Locating the Production Sites of High-Energy Neutrinos in Blazar Jets

Abstract

The production sites of high-energy neutrinos in blazar jets remain poorly constrained. In this work, we investigate the physical conditions required for efficient neutrino production by using radio-constrained jet properties to evaluate the radial evolution of the external-to-magnetic energy density ratio (Compton dominance $Y$). We identify $Y \gg 1$ as the key physical condition for efficient neutrino production, as it simultaneously enhances photohadronic interactions and suppresses synchrotron radiation from secondary pairs, thereby avoiding an excess of hard X-ray emission. We find that such large values of $Y$ are most readily achieved near or within the broad-line region. This large-$Y$ condition is generally incompatible with reproducing the observed broadband spectral energy distribution within a single emission region, naturally indicating that the neutrino-emitting region is physically distinct from the dominant electromagnetic emission zone. We further show that such a scenario can be realized either if the jet completes its acceleration within sub-parsec scales or if the bulk Lorentz factor is intrinsically large, both of which appear uncommon based on current observations. These results offer a physically motivated framework for identifying neutrino production sites, provide a natural explanation for the rarity of blazar--neutrino associations, and underscore the importance of constraining jet property at sub-parsec scales in the search for neutrino-emitting blazars.