Quasi-Periodic Polarized Emissions from Kink Structure in Magnetized Relativistic Jets

TL;DR

This work addresses how current-driven kink instabilities in magnetized relativistic jets produce quasi-periodic, multi-wavelength polarization signals in blazars. It introduces RaptorP, a polarized SRRT module built on RAPTOR, to perform self-consistent radiative transfer with non-thermal electron distributions derived from PIC simulations, including Faraday effects. Using 3D SRMHD simulations of over-pressured jets, the study shows frequency- and viewing-angle–dependent polarization structures, with inner jet regions displaying toroidal-field–driven EVPA and outer/ambient regions showing poloidal-field signatures; it also identifies QPOs in intensity and polarization, linked to the kink’s rotation, across radio to X-ray bands. The findings provide predictive, observable signatures (e.g., Q–U loops and frequency-dependent correlations) that can help interpret blazar polarization observations and constrain jet magnetization and structure, while noting caveats such as parameter sensitivity, fast-light approximation, and absence of inverse-Compton emission in this analysis.

Abstract

Recent polarimetric observations of blazars indicate the development of current-driven (CD) kink instability after passing the recollimation shocks in the relativistic jets and association with quasi-periodic oscillations (QPOs). To investigate multi-wavelength polarized features of CD kink instability in jets, we develop {\tt RaptorP}, a new special relativistic module of the polarized General Relativistic Radiative Transfer (GRRT) code {\tt RAPTOR}. Based on 3D SRMHD simulations of over-pressured magnetized jets, we find that jet images vary at different frequencies. At low frequencies, the emission comes from the turbulent ambient medium surrounding the jet that obscures the inner jet structure. Electronic Vector Position Angle (EVPA) patterns are perpendicular to the jet propagation direction, indicating a dominance of the poloidal magnetic field. At high frequencies, bright knots and twisted kink structures appear, and EVPA patterns are consistent with a toroidal magnetic field. We also find that QPOs in light curves of intensity and linear polarization (degree and angle). The peak frequency in Power Spectral Densities (PSDs) is well-matched with the rotation period of the kink structure in relativistic jets. It shows an anti-correlation between total intensity and the degree of polarization at a lower inclination angle. Our findings, based on realistic polarized radiation calculations, will explain the observational signatures seen in blazars.

Figures (11)

• Figure 1: 2D axial distribution of (a) logarithmic density, (b) magnetization of over-pressured magnetized jet at $t_s=400$.
• Figure 2: Normalized ray-Traced snapshot images of over-pressured magnetized jet of 86 GHz frequency at $t_s=400$ with inclination angles $i=1^\circ$ ( left upper) , $10^\circ$ ( left lower) and $90^\circ$ ( right) for thermal ( left), $\kappa$ ( middle), and broken power-law distribution ( right). In each panel, we normalize with the maximum intensity. White lines mark EVPA patterns (shown only degrees of polarization exceeding 10%).
• Figure 3: Ray-Traced intensity images of over-pressure magnetized jet at $t_s=400$ with an inclination angle $i=10^\circ$ at the frequencies of (a) 1.5 GHz, (b) 521 nm, (c) 1.2 keV, (d) 86 GHz with a tracer to exclude the environment. Here, we use the $\kappa$ distribution.
• Figure 4: Time-average spectrum energy distribution at an inclination angle $i=10^\circ$. We make an average every 5 frequencies and mark the red horizontal lines. The whole jet region and the specified kink region are plotted in blue and green dashed lines, respectively. The light blue (green) shaded region indicates systematic uncertainty due to time variability and frequency bins.
• Figure 5: Panel (a) shows the light curves of normalized intensity at four representative frequencies: 1.5 GHz (pink), 219 GHz (orange), 521 nm (green), and 1.2 keV (blue). Panel (b) presents PSDs of light curves, where a Hanning window of 228 segments is applied to smooth the signals. The vertical dashed red lines indicate the peak frequencies, which are 20 mHz, 29 mHz, and 44 mHz, respectively. Panel (c) plots the time evolution of the total polarization degree of the kink region at four frequencies. Panel (d) shows the corresponding PSDs of degrees of polarization with a Hanning window of 178 segments. The vertical dashed red line marks 44 mHz. All of the PSDs are normalized at the lowest frequency. Panel (e) shows the intensity and degree of polarization at 521 nm during a time interval with $t_s=428$-$456$. The grey shaded interval ($t_s=430$-$454$) is used for plotting panel (f). Panel (f) plots the Q-U plane for 521 nm. The colors of scattered points change over time.