On the Maximum Neutrino Flux of Blazars in the One-zone Leptohadronic Model

Abstract The origin of extragalactic high-energy neutrinos remains a major mystery in astrophysics, with blazars as the leading candidate sources. The widely adopted one-zone leptohadronic jet model, however, faces severe challenges from stringent X-ray observational constraints. In this work, we present an analytical approach that derives the maximum neutrino flux as a function of the observed X-ray flux and the corresponding physical parameters attainable within the one-zone leptohadronic framework. Applying this approach to a sample of neutrino candidate blazars, we further perform numerical modeling and find agreement between analytical and numerical results. Both approaches consistently show that the model-predicted neutrino fluxes do not significantly exceed those obtained in previous one-zone studies and remain below the flux levels inferred from IceCube observations, suggesting that the one-zone scenario alone is unlikely to fully account for high-energy neutrino-blazar associations. This highlights the importance of considering multizone models or alternative production sites (e.g., jet base, hot corona) to better explain high-energy neutrino origins in blazars.