Investigating Differences in the Palomar-Green Blazar Population Using Polarization

TL;DR

This study uses VLA polarization imaging to analyze the magnetic field structures and activity patterns in a complete sample of PG blazars, revealing insights into jet orientation, interaction, and reorientation mechanisms.

Contribution

It provides the first comprehensive polarization analysis of an optically-selected, volume-limited PG blazar sample, highlighting jet behavior, environmental interactions, and reorientation evidence.

Findings

Kpc-scale polarization observed in all blazars' cores and jets.

Quasar jets align with parsec-scale jets and show magnetic field compression.

BL Lacs exhibit multiple jet reorientations and features consistent with strong jet sources.

Abstract

We present polarization images with the Karl G. Jansky Very Large Array (VLA) in A and B-array configurations at 6 GHz of 7 radio-loud (RL) quasars and 8 BL Lac objects belonging to the Palomar-Green (PG) `blazar' sample. This completes our arcsecond-scale polarization study of an optically-selected volume-limited blazar sample comprising 16 radio-loud quasars and 8 BL Lac objects. Using the VLA, we identify kpc-scale polarization in the cores and jets/lobes of all the blazars, with fractional polarization varying from around $0.8 \pm 0.3$% to $37 \pm 6$%. The kpc-scale jets in PG RL quasars are typically aligned along their parsec-scale jets and show apparent magnetic fields parallel to jet directions in their jets/cores and magnetic field compression in their hotspots. The quasars show evidence of interaction with their environment as well as restarted AGN activity through morphology, polarization and spectral indices. These quasi-periodic jet modulations and restarted activity may be indicative of an unstable accretion disk undergoing transition. We find that the polarization characteristics of the BL Lacs are consistent with their jets being reoriented multiple times, with no correlation between their core apparent magnetic field orientations and pc-scale jet directions. We find that the low synchrotron peaked BL Lacs show polarization and radio morphology features typical of `strong' jet sources as defined by Meyer et al. (2011) for the `blazar envelope scenario', which posits a division based on jet profiles and velocity gradients rather than total jet power.