Energy-Dependent Polarization Angle Variability as a Robust Diagnostic for Blazar Flaring Mechanisms

TL;DR

This paper introduces a new method using the energy dependence of polarization angle variability to distinguish between different blazar flaring mechanisms, validated through simulations and observational data.

Contribution

It demonstrates that the energy dependence of polarization variability can reliably differentiate magnetic reconnection from turbulence in blazar flares.

Findings

Reconnection predicts higher polarization angle variability at higher energies.

Turbulence results in nearly flat polarization variability across the spectrum.

Application to real data indicates reconnection-driven flares in observed blazars.

Abstract

Identifying the physical mechanism driving blazar flares remains a central challenge in high-energy astrophysics. We show that the energy dependence of the standard deviation of the polarization angle variability ($\sigma_\text{PA}$) provides a powerful and robust discriminator of blazar flaring mechanisms. Using particle-in-cell-integrated polarized radiative transfer simulations, we perform to-date the most rigorous statistical analyses of polarization variability. We demonstrate that magnetic reconnection and magnetized turbulence imprint qualitatively distinct energy dependence of $\sigma_\text{PA}$ that directly reflect their different magnetic field evolution and particle transport. Reconnection predicts higher $\sigma_\text{PA}$ with higher photon energy till the synchrotron spectral peak, whereas turbulence produces nearly flat $\sigma_\text{PA}$ across the synchrotron spectral component. These trends are resilient to realistic observational limitations. Applying our results to optical and IXPE data of Mrk~421 and 1ES~1959+650, we find strong evidence for reconnection-driven flares embedded in a turbulent blazar zone. Energy-dependent $\sigma_\text{PA}$ emerges as a decisive new probe of particle acceleration in relativistic jets.