PAGaN I: Multi-Frequency Polarimetry of AGN Jets with KVN

TL;DR

This study presents multi-frequency polarimetric VLBI observations of seven AGN jets using KVN, revealing high brightness temperatures, generally low polarization due to turbulence, and an exceptional polarization in BL Lac explained by shock models.

Contribution

First-epoch multi-frequency polarimetric VLBI data of AGN jets from KVN, providing insights into jet magnetic fields and turbulence effects.

Findings

Brightness temperatures >10^8 K in jets and >10^9.86 K in cores.

Overall low polarization (<10%), indicating turbulence.

High polarization (~40%) in BL Lac's jet component at 43 GHz.

Abstract

Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to $>10^{8.01}$ K and $>10^{9.86}$ K, respectively. Degrees of linear polarization $m_{L}$ are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with $m_{L} \sim$ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.