Polarization of synchrotron emission from relativistic reconfinement shocks

We study the polarization properties of relativistic reconfinement shocks with chaotic magnetic fields. Using our hydrodynamical model of their structure, we calculate synthetic polarization maps, longitudinal polarization profiles and discuss the spatially averaged polarization degree as a function of jet half-opening angle Theta_j, jet Lorentz factor Gamma_j and observer inclination angle to the jet axis theta_{obs}. We find, that for theta_{obs} <= Theta_j the wave electric vectors are parallel in the vicinity of the structure ends and perpendicular in between, while for theta_{obs} > Theta_j the polarization can only be perpendicular. The spatially averaged polarization degree does not exceed 30%. Parallel average polarization, with polarization degrees lower than 10%, have been found for theta_{obs} < Theta_j under the condition Gamma_j * Theta_j > 1. As earlier works predicted the parallel polarization from relativistic conical shocks, we explain our results by discussing conical shocks with divergent upstream flow.

💡 Research Summary

This paper investigates the linear polarization characteristics of synchrotron radiation emerging from relativistic reconfinement shocks in active galactic nucleus (AGN) jets, assuming a chaotic magnetic field within the shocked region. The authors build upon a previously developed axisymmetric hydrodynamic model that describes the geometry, pressure, and density distribution of a reconfinement shock as a function of the jet half‑opening angle (Θ j) and the jet bulk Lorentz factor (Γ j). By embedding an isotropic, tangled magnetic field into each computational cell and adopting a power‑law electron energy distribution, they compute the synchrotron emissivity and Stokes parameters in the fluid rest frame. These quantities are then Lorentz‑transformed to the observer’s frame for a range of viewing angles (θ obs) relative to the jet axis.

Synthetic polarization maps and longitudinal (along‑jet) polarization profiles are produced for a systematic grid of (Θ j, Γ j, θ obs). The results reveal two distinct regimes. When the observer’s line of sight lies inside the jet cone (θ obs ≤ Θ j), the electric‑vector position angle (EVPA) is parallel to the jet axis near the upstream and downstream ends of the shock, but becomes perpendicular in the central region. Consequently, the map exhibits a “parallel–perpendicular–parallel” pattern along the shock. In contrast, for observers outside the cone (θ obs > Θ j) the EVPA is uniformly perpendicular across the entire shocked structure. The spatially averaged polarization degree never exceeds ~30 %. Moreover, when the product Γ j Θ j > 1 (i.e., a fast, wide jet) and θ obs < Θ j, the average polarization drops below 10 % and is dominated by the weak parallel component. For Γ j Θ j < 1 the average polarization can reach 20–30 %, still well below the theoretical maximum for a perfectly ordered field.

These findings differ from earlier predictions based on idealized conical shocks with a uniform upstream flow, which generally produce strong parallel polarization. The authors argue that realistic reconfinement shocks involve a divergent upstream flow: the jet expands conically before being squeezed by external pressure, creating a shock surface that is not purely conical but curved. This curvature, together with relativistic aberration, naturally yields the mixed parallel/perpendicular EVPA pattern and limits the net polarization.

The paper discusses observational implications. High‑resolution VLBI polarization images of blazar jets often show a transition from parallel EVPA near the core to perpendicular EVPA downstream, a behavior reproduced by the model. The modest polarization degrees (≤30 %) are consistent with a tangled magnetic field, supporting the notion that turbulence dominates the jet interior on scales probed by current interferometers. The authors suggest that future work should incorporate full 3‑D magnetohydrodynamic simulations, include particle acceleration physics, and compare multi‑wavelength polarization data (radio, optical, X‑ray) to further test the reconfinement‑shock scenario.