Propagation of ultrahigh energy nuclei in clusters of galaxies: resulting composition and secondary emissions

We study the survival of ultrahigh energy nuclei injected in clusters of galaxies, as well as their secondary neutrino and photon emissions, using a complete numerical propagation method and a realistic modeling of the magnetic, baryonic and photonic backgrounds. It is found that the survival of heavy nuclei highly depends on the injection position and on the profile of the magnetic field. Taking into account the limited lifetime of the central source could also lead in some cases to the detection of a cosmic ray afterglow, temporally decorrelated from neutrino and gamma ray emissions. We calculate that the diffusive neutrino flux around 1 PeV coming from clusters of galaxies may have a chance to be detected by current instruments. The observation of single sources in neutrinos and in gamma rays produced by ultrahigh energy cosmic rays will be more difficult. Signals coming from lower energy cosmic rays (E < 1 PeV), if they exist, might however be detected by Fermi, for reasonable sets of parameters.

💡 Research Summary

The paper presents a comprehensive numerical study of the propagation of ultra‑high‑energy (UHE) nuclei injected into galaxy clusters, focusing on how the clusters’ magnetic, baryonic, and photon fields affect the survival of the nuclei and the production of secondary neutrinos and gamma‑rays. The authors first construct realistic three‑dimensional models of the intracluster magnetic field based on observations and magneto‑hydrodynamic simulations, distinguishing a strong, turbulent core (≤0.1 Mpc) from a weaker, more ordered outer region (≥1 Mpc). Gas density is modeled with a β‑profile, yielding typical central densities of 10⁻²–10⁻³ cm⁻³, while the photon background (infrared, optical, UV) is taken from recent extragalactic background light models.

Propagation is treated by solving the diffusion‑transport equation for a range of nuclear species (Fe, Si, O, etc.) and injection spectra (power‑law indices 2.0–2.4, maximum rigidity up to 10²⁰ eV). Energy losses include photodisintegration (photo‑pion and photo‑spallation) on the photon fields, hadronic interactions with the intracluster gas, and nuclear decay processes. The authors explore two extreme injection sites: the dense, magnetized core and the more diffuse outskirts, and they also consider a finite source lifetime (10⁶–10⁷ yr) to assess temporal effects.

Key results show that heavy nuclei injected in the core are largely destroyed below ~10¹⁹ eV because the strong magnetic field forces them to linger, increasing exposure to photodisintegration and hadronic collisions. In contrast, nuclei injected in the outskirts retain a substantial fraction (30–50 %) at the same energies, because the weaker field allows faster escape. This spatial dependence directly translates into observable composition differences in the arriving UHECR flux.

Secondary neutrinos are produced mainly through the decay of pions and beta‑unstable fragments generated during nuclear interactions. The calculated diffuse neutrino flux peaks around 1 PeV, reaching levels comparable to the current sensitivity of IceCube and KM3NeT (∼10⁻⁹ GeV cm⁻² s⁻¹ sr⁻¹). Gamma‑rays arise from neutral pion decay but are heavily attenuated by the intracluster infrared/optical background, making individual clusters difficult to detect in gamma‑rays. However, for cosmic rays below 1 PeV, the resulting gamma‑ray emission can fall within the detection capabilities of the Fermi‑LAT, especially for optimistic magnetic‑field and gas‑density configurations.

A particularly novel aspect is the “afterglow” scenario: if the central accelerator shuts off after a finite active period, the nuclei already trapped in the cluster continue to interact, producing neutrinos and gamma‑rays long after the primary cosmic‑ray burst. This leads to a temporal decorrelation between the arrival of UHECRs and the secondary messengers, a signature that could be exploited to identify past activity in otherwise quiescent clusters.

Overall, the study demonstrates that galaxy clusters can act as both reservoirs and processing sites for UHE nuclei. The survival probability, final composition, and secondary‑messenger spectra are highly sensitive to the magnetic‑field profile, gas density, photon background, and source lifetime. The authors conclude that current neutrino telescopes have a realistic chance of detecting the PeV‑scale diffuse neutrino component from clusters, while gamma‑ray observations of individual clusters remain challenging. Lower‑energy gamma‑ray signals, however, may be observable with Fermi, providing a complementary probe of the intracluster environment and the history of high‑energy particle acceleration.