Explaining the UHECR spectrum, composition and large-scale anisotropies with radio galaxies

Abstract

Radio galaxies are promising candidates as the sources of ultrahigh energy cosmic rays (UHECRs). In this work, we examine if the stringent constraints imposed by the dipole and quadropole anisotropies as well as the UHECR spectrum and composition allow that radio galaxies are the dominant extragalactic cosmic ray sources. In order to calculate the UHECR flux emitted by individual radio galaxies, we constrain their properties using information from the radio-CR correlation and a dynamical evolution model. In addition to the UHECR flux from individual, local sources, we include the diffuse flux emitted by the bulk of non-local radio galaxies based on their radio luminosity distribution. Analyzing the source parameters within a range around their expected properties, we finally determine the configurations of local sources describing well the UHECR spectrum, composition and large-scale anisotropies. We obtain a good description of all data even in the case that we include only a small number of local sources. In particular, we find that scenarios where few sources like Fornax A and Virgo A dominate the flux above the ankle, while low-luminosity radio galaxies contribute an isotropic background dominating below the ankle, provide a good fit to the data.

The UHECR contribution of radio galaxies with a finite life-time

“…The so-called radio-jet power correlation, se e.g. [22], provides an order-of-magnitude estimate of their total jet power 𝑄 jet , whereof some fraction is emitted in form of CRs, yielding [11]…”

“…2) using the resulting jet power. Furthermore, some part of the remaining fraction is stored in the magnetic field, which can be used to constrain the maximal rigidity [11]…”

“…In addition, we use a minimal CR rigidity Ř = 1 GV as expected from the thermal-leakage models and used previously by Ref. [11]. Note that the necessary CR power increases with decreasing Ř, but for 𝛼 ≃ 2 its impact is marginal.…”

“…Here we incorporated that the nuclei-dependent modification factor (that accounts for the interactions with the photon fields) can be approximated by a nuclei averaged modification factor η according to 𝜂 𝑖 ≃ 𝐴 𝑖 η ≃ 2𝑍 𝑖 η (see Ref. [11] for more details). Note that also η < 1, so that the inequality in Eq.…”

“…But despite multiple efforts (see e.g. [10][11][12][13]) the original quest remains unsolved (or at most ambiguously solved), which is predominantly a consequence of the lack of knowledge on the Galactic and extragalactic magnetic field (EGMF) as well as on the individual source characteristics.…”