Extragalactic TeV Astronomy

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (30 November 2021) | Viewed by 24690

Special Issue Editors

School of Physics, University College Dublin, Belfield, Dublin D04 V1W8, Ireland
Interests: extragalactic astrophysics; relativistic jets; instrumentation; software for high-energy astrophysics; cosmology; dark matter; blazar modelling

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Guest Editor
Laboratoire Leprince Ringuet , Ecole Polytechnique, CNRS /IN2P3, 91128 Palaiseau, France
Interests: gamma-ray blazars; GeV-TeV connection; catalogue of the TeV sky

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Guest Editor
Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
Interests: extragalactic science; gamma-ray cosmology; fundamental physics

Special Issue Information

Dear Colleagues,

The first detection of an extragalactic TeV source, Markarian 421, in 1992, by the imaging atmospheric Cherenkov telescopes (IACTs), marked the beginning of a vibrant new era in extragalactic astrophysics. With the current generation of leading IACT telescopes (H.E.S.S., MAGIC, and VERITAS), more than 80 extragalactic objects have been detected, including blazars, radio galaxies, starburst galaxies and, most recently, gamma-ray bursts. These TeV detections have come during a new era of multi-wavelength observations, enabled by the extensive coverage provided by instruments such as those on-board Fermi and Swift, as well as a large cohort of ground-based telescopes. The GeV–TeV connection afforded by the simultaneous operations of the IACTs and the Fermi-LAT has proved particularly fruitful; both from the perspective of determining promising candidates for TeV emissions, and in allowing complementary spectral measurements of gamma-ray sources.

The TeV observations provide a key component of the broadband multi-wavelength spectra of these objects, essential to understanding the physical processes at work, and the measurements which have also been used to probe fundamental physics. The recent detections of extragalactic gamma-ray source activity in coincidence with IceCube neutrino events, in addition to the recent TeV detections of gamma-ray bursts, has further highlighted the critical role which gamma-ray observations play in the multi-messenger context.

The aim of this Special Issue is to review the current state of the art and explore prospects for TeV extragalactic astrophysics. It is particularly timely, because the current generation of IACTs will, in the near future, be superseded by the next-generation IACT observatory, the Cherenkov Telescope Array (CTA), and the multi-wavelength and multi-messenger landscapes are also evolving. The goal is to address certain key questions in TeV extragalactic astrophysics: What have we learned? What are the outstanding issues? What observations, both in the TeV and multi-wavelength contexts, are needed to address those issues? 

In this Special Issue, original research articles and reviews are welcome. We look forward to receiving your contributions.

Dr. John Quinn
Dr. Deirdre Horan
Dr. Elisa Pueschel
Guest Editors

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Keywords

  • TeV
  • VHE
  • gamma ray
  • extragalactic
  • blazar
  • radio galaxy
  • gamma-ray burst

Published Papers (9 papers)

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Review

28 pages, 2528 KiB  
Review
Blazars at Very High Energies: Emission Modelling
by Hélène Sol and Andreas Zech
Galaxies 2022, 10(6), 105; https://doi.org/10.3390/galaxies10060105 - 03 Nov 2022
Cited by 3 | Viewed by 1887
Abstract
Blazars are very broadband cosmic sources with spectra spanning over twenty orders of magnitude in frequency, down to the 100 MHz regime in the radio range, up to VHE at several tens of TeV. The modelling of their spectral energy distribution at high [...] Read more.
Blazars are very broadband cosmic sources with spectra spanning over twenty orders of magnitude in frequency, down to the 100 MHz regime in the radio range, up to VHE at several tens of TeV. The modelling of their spectral energy distribution at high energies currently considers two main classes of models, leptonic and lepto-hadronic, which both succeed fairly well in describing the observed spectra for the two populations of blazars, namely BL Lac objects (BL Lacs) and flat spectrum radio quasars (FSRQs). However they are both confronted with difficulties, in particular to reproduce flaring phenomena monitored with a good multi-spectral and temporal coverage, or to reproduce extreme sources which challenge the basic descriptions. Such a situation has led to a diversity of specific scenarios, the positioning of which in relation to the general context of the sources is generally not clearly fixed. The identification of the dominant particle acceleration mechanism at work and a better understanding of the location of the TeV emitting zone would make it possible to break the degeneracies between models. Multi-wavelength and multi-messenger studies should also help in this regard, with the perspective to elaborate a general reference scenario of blazars and AGNs. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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29 pages, 3386 KiB  
Review
TeV Dark Matter Searches in the Extragalactic Gamma-ray Sky
by Moritz Hütten and Daniel Kerszberg
Galaxies 2022, 10(5), 92; https://doi.org/10.3390/galaxies10050092 - 29 Aug 2022
Cited by 4 | Viewed by 2217
Abstract
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray [...] Read more.
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray signature is expected to be universal from various targets, uncertainties of astrophysical origin strongly affect and weaken the limits. At the same time, spurious signals may originate from non-DM related processes. The many gamma-ray targets in the extragalactic sky being searched for DM play a crucial role to keep these uncertainties under control and to ultimately achieve an unambiguous DM detection. Lately, a large progress has been made in combined analyses of TeV DM candidates towards different targets by using data from various instruments and over a wide range of gamma-ray energies. These approaches not only resulted in an optimal exploitation of existing data and an improved sensitivity, but also helped to level out target- and instrument-related uncertainties. This review gathers all searches in the extragalactic sky performed so far with the space-borne Fermi-Large Area Telescope, the ground-based imaging atmospheric Cherenkov telescopes, and the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC). We discuss the different target classes and provide a complete list of all analyses so far. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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23 pages, 1798 KiB  
Review
Open-Source Radiative Modeling Tools for Extragalactic VHE Gamma-ray Sources
by Cosimo Nigro and Andrea Tramacere
Galaxies 2022, 10(4), 85; https://doi.org/10.3390/galaxies10040085 - 31 Jul 2022
Viewed by 1987
Abstract
In this review, we discuss various open-source software for modeling the broadband emission of extragalactic sources from radio up to the highest gamma-ray energies. As we provide an overview of the different tools available, we discuss the physical processes that such tools implement [...] Read more.
In this review, we discuss various open-source software for modeling the broadband emission of extragalactic sources from radio up to the highest gamma-ray energies. As we provide an overview of the different tools available, we discuss the physical processes that such tools implement and detail the computations they can perform. We also examine their conformity with modern good software practices. After considering the currently available software as a first generation of open-source modeling tools, we outline some desirable characteristics for the next generation. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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34 pages, 2096 KiB  
Review
Gamma-Ray Bursts at TeV Energies: Theoretical Considerations
by Ramandeep Gill and Jonathan Granot
Galaxies 2022, 10(3), 74; https://doi.org/10.3390/galaxies10030074 - 30 May 2022
Cited by 14 | Viewed by 2580
Abstract
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt γ-ray emission briefly outshines the rest of the γ-ray sky, making them detectable from cosmological distances. A burst is followed by, and [...] Read more.
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt γ-ray emission briefly outshines the rest of the γ-ray sky, making them detectable from cosmological distances. A burst is followed by, and sometimes partially overlaps with, a similarly energetic but very broadband and longer-lasting afterglow emission. While most GRBs are detected below a few MeV, over 100 have been detected at high (≳0.1 GeV) energies, and several have now been observed up to tens of GeV with the Fermi Large Area Telescope (LAT). A new electromagnetic window in the very-high-energy (VHE) domain (≳0.1 TeV) was recently opened with the detection of an afterglow emission in the (0.11)TeV energy band by ground-based imaging atmospheric Cherenkov telescopes. The emission mechanism for the VHE spectral component is not fully understood, and its detection offers important constraints for GRB physics. This review provides a brief overview of the different leptonic and hadronic mechanisms capable of producing a VHE emission in GRBs. The same mechanisms possibly give rise to the high-energy spectral component seen during the prompt emission of many Fermi-LAT GRBs. Possible origins of its delayed onset and long duration well into the afterglow phase, with implications for the emission region and relativistic collisionless shock physics, are discussed. Key results for using GRBs as ideal probes for constraining models of extra-galactic background light and intergalactic magnetic fields, as well as for testing Lorentz invariance violation, are presented. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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27 pages, 2685 KiB  
Review
Radio Galaxies at TeV Energies
by Cameron Rulten
Galaxies 2022, 10(3), 61; https://doi.org/10.3390/galaxies10030061 - 22 Apr 2022
Cited by 4 | Viewed by 2975
Abstract
Unlike blazars, radio galaxies have jets that are misaligned relative to our line-of-sight. This misaligned geometry provides us with a unique view of both the jet and super massive black hole. To date, four radio galaxies have been detected at TeV energies with [...] Read more.
Unlike blazars, radio galaxies have jets that are misaligned relative to our line-of-sight. This misaligned geometry provides us with a unique view of both the jet and super massive black hole. To date, four radio galaxies have been detected at TeV energies with an additional two active galactic nuclei shown to exhibit both radio galaxy and BL Lac-type properties. TeV observations of radio galaxies have revealed these objects to be fascinating, displaying ultra-fast variability and often relatively hard spectral energy distributions. This work aims to provide a review of the current state of radio galaxy observations within the context of very-high-energy γ-ray astronomy, while also highlighting that radio galaxies are excellent targets for multi-wavelength observations. A number of motivations for the continued study of radio galaxies are provided, and these are discussed with a focus on the key observational results, including implications for future observations with next-generation instruments soon to be operational. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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28 pages, 990 KiB  
Review
Gamma-Ray Cosmology and Tests of Fundamental Physics
by Jonathan Biteau and Manuel Meyer
Galaxies 2022, 10(2), 39; https://doi.org/10.3390/galaxies10020039 - 22 Feb 2022
Cited by 13 | Viewed by 3164
Abstract
The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and [...] Read more.
The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and positrons on the cosmic optical and infrared backgrounds. Electrons and positrons cool in the intergalactic medium while gyrating in its magnetic fields, which could cause either its global heating or the production of lower-energy secondary gamma-rays. The energy distribution of gamma-rays surviving the cosmological journey carries observed absorption features that gauge the emissivity of baryonic matter over cosmic time, constrain the distance scale of ΛCDM cosmology, and limit the alterations of the interaction cross section. Competitive constraints are, in particular, placed on the cosmic star-formation history as well as on phenomena expected from quantum gravity and string theory, such as the coupling to hypothetical axion-like particles or the violation of Lorentz invariance. Recent theoretical and observational advances offer a glimpse of the multi-wavelength and multi-messenger path that the new generation of gamma-ray observatories is about to open. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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18 pages, 3178 KiB  
Review
The Blazar Sequence and Its Physical Understanding
by Elisa Prandini and Gabriele Ghisellini
Galaxies 2022, 10(1), 35; https://doi.org/10.3390/galaxies10010035 - 15 Feb 2022
Cited by 27 | Viewed by 3321
Abstract
Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as the physical interpretation of the overall emission from these [...] Read more.
Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as the physical interpretation of the overall emission from these galaxies. In this review, we describe the evolution of the sequence along with the tremendous advances in the observational field, in particular in the gamma-ray band. A new version of the sequence built on TeV-detected objects is also presented. Two extreme classes of objects (MeV and hard-TeV blazars) are included in the discussion, given their relevance for future observatories. Finally, the current physical understanding at the base of the sequence is presented along with the major criticisms to the blazar sequence. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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21 pages, 536 KiB  
Review
TeV Instrumentation: Current and Future
by Julian Sitarek
Galaxies 2022, 10(1), 21; https://doi.org/10.3390/galaxies10010021 - 27 Jan 2022
Cited by 9 | Viewed by 2608
Abstract
During the last 20 years, TeV astronomy has turned from a fledgling field, with only a handful of sources, into a fully-developed astronomy discipline, broadening our knowledge on a variety of types of TeV gamma-ray sources. This progress has been mainly achieved due [...] Read more.
During the last 20 years, TeV astronomy has turned from a fledgling field, with only a handful of sources, into a fully-developed astronomy discipline, broadening our knowledge on a variety of types of TeV gamma-ray sources. This progress has been mainly achieved due to the currently operating instruments: imaging atmospheric Cherenkov telescopes, surface arrays and water Cherenkov detectors. Moreover, we are at the brink of a next generation of instruments, with a considerable leap in performance parameters. This review summarizes the current status of the TeV astronomy instrumentation, mainly focusing on the comparison of the different types of instruments and analysis challenges, as well as providing an outlook into the future installations. The capabilities and limitations of different techniques of observations of TeV gamma rays are discussed, as well as synergies to other bands and messengers. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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12 pages, 657 KiB  
Review
Gamma-Ray Bursts at TeV Energies: Observational Status
by Koji Noda and Robert Daniel Parsons
Galaxies 2022, 10(1), 7; https://doi.org/10.3390/galaxies10010007 - 05 Jan 2022
Cited by 9 | Viewed by 2483
Abstract
Gamma-ray bursts (GRBs) are some of the most energetic events in the Universe and are potential sites of cosmic ray acceleration up to the highest energies. GRBs have therefore been a target of interest for very high energy gamma-ray observatories for many years, [...] Read more.
Gamma-ray bursts (GRBs) are some of the most energetic events in the Universe and are potential sites of cosmic ray acceleration up to the highest energies. GRBs have therefore been a target of interest for very high energy gamma-ray observatories for many years, leading to the recent discovery of a number of bursts with photons reaching energies above 100 GeV. We summarize the GRB observational campaigns of the current generation of very high energy gamma-ray observatories as well as describing the observations and properties of the GRBs discovered so far. We compare the properties of the very high energy bursts to the total GRB distribution and make predictions for the next generation of very high energy gamma-ray observations. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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