# Multi-frequency VLBA Polarimetry of the high-redshift GPS Quasar OQ172

## TL;DR

This study presents multi-frequency VLBA polarimetry of the high-redshift GPS quasar OQ172, revealing complex polarization, Faraday rotation, and jet-medium interactions, providing insights into the source's magnetic environment and nuclear medium.

## Contribution

First multi-frequency VLBA polarimetry of OQ172 analyzing Faraday rotation, depolarization, and spectral models, linking jet behavior with nuclear medium properties.

## Key findings

- Faraday Rotation measures are ~2000 rad/m^2 in the core and ~700 rad/m^2 in the jet.
- Depolarization likely caused by internal medium within the source.
- Jet interaction with NLR may explain broad emission lines and jet bending.

## Abstract

Multi-frequency Very Long Baseline Array (VLBA) polarimetry observation of the GHz-Peaked Spectrum (GPS) quasar OQ172 (J1445+0958) has been performed at 1.6, 2.2, 4.8, 8.3 and 15.3 GHz in 2005. Core-jet structures are detected in all bands with the jet strongly bent at about 3 mas from the core. The radio emission of the source is polarised at all five bands. We study the Faraday Rotation in the core and jet components at all five bands, and find good linear fits of Faraday Rotation in the core and jet components at 4.8 and 8.3 GHz. At these two frequencies, the Rotation Measure (RM) is $\sim 2000~\rm rad~m^{-2}$ in the core and $\sim 700~\rm rad~m^{-2}$ in the inner jet components and continues to decrease at the outer jet parts. We find that the depolarisation at 4.8 and 8.3 GHz might be caused by the internal medium in the source. We investigate consistency of the turnover spectra of VLBI components with the Synchrotron Self-Absorption (SSA) and Free-Free Absorption (FFA) models. Although these two models can not be easily distinguished due to the lack of low-frequency data, the physical parameters can be constrained for each model. We find that the large width of the $\rm [OIII]_{5007}$ line is likely caused by a jet interaction with a Narrow Line Region (NLR) medium. The jet bending, significant RM variations, Faraday depolarisation, spectral turnover, and broad line width of $\rm [OIII]_{5007}$ could be closely related, likely caused by the same nucleus medium, presumably NLR.