A new method for estimating frequency-dependent core shifts in active galactic nucleus jets

TL;DR

This paper introduces a novel method to estimate frequency-dependent core shifts in active galactic nucleus jets using flare time lags, validated on quasar 3C 345, enabling insights into jet physics and magnetic fields.

Contribution

The paper presents a new approach combining single-dish flare observations with VLBI data to measure core shifts and infer jet physical parameters.

Findings

Core magnetic field ~0.1 G in 3C 345

Magnetic field at 1 pc ~0.4 G

Good agreement between VLBI and flare-based core shift measurements

Abstract

We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.