Measurements of magnetic fields in the jets of active galactic nuclei - A tool for testing black hole jet launching models

Research output: ThesisDoctoral ThesisCollection of Articles


Active galactic nuclei (AGNs) are accreting supermassive black holes residing at the centres of galaxies that can launch jets made of magnetised plasma, reaching speeds close to the speed of light. In the context of the Blandford-Znajek mechanism, the magnetic flux paradigm of Sikora and Begelman (2013) can explain the efficient jet production of AGNs via the large-scale poloidal magnetic flux accumulation around a fast-spinning black hole that gives rise to the formation of a magnetically arrested disc (MAD). In the above scenario, magnetic fields play an important role in launching relativistic jets in AGNs and possibly producing variability in their radio emission. This thesis aims to test the magnetic flux paradigm observationally by using the so-called core-shift method to estimate the parsec-scale jet magnetic fields close to the black hole. Furthermore, it addresses how to reliably infer magnetic field strengths by measuring core-shift (rcore ∝ ν−1/kr) variability. The magnetic flux paradigm has been effectively tested with a sample of radio-loud AGNs, not including radio-intermediate and radio-quiet AGNs. In this thesis, the model was tested for the first time with a radio-intermediate quasar III Zw 2. For this, the study utilised (single epoch) multi-frequency Very Long Baseline Array (VLBA) observations to infer the magnetic flux and joint XMM-Newton and NuSTAR X-ray observations to measure the black hole spin by fitting a relativistic reflection model to the reflection spectrum. The results on III Zw 2 revealed that the magnetic flux value predicted by the MAD model is higher by a factor of five compared to the upper limit of the magnetic flux. Thus, the source did not reach the MAD state despite harbouring a fast-spinning black hole. These findings show that the magnetic flux controls the jet production efficiency, as predicted by the magnetic flux paradigm. Our study on III Zw 2 presents a new observational method for testing the magnetic flux paradigm and proposes extending such a study to radio-quiet AGNs. The investigation of the core-shift time variability was performed on the blazar 3C454.3. For this purpose, multi-frequency and multi-epoch (19 epochs) VLBA observations were utilised. Additionally, radio, near-infrared and optical observations on 3C454.3 were employed to study variability and waveband correlations. Our results on 3C454.3 show significant time variability of the core-shift magnitude, confirming a previous study. We have demonstrated for the first time the variability of the core-shift index kr also finding kr ≤ 1 during flaring and quiescent states in the study period. These results indicate deviations from the ideal conditions of equipartition and conical jet shape (kr = 1) of the Blandford and K ̈onigl jet model. Hence, both conditions might not hold simultaneously. Our study suggests that the derived magnetic field parameters are reliable only as long as the core-shift observations confirm kr = 1.
Translated title of the contributionMeasurements of magnetic fields in the jets of active galactic nuclei - A tool for testing black hole jet launching models
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Lähteenmäki, Anne, Supervising Professor
  • Savolainen, Tuomas, Thesis Advisor
Print ISBNs978-952-64-0969-6
Electronic ISBNs978-952-64-0970-2
Publication statusPublished - 2022
MoE publication typeG5 Doctoral dissertation (article)


  • active galactic nuclei
  • relativistic jets
  • black hole spin
  • synchrotron emission
  • parsec-scale magnetic fields
  • radio astronomy
  • very long baseline interferometry
  • core-shift effect


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