# High Energy Cosmic Rays from Fanaroff-Riley Radio Galaxies

## TL;DR

This paper investigates the role of Fanaroff-Riley radio galaxies as sources of high energy cosmic rays, analyzing magnetic field models, source contributions, and the impact of radio-to-CR correlations on cosmic ray fluxes.

## Contribution

It introduces an inverted simulation setup to test isotropy assumptions and evaluates the contributions of different RG types based on recent radio-CR correlations.

## Key findings

- FR-II RGs may explain the spectral hardening between 3-30 EeV.
- FR-I RGs can contribute significantly if acceleration efficiency and correlation slope are high.
- A primordial magnetic field or numerous isotropic sources are needed for isotropy at energies below 8 EeV.

## Abstract

The extended jet structures of radio galaxies (RGs) represent an ideal acceleration site for High Energy Cosmic Rays (HECRs) and a recent model showed that the HECR data can be explained by these sources, if the arrival directions of HECRs at energies $\lesssim 8\,\text{EeV}$ from a certain RG, Cygnus A, are isotropized. First, this work introduces the inverted simulation setup in order to probe the isotropy assumption. Here, different extragalactic magnetic field models are compared showing that either a magnetic field of primordial origin that yields a high field strength in the large scale structures of the Universe is needed, or a significant contribution by a multitude of isotropically distributed sources. Secondly, the HECRs contribution by the bulk of RGs of different Fanaroff-Riley (FR) type is determined. Here, the most recent FR-type dependent radio-to-CR correlations $Q_{\rm cr}\propto L_{\rm radio}^{\beta_L}$ are used, and the impact of the slope $\beta_L$ on the HECRs is analyzed in detail. Finally, it is carved out that FR-II RGs provide a promising spectral behavior at the hardening part of the CR flux, between about $3\,\text{EeV}$ and $30\,\text{EeV}$, but most likely not enough CR power. At these energies, FR-I RGs can only provide an appropriate flux in the case of a high acceleration efficiency and $\beta_L\gtrsim 0.9$, otherwise these sources rather contribute below $3\,\text{EeV}$. Further, the required acceleration efficiency for a significant HECR contribution is exposed dependent on $\beta_L$ and the CR spectrum at the acceleration site.