We study parity-violating birefringence induced by an asymmetric neutrino background in plasmas associated with active galactic nuclei (AGN). We derive a directionality factor arising from the relative bulk motion between the neutrino medium and plasma, and show that it can produce an anomalous frequency dependence of the polarization-rotation angle, distinct from the ${\omega }^{-2}$ scaling of Faraday rotation. This anomalous scaling can occur either at the resonance plasma frequency condition $\omega \simeq {\omega }_p$, or when $E_{\nu }^0\simeq m_{\nu }\omega /{\omega }_p$ lies within the range of the neutrino energy spectrum. We estimate the effect for three scenarios: jets propagating through the cosmic neutrino background ($\mathrm{C}\nu \mathrm{B}$), jets with an internal flux of high-energy neutrinos, and accretion-disk plasma permeated by the $\mathrm{C}\nu \mathrm{B}$. Of the three scenarios, the latter gives the largest rotation angle ${\varphi }_{\mathrm{d}}\sim {10}^{-35}\mathrm{rad}$, at x-ray frequencies. Although the predicted rotation angles are below current polarimetric sensitivity, the identified spectral signatures provide a theoretical framework for probing neutrino asymmetries and AGN plasma properties independent of magnetic field models.

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• Birefringence
• Particle astrophysics
• Particle interactions
• Neutrinos