ULTRA-HIGH-ENERGY COSMIC RAYS ACCELERATED IN RELATIVISTIC JETS FROM RADIO GALAXIES

Abstract

Ultra-High-Energy Cosmic Rays Accelerated in Relativistic Jets from Radio Galaxies Hyesung Kang^1, Dongsu Ryu^2, Jeongbhin Seo^3

1Depart. of Earth Sciences, Pusan National University, Rep. of Korea 2Depart. of Physics, College of Natural Sciences, UNIST, Rep. of Korea 3Los Alamos National Laboratory, Los Alamos, United States

While the origin of UHECRs remains rather elusive, many studies point out that relativistic jets from radio galaxies are promising candidates. Using relativistic hydrodynamics simulations of relativistic jets and test-particle cosmic-ray transport simulations, we show that radio galaxy jets can energize UHECRs above 100 EeV, mainly through shock acceleration within the jet-induced flows and shear acceleration at the jet-backflow interface. Our results demonstrate that the energy spectrum of particles escaping from simulated jets can be represented by a double power law with an exponential cutoff. Moreover, more relativistic jets with a higher bulk Lorentz factor can generate the energy spectrum, extending to higher energies. If the jets of Virgo A are faster and stronger than those of Centaurus A, our model predicts that UHECRs observed in the northern hemisphere will have a higher flux and a lighter mass composition in the high-energy tail than those seen in the southern hemisphere. These findings may offer a plausible explanation for observed discrepancies in particle flux and mass composition at the highest energy range, particularly between the Telescope Array and Pierre Auger Observatory. This study offers valuable insights into the potential role of nearby radio galaxies as dominant sources of UHECRs and may help understand the north-south asymmetries observed in UHECR data.