Neutrino-Induced Polarization Rotation in Active Galactic Nuclei Plasmas

TL;DR

We address parity-violating birefringence arising from asymmetric neutrino backgrounds in AGN plasmas and develop a directionality factor that accounts for relative bulk motion between neutrinos and plasma. The authors derive a general rotary power φ/l governed by neutrino–antineutrino asymmetries, plasma properties, and the directionality factor, revealing an anomalous frequency dependence distinct from Faraday rotation under certain conditions. Applying the framework to AGN jets and accretion disks, they find φ_j ≈ $10^{-41}$ rad (CνB, radio) and φ_d ≈ $10^{-35}$ rad (accretion disks, X-ray), with jet-produced UHE neutrinos contributing only modestly unless extreme fluxes occur. The results offer a magnetic-field-independent probe of neutrino asymmetries and AGN plasmas, outlining spectral signatures that could guide future polarimetric observations as sensitivities improve.

Abstract

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 $ω^{-2}$ scaling of Faraday rotation. This anomalous scaling can occur either at the resonance plasma frequency condition $ω\simeq ω_p$, or when $E_ν^{0}\simeq m_νω/ω_p$ lies within the range of the neutrino energy spectrum. We estimate the effect for three scenarios: jets propagating through the cosmic neutrino background (C$ν$B), jets with an internal flux of high-energy neutrinos, and accretion-disk plasma permeated by the C$ν$B. Of the three scenarios, the latter gives the largest rotation angle $φ_{\rm d} \sim 10^{-35}\,\mathrm{rad}$, 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.

Figures (1)

• Figure 1: Birefringence angle $\phi$ in terms of frequency $\omega$. Rotary power induced by neutrinos produced in the blazar source TXS 0506+056 identified by IceCube IceCube:2024fxo, for a plasma density $n_e = 0.1$ cm$^{-3}$. Total angle $\phi$ is given by the black curve, while its constant $\phi_C$ (quadratic $\phi_2$) [anomalous $\phi_A$] component, given by Eq. \ref{['eq:zerocte']} (Eq. \ref{['eq:zero2']}) [Eq. \ref{['eq:zeroA']}], is shown in blue (red) [green]. Orange shaded region corresponds to frequencies probed by IXPE galaxies12050054. To the left of the vertical dashed line $E_\nu^0$\ref{['eq:Enu0']} is no longer within $(E^{\mathrm{min}}_\nu,E^{\mathrm{max}}_\nu)$.