Abstract

Accreting supermassive black holes in active galactic nuclei (AGN) produce powerful relativistic jets that shine from radio to GeV/TeV γ-rays. Over the past decade, AGN jets have extensively been studied in various energy bands and our knowledge about the broadband emission and rapid flares are now significantly updated. Meanwhile, the progress of magnetohydrodynamic simulations with a rotating black hole have greatly improved our theoretical understanding of powerful jet production. Nevertheless, it is still challenging to observationally resolve such flaring sites or jet formation regions since the relevant spatial scales are tiny. Observations with very long baseline interferometry (VLBI) are currently the only way to directly access such compact scales. Here we overview some recent progress of VLBI studies of AGN jets. As represented by the successful black hole shadow imaging with the Event Horizon Telescope, the recent rapid expansion of VLBI capability is remarkable. The last decade has also seen a variety of advances thanks to the advent of RadioAstron, GMVA, new VLBI facilities in East Asia as well as to the continued upgrade of VLBA. These instruments have resolved the innermost regions of relativistic jets for a number of objects covering a variety of jetted AGN classes (radio galaxies, blazars, and narrow-line Seyfert 1 galaxies), and the accumulated results start to establish some concrete (and likely universal) picture on the collimation, acceleration, recollimation shocks, magnetic field topology, and the connection to high-energy flares in the innermost part of AGN jets.

Highlights

  • A small fraction of galaxies release an enormous amount of energy from a tiny volume of the central regions that outshine the whole light of the host galaxies

  • Recent mm/submm rotation measure (RM) measurements of blazars consistently show a systematic increase of RM magnitude towards higher frequencies [162,183,184], reaching as high as 105−8 rad m−2, which are at least ∼2–5 orders of magnitude higher than those typically measured at low frequencies

  • We summarized the recent progress of very long baseline interferometry (VLBI) studies on active galactic nuclei (AGN) jets

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Summary

Introduction

A small fraction of galaxies release an enormous amount of energy from a tiny volume of the central regions that outshine the whole light of the host galaxies. At the highest frequencies, the rapid expansion of the Event Horizon Telescope (EHT) , a global 230 GHz (and 350 GHz) VLBI array, is touching the horizon-scale structures of nearby AGN thanks to its unprecedented high angular resolution (∼20 μas) and reduced optical depth towards the radio core, opening a new frontier of observational studies of AGN/SMBH. The following three sections are divided by three different classes of AGN: (1) radio galaxies, (2) blazars, and (3) narrow-line Seyfert 1 (NLSy1) galaxies These three classes of AGN are firmly established as the ones with powerful relativistic jets thanks to the recent accumulated data from the Fermi high-energy γ-ray satellite [22].

Radio Galaxies
Black Hole Shadow
Jet Kinematics and Velocity Profile
Accretion Flows and Winds
Connection to High-Energy Emission
Other Radio Galaxies
Blazars
Parsec-Scale Jets in Blazars
The Innermost Regions of Blazars
Core-Shift
Polarimetric Properties
Narrow-Line Seyfert 1 Galaxies
Parsec-Scale Radio Properties of NLSy1s
A Case Study
MWL Properties
Findings
Conclusions and Future Prospects
Full Text
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