Observations of Ultra-High Energy Cosmic Rays and Radio Galaxies as their Origin

Abstract

Observations of Ultra-High Energy Cosmic Rays and Radio Galaxies as their Origin

Dongsu Ryu Department of Physics, College of Natural Sciences, UNIST, Ulsan 44919, Korea

Ultra-high-energy cosmic rays (UHECRs) are currently being observed in two leading experiments, the Pierre Auger Observatory (PAO) in the southern hemisphere and the Telescope Array (TA) in the northern hemisphere. Despite similar experimental methodologies, the two experiments report discrepancies in their observations, particularly at energies above a few tens EeV. Specifically, the particle flux observed by PAO is lower than that by TA, and the mass composition appears to be heavier at PAO than at TA. Radio galaxies (RGs) are the most promising sources for UHECRs, and we investigated the RG origin of UHECRs through relativistic hydrodynamic simulations of FR I/II type jets and Monte Carlo simulations of particle transport and acceleration. While diffusive shock acceleration is the dominant process at energies below EeV, relativistic shear acceleration at the jet-cocoon interface is more important at lower energies. The resulting energy spectrum is best described by a double power law with an “extended” exponential cutoff. We then simulated the propagation of UHECRs from RGs both in the local universe and at cosmological distances, through the intergalactic space to Earth. Our results suggest that UHECRs in the northern hemisphere exhibit a higher flux and a lighter composition at the highest energies than in the southern hemisphere if they are originated from RGs. We attribute this to the fact that Virgo A has a jet with a higher Lorentz factor compared to Centaurus A and Fornax A. These findings offer a potential explanation for the discrepancies in UHECR flux and mass composition observed by the PAO and TA experiments.