Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets

TL;DR

This study measures magnetic fields and spectral properties of six active galactic nuclei jets using VLBA observations across multiple frequencies, confirming core-shift behavior consistent with jet models and estimating magnetic field strengths near the cores.

Contribution

It provides new measurements of core-shifts and magnetic fields in AGN jets, supporting the equipartition and Blandford-Konigl jet models with empirical data.

Findings

Core-shift measurements align with equipartition jet models.

Magnetic field strengths in cores are 10s to 100s of milligauss.

Results are consistent with theoretical jet-launching magnetic fields.

Abstract

We use observations of six "blazars" with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2, 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ("core-shift") and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multi-frequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows $r_{core}\propto\nu^{-1}$. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10's to 100's of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disk and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets.