Nuclear and electromagnetic cascades induced by ultrahigh-energy cosmic rays in radio galaxies: implications for Centaurus A

Abstract

Very-high-energy (VHE) $\gamma$-rays ($\gtrsim 0.1\rm~TeV$) and neutrinos are crucial for identifying accelerators of ultrahigh-energy cosmic rays (UHECRs), but this is challenging especially for UHECR nuclei. In this work, we develop a numerical code to solve the transport equation for UHECRs and their secondaries, where both nuclear and electromagnetic cascades are taken into account self-consistently, considering steady UHECR accelerators such as radio galaxies. In particular, we focus on Centaurus A, which has been proposed as one of the most promising UHECR sources in the local universe. Motivated by observations of extended VHE $\gamma$-ray emission from its kiloparsec-scale jet by the H.E.S.S. telescope, we study interactions between UHECRs accelerated in the large-scale jet and various target photon fields including blazar-like beamed core emission, and present a quantitative study on VHE $\gamma$-ray signatures of UHECR nuclei, including the photodisintegration and Bethe-Heitler pair-production processes. We show that VHE $\gamma$-rays from UHECR nuclei could be detected by the ground-based $\gamma$-ray telescopes given that the dominant composition of UHECRs consists of intermediate-mass (such as oxygen) nuclei