Ultra-high-energy cosmic rays from radio galaxies

Abstract

Radio galaxies are intensively discussed as the sources of cosmic rays observed above about 3 × 10 18 eV, called ultra-high energy cosmic rays (UHECRs). We present a first, systematic approach that takes the individual characteristics of these sources into account, as well as the impact of the extragalactic magnetic-field structures up to a distance of 120 Mpc. We use a mixed simulation setup, based on 3D simulations of UHECRs ejected by observed, individual radio galaxies taken out to a distance of 120 Mpc, and on 1D simulations over a continuous source distribution contributing from beyond 120 Mpc. Additionally, we include the ultra-luminous radio galaxy Cygnus A at a distance of about 250 Mpc, as its contribution is so strong that it must be considered as an individual point source. The implementation of the UHECR ejection in our simulation setup, both that of individual radio galaxies and the continuous source function, is based on a detailed consideration of the physics of radio jets and standard first-order Fermi acceleration. This allows to derive the spectrum of ejected UHECR as a function of radio luminosity, and at the same time provides an absolute normalization of the problem involving only a small set of parameters adjustable within narrow constraints.We show that the average contribution of radio galaxies taken over a very large volume cannot explain the observed features of UHECRs measured at Earth. However, we obtain excellent agreement with the spectrum, composition, and arrival-direction distribution of UHECRs measured by the Pierre Auger Observatory, if we assume that most UHECRs observed arise from only two sources: The ultra-luminous radio galaxy Cygnus A, providing a mostly light composition of nuclear species dominating up to about 6 × 10 19 eV, and the nearest radio galaxy Centaurus A, providing a heavy composition dominating above 6 × 10 19 eV.Here we have to assume that extragalactic magnetic fields out to 250 Mpc, which we did not include in the simulation, are able to isotropize the UHECR events at about 8 EeV arriving from Cygnus A. Even in this case, significant anisotropy correlated with Cygnus A and Centaurus A could be present at higher energies, and thus allow for differences in UHECR spectrum and composition between the northern and southern hemispheres. If this scenario can be confirmed, it would also imply that the UHECR flux in our local cosmic environment is significantly above the average throughout the universe.

A New View on Auger Data and Cosmogenic Neutrinos in Light of Different Nuclear Disintegration and Air-shower Models

“…Another impacting assumption is that of single dominant source population. The complexity can be increased by accounting for an additional proton component with higher rigidity (van Vliet et al 2019) or even by a detailed modelling of individual nearby sources (Eichmann et al 2018). This however also vastly increases the degrees of freedom, making a global fit of all free parameters unfeasible given the current statistics of the UHECR data.…”

The High Energy View of FR0 Radio Galaxies

“…Although misaligned RGs represent 2% of the entire extra-galactic γ-ray population that is dominated by blazars [77,78], they are extremely relevant in addressing problems related to the jet structure, the particle acceleration mechanism and the location of the acceleration sites. Moreover, RGs have been known to significantly contribute to the extra-galactic diffuse γ-ray background [79,80] and, interestingly, they are often invoked as possible sources of high-energy neutrinos and ultra-high energy cosmic-rays [81,82].…”

Cosmic ray acceleration to ultrahigh energy in radio galaxies

“…They indeed find that radio galaxies are plausible UHECR sources. However, there are very few powerful radio galaxies within a canonical GZK horizon of ∼ 100Mpc that meet the power requirement from section 2 [3,16,33]. It is therefore natural to explore whether the jet powers in local radio galaxies could have been higher in the past.…”

“…It might be interesting in future studies to modify the assumptions of the analysis. For example, one could envision making the search radius dependent on source distance, d, as might be expected if the deflection is extragalactic (e.g., δθ ∼ (l c d) 1/2 E −1 ZB EG , where B EG and l c are the strength and coherence length of the extragalactic magnetic field [33,46]). However, it is extremely difficult to account for magnetic deflection in a statistically rigorous and physically motivated way.…”