Testing the magnetic flux paradigm for AGN radio loudness with a radio-intermediate quasar

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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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Astronomy and Astrophysics, Volume 652
For understanding the diversity of jetted active galactic nuclei (AGN) and especially the puzzling wide range in their radio loudness, it is important to understand what role the magnetic fields play in setting the power of relativistic jets in AGN. We have performed VLBA phase-referencing observations of the radio-intermediate quasar IIIZw 2 to estimate jet magnetic flux by measuring the core-shift effect. Multi-frequency observations at 4 GHz, 8 GHz, 15 GHz, and 24 GHz were made using three nearby calibrators as reference sources. By combining the self-referencing core shift of each calibrator with the phase-referencing core shifts, we obtained an upper limit of 0.16 mas for the core shift between 4 and 24 GHz in IIIZw 2. By assuming equipartition between magnetic and particle energy densities and adopting the flux-freezing approximation, we further estimated the upper limit for both the magnetic field strength and poloidal magnetic flux threading the black hole. We find that the upper limit to the measuredmagnetic flux is smaller by at least a factor of five compared to the value predicted by the magnetically arrested disk (MAD) model. An alternative way to derive the jet magnetic field strength from the turnover of the synchrotron spectrum leads to an even smaller upper limit. Hence, the central engine of IIIZw 2 has not reached the MAD state, which could explain why it has failed to develop a powerful jet even though the source harbours a fast-spinning black hole. However, it generates an intermittent jet, which is possibly triggered by small-scale magnetic field fluctuations, as predicted by the magnetic flux paradigm. We propose here that combining black hole spin measurements with magnetic field measurements from the very-long-baseline-interferometry core-shift observations of AGN over a range of jet powers could provide a strong test for the dominant factor that sets the jet power relative to the available accretion power.
Funding Information: Acknowledgements. The authors thank Karri Koljonen for his help in the preparation of the observing proposals, Maria Rioja, Richard Dodson, Andreas Brun-thaler and Andrzej Zdziarski for helpful discussions, and Eduardo Ros for careful reading of the manuscript. This work was supported partly by the Academy of Finland projects 274477 and 315721. W. Chamani is thankful for the visiting joint research grant supported by the National Astronomical Observatory of Japan (NAOJ). The Very Long Baseline Array (VLBA) is an instrument of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence. This publication makes use of data obtained at the Metsähovi Radio Observatory, operated by the Aalto University in Finland. Publisher Copyright: © 2021 ESO.
Astrometry, Galaxies: active, Galaxies: jets, Galaxies: magnetic fields, Galaxies: Seyfert, Techniques: high angular resolution
Chamani , W , Savolainen , T , Hada , K & Xu , M H 2021 , ' Testing the magnetic flux paradigm for AGN radio loudness with a radio-intermediate quasar ' , Astronomy and Astrophysics , vol. 652 , A14 . https://doi.org/10.1051/0004-6361/202140676