Polarization Dynamics of X-Ray Synchrotron Emission from a Multi-Zone Blazar Jet

TL;DR

This work tackles the problem of identifying the dominant particle acceleration mechanism in blazar jets from time-resolved X-ray polarization. It develops a multi-zone framework where each emitting cell is governed by PIC simulations of magnetic reconnection or magnetized turbulence, with their outputs combined and radiatively transferred to produce Stokes I,Q,U light curves that can be compared to IXPE data for Mrk 421. Using five variability metrics, the study shows that a reconnection-dominated, small-N configuration (specifically $N=15$, $α=2.5$, $\{θ_v\}={15°,30°}$) best reproduces both the flux and polarization statistics, while turbulence alone underpredicts variability; a mixed model with a substantial reconnection fraction also fits well. The results provide a quantitative framework to test theoretical models against IXPE observations and can be extended to other high-synchrotron-peaked blazars to infer the governing acceleration processes.

Abstract

The polarization of X-ray synchrotron emission in blazars directly probes the magnetic field geometry and particle acceleration processes in relativistic jets. We use particle-in-cell simulations of magnetic reconnection and magnetized turbulence, coupled to polarization-sensitive radiative transfer code, to interpret IXPE observations of Mrk 421 during a high flux state recorded in December of 2023. To evaluate the fitness of the two theoretical scenarios, we rely on a quantitative comparison of the statistical properties of simulated and observed X-ray flux and polarization light curves using five evaluation metrics, rather than attempting to fit individual data points. We propose a turbulence-driven multi-zone model where jet emission is represented as the sum of the radiative output of N independent cells, each described by a particle-in-cell simulation. Comparison of ensembles of simulated Stokes-parameter light curves with IXPE data shows that magnetic reconnection dominated models provide the best match to the observed X-ray flux and polarization dynamics. The optimal configuration corresponds to N = 15 emitting cells, which reproduces the observed amplitudes and timescales of the X-ray flux and polarization variations. Magnetized turbulence models underpredict both the flux and polarization variability. Our results indicate that a multi-zone, reconnection-powered emission scenario can describe the X-ray polarization behavior of Mrk 421 and establish a quantitative framework for testing theoretical models against IXPE observations of other high-synchrotron-peaked blazars.

Figures (11)

• Figure 1: Diagram representing the physical picture of the multi-cell blazar zone model. The jet is discretized into multiple independent cells in an embedded toroidal magnetic field, as seen in the diagram adapted from Marscher_2014. The model assumes that some subset of the total jet represented by $N$ emitting cells responsible for the observed radiative output. Each cell is represented by a particle-in-cell simulation of either magnetic reconnection (left) or magnetized turbulence (right) viewed at some angle $\theta_{\mathrm{v}}$.
• Figure 2: Radiative output for individual magnetized turbulence and magnetic reconnection PIC simulations at $0^\circ$ viewing angles. The plots show the X-ray flux (top), polarization angle (middle), and degree of polarization (bottom).
• Figure 3: Light curves for the set of turbulence parameters that best approximates all five evaluation metrics simultaneously. This corresponds to parameters $N = 5$, $\alpha = 2.5$, and $\{ \theta_{\mathrm{v}}\} = \{0^\circ\}$. Simulated light curves are compared to the observational IXPE data. Sorting by the total distance metric, the 10th and 100th best-matching scenarios(out of 1000 simulated outputs) are plotted.
• Figure 4: Light curves for the set of turbulence parameters that best approximates all five evaluation metrics simultaneously. This corresponds to parameters $N = 15$, $\alpha = 2.5$, $\{\theta_{\mathrm{v}}\} = \{15^\circ, 30^\circ\}$. Simulated light curves are compared to the observational IXPE data. Sorting by the total distance metric, the 10th and 100th best-matching scenarios (out of 1000 simulated outputs).
• Figure 5: Evaluation metric distributions for the set of magnetized turbulence parameters that best approximates the data. The parameters, $N = 5$, $\alpha = 2.5$, and $\{ \theta_{\mathrm{v}}\} = \{0^\circ\}$, represent the combination of 5 magnetized turbulence simulations with flux weights sampled from a powerlaw distribution $\propto A^{-\alpha}$ with $\alpha=2.5$ and only viewed at a viewing angle of $0^\circ$.