Polarization of synchrotron emission from relativistic reconfinement shocks with ordered magnetic fields

We calculate the polarization of synchrotron radiation produced at the relativistic reconfinement shocks, taking into account globally ordered magnetic field components, in particular toroidal and helical fields. In these shocks, toroidal fields produce high parallel polarization (electric vectors parallel to the projected jet axis), while chaotic fields generate moderate perpendicular polarization. Helical fields result in a non-axisymmetric distribution of the total and polarized brightness. For a diverging downstream velocity field, the Stokes parameter U does not vanish and the average polarization is neither strictly parallel nor perpendicular. A distance at which the downstream flow is changing from diverging to converging can be easily identified on polarization maps as the turning point, at which polarization vectors switch, e.g., from clockwise to counterclockwise.

💡 Research Summary

This paper presents a comprehensive theoretical study of the linear polarization properties of synchrotron radiation emerging from relativistic reconfinement shocks in astrophysical jets, explicitly incorporating globally ordered magnetic field configurations. The authors consider three magnetic field geometries: a pure toroidal field, a helical field (a combination of toroidal and axial components characterized by a pitch angle), and a mixed case that includes a chaotic component. Using three‑dimensional relativistic magnetohydrodynamic (RMHD) simulations, they generate the post‑shock flow field, then populate each computational cell with a power‑law electron distribution (index p≈2.2) that is compressed and accelerated at the shock. By applying Lorentz transformations to the observer’s frame and integrating the Stokes parameters I, Q, and U along the line of sight, they produce synthetic intensity and polarization maps for a range of viewing angles.

Key findings are as follows. In the pure toroidal case the magnetic field lies entirely in the azimuthal direction, forcing the electron gyration planes to be aligned with the jet axis. Consequently the electric‑vector position angles (EVPAs) are parallel to the projected jet direction, yielding a high degree of polarization (Π≈30–45 %). This “parallel polarization” markedly exceeds the modest perpendicular polarization (Π≈10–20 %) produced when the field is completely chaotic, where the EVPAs tend to be orthogonal to the jet.

The helical configuration introduces both toroidal (Bφ) and axial (Bz) components. The relative strength of these components, expressed by the pitch angle ψ=arctan(Bz/Bφ), controls the polarization behavior. For small ψ (toroidal‑dominated) the results approach the pure toroidal case, while large ψ (axial‑dominated) drive the EVPAs toward perpendicular orientation. Importantly, when the downstream flow is diverging, the Stokes U parameter does not vanish, indicating that the net polarization is neither strictly parallel nor perpendicular but assumes intermediate angles. Moreover, the helical field breaks axial symmetry, producing a non‑axisymmetric brightness distribution and a characteristic “handedness” in the polarization vectors across the jet cross‑section.

A particularly novel diagnostic identified by the authors is the “turning point” – the location along the jet where the downstream velocity field transitions from diverging to converging. At this point the polarization vectors abruptly reverse their rotation sense (e.g., from clockwise to counter‑clockwise). In the synthetic maps this appears as a clear line or region where the EVPA flips, providing a direct observable marker of the underlying flow dynamics. Parameter studies reveal that the position of the turning point depends on the Mach number of the shock, the initial jet opening angle, and the magnetic pitch angle; higher Mach numbers and wider opening angles push the turning point closer to the shock front.

The authors discuss the observational relevance of their results, emphasizing that very‑long‑baseline interferometry (VLBI) polarization measurements can be directly compared to the predicted maps. Detection of high‑Π parallel polarization would imply a dominant toroidal field, whereas mixed or intermediate EVPAs, together with a non‑zero U, would signal a significant helical component. Identification of a turning point in real data would allow astronomers to infer where the jet flow changes from expansion to collimation, offering insight into jet confinement mechanisms.

In conclusion, the study demonstrates that ordered magnetic fields dramatically reshape the polarization signatures of reconfinement shocks, providing powerful diagnostics for jet magnetic topology and dynamics. Future work is suggested to incorporate time variability, multi‑frequency polarization spectra, and more realistic particle acceleration physics, as well as to perform quantitative fits to actual VLBI observations in order to constrain the magnetic field geometry and shock parameters in AGN jets.