Measurements of magnetic fields in the jets of active galactic nuclei - A tool for testing black hole jet launching models
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School of Electrical Engineering |
Doctoral thesis (article-based)
| Defence date: 2022-11-18
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Authors
Date
2022
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Mcode
Degree programme
Language
en
Pages
108 + app. 126
Series
Aalto University publication series DOCTORAL THESES, 144/2022
Abstract
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.Description
Supervising professor
Lähteenmäki, Anne, Prof., Aalto University Metsähovi Radio Observatory, FinlandThesis advisor
Savolainen, Tuomas, Dr., Aalto University Metsähovi Radio Observatory, FinlandKeywords
active galactic nuclei, relativistic jets, black hole spin, synchrotron emission, parsec-scale magnetic fields, radio astronomy, very long baseline interferometry, core-shift effect
Other note
Parts
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[Publication 1]: A. Lähteenmäki, E. Järvelä, V. Ramakrishnan, Merja Tornikoski, J. Tammi, R. J. C. Vera, and W. Chamani. Radio jets and gamma-ray emission in radio-silent narrow-line Seyfert 1 galaxies. Astronomy & Astrophysics, 614, L1, 6 pages, June 2018.
Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201808014111DOI: 10.1051/0004-6361/201833378 View at publisher
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[Publication 2]: A. Sarkar, V. R. Chitnis, A. C. Gupta, H. Gaur, S. R. Patel, P. J. Wiita, A. E. Volvach, M. Tornikoski, W. Chamani, S. Enestam, A. Lähteenmäki, J. Tammi, R. J. C Vera, and L. N. Volvach. Long-term Variability and Correlation Study of the Blazar 3C 454.3 in the Radio, NIR, and Optical Wavebands. The Astrophysical Journal, 887, 185, 14 pages, December 2019.
Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202001171618DOI: 10.3847/1538-4357/ab5281 View at publisher
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[Publication 3]: Wara Chamani, Karri Koljonen, and Tuomas Savolainen. Joint XMM-Newton and NuSTAR observations of the reflection spectrum of IIIZw2. Astronomy & Astrophysics, 635, A172, 11 pages, March 2020.
Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202004282885DOI: 10.1051/0004-6361/201936992 View at publisher
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[Publication 4]: Wara Chamani, Tuomas Savolainen, Kazuhiro Hada and Minghui Xu. Testing the magnetic flux paradigm for AGN radio loudness with a radio-intermediate quasar. Astronomy & Astrophysics, 652, A14, 28 pages, June 2021.
Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202108258421DOI: 10.1051/0004-6361/202140676 View at publisher
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[Publication 5]: Wara Chamani, Tuomas Savolainen, Eduardo Ros, Yuri Y. Kovalev, Kaj Wiik, Anne Lähteenmäki, Merja Tornikoski and Joni Tammi. Time variability of the core-shift effect in the blazar 3C 454.3. Submitted to Astronomy & Astrophysics , February 2022.
DOI: 10.48550/arXiv.2209.13301 View at publisher