Editorial

3 pages, 665 KiB

Open AccessEditorial

by Dorit Glawion

Galaxies 2020, 8(1), 18; https://doi.org/10.3390/galaxies8010018 - 22 Feb 2020

Cited by 1 | Viewed by 1778

The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger [...] Read more.

The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger viewing angle have been detected in the TeV range. These objects also show fascinating properties, such as fast variability or spectral features and are called “radio galaxies”. The TeV radio galaxies provide a unique laboratory for studying key aspects of active galactic nuclei. This Special Issue of Galaxies targets these exciting objects. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

Research

Jump to: Editorial, Review

31 pages, 3352 KiB

Open AccessArticle

3C 84: Observational Evidence for Precession and a Possible Relation to TeV Emission

by Silke Britzen, Christian Fendt, Michal Zajaček, Frédéric Jaron, Ilya Pashchenko, Margo F. Aller and Hugh D. Aller

Galaxies 2019, 7(3), 72; https://doi.org/10.3390/galaxies7030072 - 14 Aug 2019

Cited by 19 | Viewed by 3782

Abstract

3C 84 (NGC 1275, Perseus A) is a bright radio source at the center of an ongoing merger, where HST observations show two colliding spiral galaxies. 3C 84 holds promise to improve our understanding about how of the activity of active galactic nuclei, [...] Read more.

3C 84 (NGC 1275, Perseus A) is a bright radio source at the center of an ongoing merger, where HST observations show two colliding spiral galaxies. 3C 84 holds promise to improve our understanding about how of the activity of active galactic nuclei, the formation of supermassive binary black holes, feedback processes, and galaxy collisions are interrelated. 3C,84 is one of only six radio galaxies, which reveal TeV emission. The origin of this TeV emission is still a matter of debate. Our present study is based on high resolution radio interferometric observations (15 GHz) of the pc-scale jet in this complex radio galaxy. We have re-modeled and re-analyzed 42 VLBA observations of 3C 84, performed between 1999.99 and 2017.65. In order to enable a proper alignment of the VLBA observations, we developed a method of a “differential” alignment whereby we select one reference point and minimize the deviations from this reference point in the remaining epochs. As a result, we find strong indication for a precession of the 3C 84 jet—not only for its central regions, but also for the outer lobe at 10 mas distance. These findings are further supported by our kinematic precession modeling of the radio flux-density monitoring data provided by the University of Michigan Radio Observatory and the Owens Valley Radio Observatory, which yields a precession time scale of about 40 yr. This time scale is further supported by literature maps obtained about 40 yr ago (1973 and 1974.1) which reveal a similar central radio structure. We suggest that the TeV flare detected by MAGIC may correlate with the precession of 3C 84, as we disentangle a projected reversal point of the precessing motion that correlates with the flaring time. This may physically be explained by a precessing jet sweeping over a new region of so far undisturbed X-ray gas which would then lead to shock-produced TeV-emission. In addition, we perform a correlation analysis between the radio data and GeV data obtained by the Fermi Gamma-ray Space Telescope and find that the

γ

-ray data are lagging the radio data by 300–400 days. A possible explanation could be that the radio and the GeV data stem from different emission regions. We discuss our findings and propose that the detected jet precession can also account for the observed cavities in the X-ray emission on kpc-scales. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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8 pages, 531 KiB

Open AccessArticle

Centaurus A: Hard X-ray and High-Energy Gamma-Ray Light Curve Correlation

by Isak Delberth Davids, Markus Böttcher and Michael Backes

Galaxies 2019, 7(2), 44; https://doi.org/10.3390/galaxies7020044 - 04 Apr 2019

Cited by 2 | Viewed by 3161

Abstract

Centaurus A, powered by a 55 million solar mass supermassive black hole, has been intensively monitored in all accessible wavelength ranges of the electromagnetic spectrum. However, its very-high energy gamma (

γ

) ray flux (TeV photons), obtained from H.E.S.S. is relatively faint, [...] Read more.

Centaurus A, powered by a 55 million solar mass supermassive black hole, has been intensively monitored in all accessible wavelength ranges of the electromagnetic spectrum. However, its very-high energy gamma (

γ

) ray flux (TeV photons), obtained from H.E.S.S. is relatively faint, hampering detailed light curve analyses in the most energetic energy band. Yet, the extensive long-term light curve data from Fermi-LAT and Swift-BAT (hard X-rays) allows for cross-correlation studies. We find a hint that X-ray emission from Centaurus A precedes the

γ

rays by

25 \pm 125

days. If this lag is real and related to a

γ γ

absorption effect in the broad-line region (BLR) around the central source, we can constrain the size of the BLR using light-travel time arguments. These are first results of extended light curve correlation studies between high-energy

γ

rays and X-rays from Centaurus A. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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Review

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19 pages, 4767 KiB

Open AccessReview

The High Energy View of FR0 Radio Galaxies

by Ranieri Diego Baldi, Eleonora Torresi, Giulia Migliori and Barbara Balmaverde

Galaxies 2019, 7(3), 76; https://doi.org/10.3390/galaxies7030076 - 05 Sep 2019

Cited by 16 | Viewed by 3689

Abstract

A new class of low-power compact radio sources with limited jet structures, named FR 0, is emerging from recent radio-optical surveys. This abundant population of radio galaxies, five times more numerous than FR Is in the local Universe (z < 0.05), represent a [...] Read more.

A new class of low-power compact radio sources with limited jet structures, named FR 0, is emerging from recent radio-optical surveys. This abundant population of radio galaxies, five times more numerous than FR Is in the local Universe (z < 0.05), represent a potentially interesting target at high and very-high energies (greater than 100 GeV), as demonstrated by a single case of Fermi detection. Furthermore, these radio galaxies have been recently claimed to contribute non-negligibly to the extra-galactic γ-ray background and to be possible cosmic neutrino emitters. Here, we review the radio through X-ray properties of FR 0s to predict their high-energy emission (from MeV to TeV), in light of the near-future facilities operating in this band. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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35 pages, 7420 KiB

Open AccessEditor’s ChoiceReview

Dissipative Processes and Their Role in the Evolution of Radio Galaxies

by Manel Perucho

Galaxies 2019, 7(3), 70; https://doi.org/10.3390/galaxies7030070 - 31 Jul 2019

Cited by 27 | Viewed by 3496

Abstract

Particle acceleration in relativistic jets, to very high levels of energy, occurs at the expense of the dissipation of magnetic or kinetic energy. Therefore, understanding the processes that can trigger this dissipation is key to the characterization of the energy budgets and particle [...] Read more.

Particle acceleration in relativistic jets, to very high levels of energy, occurs at the expense of the dissipation of magnetic or kinetic energy. Therefore, understanding the processes that can trigger this dissipation is key to the characterization of the energy budgets and particle acceleration mechanisms in action in active galaxies. Instabilities and entrainment are two obvious candidates to trigger dissipation. On the one hand, supersonic, relativistic flows threaded by helical fields, as expected from the standard formation models of jets in supermassive black-holes, are unstable to a series of magnetohydrodynamical instabilities, such as the Kelvin–Helmholtz, current-driven, or possibly the pressure-driven instabilities. Furthermore, in the case of expanding jets, the Rayleigh–Taylor and centrifugal instabilities may also develop. With all these destabilizing processes in action, a natural question is to ask how can some jets keep their collimated structure along hundreds of kiloparsecs. On the other hand, the interaction of the jet with stars and clouds of gas that cross the flow in their orbits around the galactic centers provides another scenario in which kinetic energy can be efficiently converted into internal energy and particles can be accelerated to non-thermal energies. In this contribution, I review the conditions under which these processes occur and their role both in jet evolution and propagation and energy dissipation. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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19 pages, 1434 KiB

Open AccessEditor’s ChoiceReview

Radio Galaxies—The TeV Challenge

by Bindu Rani

Galaxies 2019, 7(1), 23; https://doi.org/10.3390/galaxies7010023 - 22 Jan 2019

Cited by 15 | Viewed by 4821

Abstract

Over the past decade, our knowledge of the

γ

-ray sky has been revolutionized by ground- and space-based observatories by detecting photons up to several hundreds of tera-electron volt (TeV) energies. A major population of the

γ

-ray bright objects are active galactic [...] Read more.

Over the past decade, our knowledge of the

γ

-ray sky has been revolutionized by ground- and space-based observatories by detecting photons up to several hundreds of tera-electron volt (TeV) energies. A major population of the

γ

-ray bright objects are active galactic nuclei (AGN) with their relativistic jets pointed along our line-of-sight. Gamma-ray emission is also detected from nearby misaligned AGN such as radio galaxies. While the TeV-detected radio galaxies (

T e V R a d

) only form a small fraction of the

γ

-ray detected AGN, their multi-wavelength study offers a unique opportunity to probe and pinpoint the high-energy emission processes and sites. Even in the absence of substantial Doppler beaming

T e V R a d

are extremely bright objects in the TeV sky (luminosities detected up to

10^{45}

erg s

^{- 1}

), and exhibit flux variations on timescales shorter than the event-horizon scales (flux doubling timescale less than 5 min). Thanks to the recent advancement in the imaging capabilities of high-resolution radio interferometry (millimeter very long baseline interferometry, mm-VLBI), one can probe the scales down to less than 10 gravitational radii in

T e V R a d

, making it possible not only to test jet launching models but also to pinpoint the high-energy emission sites and to unravel the emission mechanisms. This review provides an overview of the high-energy observations of

T e V R a d

with a focus on the emitting sites and radiation processes. Some recent approaches in simulations are also sketched. Observations by the near-future facilities like Cherenkov Telescope Array, short millimeter-VLBI, and high-energy polarimetry instruments will be crucial for discriminating the competing high-energy emission models. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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37 pages, 6395 KiB

Open AccessReview

Very High-Energy Emission from the Direct Vicinity of Rapidly Rotating Black Holes

by Kouichi Hirotani

Galaxies 2018, 6(4), 122; https://doi.org/10.3390/galaxies6040122 - 22 Nov 2018

Cited by 8 | Viewed by 3188

Abstract

When a black hole accretes plasmas at very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere. [...] Read more.

When a black hole accretes plasmas at very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere. Such pairs efficiently screen the electric field along the magnetic field lines, when the accretion rate is typically greater than 0.03–0.3% of the Eddington rate. However, when the accretion rate becomes smaller than this value, the number density of the created pairs becomes less than the rotationally induced Goldreich–Julian density. In such a charge-starved magnetosphere, an electric field arises along the magnetic field lines to accelerate charged leptons into ultra-relativistic energies, leading to an efficient TeV emission via an inverse-Compton (IC) process, spending a portion of the extracted hole’s rotational energy. In this review, we summarize the stationary lepton accelerator models in black hole magnetospheres. We apply the model to super-massive black holes and demonstrate that nearby low-luminosity active galactic nuclei are capable of emitting detectable gamma-rays between 0.1 and 30 TeV with the Cherenkov Telescope Array. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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25 pages, 3110 KiB

Open AccessEditor’s ChoiceReview

Radio Galaxies at VHE Energies

by Frank M. Rieger and Amir Levinson

Galaxies 2018, 6(4), 116; https://doi.org/10.3390/galaxies6040116 - 15 Nov 2018

Cited by 44 | Viewed by 4226

Abstract

Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of [...] Read more.

Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of active galactic nuclei (AGN) in general. This review provides a compact summary of their observed characteristics at very high γ-ray energies (VHE; greater than 100 GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

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