}$, while the background contribution remains subdominant. The resulting scenarios yield mass-composition trends broadly consistent with observations and predict distinct levels of secondary neutrino fluxes. These results demonstrate that physically grounded, source-specific modeling of nearby radio galaxies provides a viable and predictive explanation for the origin of the highest-energy cosmic rays.

Figures (10)

• Figure 1: Results for Scenario $1.\rm b$ using Solar composition. Left: Energy spectrum of UHECRs compared with the Pierre Auger Observatory data Auger_2020. Contributions from individual nearby sources, a continuous source distribution (dashed yellow line) beyond the nearby radio galaxies and their sum are given respectively by dashed and solid lines with shaded bands (statistical uncertainties of the best-fit parameters). Right: Corresponding mean logarithmic mass number $\langle \ln A \rangle$ predicted by the summed source contribution (black solid line), compared with the Pierre Auger Observatory data Auger_2023 interpreted with different hadronic interaction models: EPOS-LHC, SIBYLL 2.3c and QGSJet II-04. Shaded vertical regions in both panels indicate energy ranges excluded from the fit.
• Figure 2: Resulting UHECR spectrum (left) and corresponding $\langle \ln A \rangle$ (right) for Scenario $1.\rm a$. The elements are the same as Figure \ref{['fig:friall_solar']}, but without the continuous source distribution beyond the nearby radio galaxies.
• Figure 3: Resulting UHECR spectrum (left) and corresponding $\langle \ln A \rangle$ (right) for Scenario $1.\rm b$. The elements are the same as Figure \ref{['fig:friall_solar']}.
• Figure 4: Resulting UHECR spectrum (left) and $\langle \ln A \rangle$ (right) for Scenario $2.\rm a$. The elements are the same as Figure \ref{['fig:frinearby']}.
• Figure 5: Resulting UHECR spectrum (left) and $\langle \ln A \rangle$ (right) for Scenario $2.\rm b$. The elements are the same as Figure \ref{['fig:friall_solar']}.