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

TL;DR

This work leverages ~15 years of POLAMI millimeter polarization data (1 mm and 3 mm) together with extensive optical polarimetry to compare blazar polarization across bands. Using Bayesian deconvolution to recover intrinsic flux and polarization statistics and circular statistics for EVPA, the study uncovers systematic differences between BL Lacs and FSRQs: BL Lacs show higher $p_0$, lower EVPA variability, and more cross-band EVPA alignment, while FSRQs exhibit stronger EVPA variability and weaker cross-band coherence; EVPA spread anti-correlates with the synchrotron peak frequency and correlates with radio/gamma-ray luminosities, supporting an energy-stratified, multi-zone jet picture with possible disk/BLR optical contributions in some FSRQs. The results point to a magnetic-field structure that is partially ordered yet turbulent, consistent with hybrid models incorporating both large-scale geometry and localized reconnection/turbulence, and have significant implications for understanding jet emission sites and magnetic-field configurations in blazars.

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.

Figures (12)

• Figure 1: Millimeter (upper hemisphere) and optical (lower hemisphere) EVPA distribution in J2232+1143. EVPA distributions at 3 mm (blue), 1 mm (yellow), and optical (grey) wavelengths are overlaid by vectors of the same color representing the mean value of the respective distribution. The red line marks the jet PA. The different circles mark the counts as a fraction of the total. Corresponding distributions for all other sources in the sample are provided in Appendix \ref{['app:A']}.
• Figure 2: Dependence of the EVPA spread on the synchrotron peak position. The vertical lines in each panel indicate the respective observing frequencies. NOTES: The trend observed at 1 and 3 mm, resembling the trend at optical wavelength, is similar to the trend in Angelakis2016 and fits with the model proposed in Potter2015. The gray line and the shadow area represent the best linear fit using a least-squares fitting method and the 1-$\sigma$ deviation, respectively.
• Figure 3: EVPAs spread in three bands as a function of radio luminosity. The gray line and the shadow area represent the best linear fit using a least-squares fitting method and the 1-$\sigma$ deviation, respectively.
• Figure 4: EVPAs spread in three bands as a function of gamma-ray luminosity. The gray line and shadow area are as in Fig. \ref{['fig:EVPAstd_vs_RadioLumin']}.
• Figure 5: The optical EVPA spread in function of the alignment between the optical EVPA and the jet PA (upper panel), the 1 mm EVPAs (middle panel), and the 3 mm EVPAs (bottom panel). The optical EVPA spread correlates with the divergence between optical EVPAs and the three quantities explored in the three panels. The gray line and the shadow area represent the best linear fit using a least-squares fitting method and the 1-$\sigma$ deviation, respectively.