POLAMI Multi-Wavelength Polarization Study of AGN Jets: A Millimeter-Optical Comparison

TL;DR

This study analyzes 14 years of multi-wavelength polarization data of AGN jets, revealing differences between BL Lacs and FSRQs, and linking polarization variability to jet magnetic field structure and luminosities.

Contribution

It provides a comprehensive multi-band polarization analysis of AGN jets, testing jet physics models with Bayesian methods and revealing new correlations between polarization properties and jet characteristics.

Findings

BL Lacs have higher fractional polarization than FSRQs.

Fractional polarization increases with frequency, indicating more ordered magnetic fields at shorter wavelengths.

EVPA variability correlates with radio and gamma-ray luminosities and inversely with synchrotron peak frequency.

Abstract

Millimeter-band polarimetry offers a powerful probe of AGN jets, accessing regions less affected by opacity and Faraday rotation than at longer radio wavelengths. As part of the POLAMI program, we have conducted 14 years of 1 mm and 3 mm polarization monitoring of a sample of gamma-ray-bright blazars with the IRAM 30-m telescope, complemented here with long-term optical polarimetric observations from multiple facilities. We aim to test whether current models of parsec-scale jet physics are consistent with observed multi-band polarization behavior. Using a Bayesian framework, we derive intrinsic mean flux densities and modulation indices for total flux and fractional polarization, and characterize EVPA variability using circular statistics. We then examine how these quantities reflecting variability properties across millimeter and optical bands relate to synchrotron peak frequency, jet orientation, and radio/gamma-ray luminosities. BL Lac objects exhibit, on average, higher fractional polarization and lower EVPA variability than FSRQs at all wavelengths. Fractional polarization increases with frequency, consistent with increasingly ordered magnetic fields at shorter wavelengths. BL Lacs also show more frequent alignment of EVPAs between optical and millimeter bands, whereas FSRQs display weaker coherence. EVPA variability correlates positively with radio and gamma-ray luminosities and negatively with synchrotron peak frequency, most strongly in the optical. We further find a positive correlation between EVPA spread and fractional polarization variability, suggesting a direct link between magnetic-field structure and polarization dynamics.