On the statistical characterization of the synchrotron multi-zone polarization of blazars

Abstract

Multiwavelength polarimetric observations of blazars reveal complex, energy-dependent polarization behavior, including a decrease in polarization fraction from X-rays to millimeter bands and significant variability in the electric vector position angle (EVPA). These trends challenge simple single-zone synchrotron models and suggest a more intricate, turbulent jet structure with multiple emission zones. We develop a statistical framework to model the observed energy-dependent polarization patterns in blazars, focusing on the behavior captured by IXPE in the X-ray band and RoboPol in the optical. The goal is to statistically characterize multi-zone models in terms of the distributions of cell size and the physical parameters of the electron energy distribution (EED). A Monte Carlo approach, implemented with the JetSeT code, is used to generate synthetic multi-zone synchrotron emission from a spherical region filled with turbulent cells with randomly distributed physical properties. Simulations explore scenarios ranging from identical cells to power-law distributions of cell sizes and EED parameters with variable cutoff and low-energy slopes. The results show that a purely turbulent, multi-zone model can reproduce the observed energy-dependent polarization without requiring correlations between cell size and EED parameters. The polarization degree is primarily determined by the effective, flux-weighted, number of emitting cells, modulated by the dispersion in cell properties, particularly the EED cutoff energy at high frequencies and the low-energy spectral index at low frequencies. With a fractional dispersion in cutoff energy of about 90% and a low-energy spectral index dispersion of ~0.5-1.5, the model reproduces the chromatic mm-to-X-ray polarization trends seen by IXPE and the optical polarization limiting envelope observed in the RoboPol dataset.

published in A&A (final version after language editing and proofs)

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