3C 403: a candidate neutrino-emitting radio galaxy

Abstract 3C 403 is a well-known FR II radio galaxy with jets extending up to kiloparsec scales. We report its identification as the second most significant candidate among more than 150 sources examined using the 15-year neutrino dataset from the ANTARES collaboration, making it one of the most promising radio-galaxy candidates for high-energy neutrino emission. Motivated by previous associations between blazars and neutrino events, we investigated the jet properties of 3C 403 and their possible role in neutrino production. Multi-scale radio observations, from parsec to kiloparsec scales, reveal a stable, two-sided jet lying close to the plane of the sky, with no evidence of strong Doppler boosting, while X-ray data indicate a dominant, heavily absorbed accretion-related component. We also examined the recently proposed correlation between neutrino and hard X-ray fluxes — originally identified in blazars and Seyfert galaxies — and find that 3C 403 occupies an intermediate location in the L ν – L hX plane between jet-dominated and corona-dominated systems. However, the current upper limit on its neutrino flux prevents a firm assessment of whether it follows the proposed relation. With radiatively efficient accretion (λ Edd ∼ 10 -2 ), strong hard X-ray emission, and a powerful but misaligned jet, 3C 403 provides a physically motivated laboratory for exploring the interplay between coronal activity and jet environments in multimessenger scenarios of neutrino production in active galaxies.