A high-resolution radio morphology and polarization of the kpc-scale X-ray jet of PKS 1127-145

TL;DR

This study presents high-resolution radio and X-ray observations of the 300 kpc jet of quasar PKS 1127-145, revealing detailed jet morphology, polarization, and magnetic field structure, and analyzing the jet's behavior during a gamma-ray flare.

Contribution

First high-resolution multi-frequency radio and X-ray imaging of PKS 1127-145's jet, revealing magnetic field structure, jet bending, and flare region localization.

Findings

Inner jet is highly polarized with magnetic fields aligned with the jet axis.

Jet bending correlates with magnetic field rotation and radio re-brightening.

No significant radio or X-ray brightening during gamma-ray flare.

Abstract

We report on new multi-frequency Very Large Array (VLA) radio observations and Chandra X-ray observations of a radio-loud quasar with a ~300 kpc long jet, PKS 1127-145, during a flaring event detected in $\gamma$-rays by the Fermi Large Area Telescope in 2020 December. The high angular resolution of the new radio images allows us to disentangle for the first time the inner kpc-scale jet from the core contribution. The inner radio jet, up to 15 kpc from the core, is highly polarized (33 per cent) and the magnetic field is parallel to the jet axis. At about 18 arcsec from the core the jet slightly bends and we observe a re-brightening of the radio emission and a 90-degree rotation of the magnetic field, likely highlighting the presence of a shock that is compressing the magnetic field to a plane perpendicular to the jet axis and where efficient particle acceleration takes place. At the same position the X-ray emission fades, suggesting a deceleration of the bulk velocity of the jet after the bend. A change in velocity and collimation of the jet is supported by the widening of the jet profile and the detection of a limb-brightened structure connecting the bending region with the jet termination. The limb-brightened structure might indicate the co-existence of both longitudinal and transverse velocity gradients at the jet bending. There is no evidence for significant brightening of the kpc-scale jet in the radio or X-ray band during the $\gamma$-ray flare. The X-ray flux, $F_{\rm 2-10\,keV} = (6.24\pm0.57)\times10^{-12}$ ergs s$^{-1}$ cm$^{-2}$, measured by Chandra from the quasar core is consistent with the flux measured by the X-Ray Telescope on board the Neil Gehrels Swift Observatory after the high-energy flare. Our results indicate that the $\gamma$-ray flaring region is located within the VLA source core.