AGN jet physics from measurements of the frequency-dependent position of the VLBI radio core

TL;DR

This study measures the frequency-dependent position shifts of VLBI radio cores to derive physical parameters like magnetic field strength and core distance, using a cross-correlation alignment method validated against phase-referencing.

Contribution

It introduces a cross-correlation technique for aligning VLBI images to accurately measure core shifts, enabling precise estimation of magnetic fields and core locations in AGN jets.

Findings

Magnetic field strength of 0.15 G in Mrk 501 at 8.4 GHz.

Estimated magnetic fields of ~1 G in millimeter VLBI cores.

Core distances ranging from 0.8 to 20 pc depending on frequency and source.

Abstract

Accurate measurement of the frequency-dependent shift of the self-absorbed radio core is required for multi-frequency analysis of VLBI data since absolute positional information is lost as a result of phase self-calibration. We use the cross-correlation technique of Croke & Gabuzda (2008) on the optically thin jet emission to align our VLBA images. Our results are consistent with those obtained from the phase-referencing method, as well as alignment by model-fitted optically thin jet components. Physical parameters of the compact jet regions, such as the magnetic field strength (B_core) and the distance of the radio core to the jet origin (r_core), can be calculated from these measurements. For the source Mrk 501, we find a magnetic field strength of 0.15\pm0.04 G in the 8.4-GHz core at a distance of 0.8\pm0.2 pc from the base of the jet. By extrapolating our 4.6 to 15.4 GHz results for BL Lac (2200+420), we estimate magnetic field strengths of the order of 1 G in the millimetre VLBI core. Using our core-shift measurement between 1.6 and 4.8 GHz for 1803+784, we find B_core(4.8 GHz) = 0.11\pm0.02 G and r_core(4.8 GHz) = 20\pm5 pc. The phase-referencing observations of this source at 8.4 and 43 GHz by Jim\'enez-Monferrer et al. (2008) imply B_core(43 GHz) = 1.0\pm0.4 G and r_core(43 GHz) = 2.0\pm0.9 pc.