High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Galaxies and Clusters".

Deadline for manuscript submissions: closed (28 February 2021) | Viewed by 44607

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Special Issue Editors

Brazilian Center for Physics Research (CBPF), Rua Dr. Xavier Sigaud 150, Rio de Janeiro 22610-010, Brazil
Interests: gamma rays; relativistic astrophysics; active galaxies; high-energy observational astrophysics
Department of Physics and Astronomy, University of Padova, Via Marzolo 8, I-35131 Padova, Italy
Interests: gamma rays; astroparticle physics; dark matter; ground-based gamma-ray instrumentation
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Special Issue Information

Dear Colleagues,

Last year we celebrated the 30-year anniversary of the first ground-based gamma-ray astronomy detection: the Crab Nebula, observed by the Whipple Telescope in Teraelectronvolts back in 1989. As we approach a new decade in ground-based gamma-ray astronomy, with the imminent start of operations of the Cherenkov Telescope Array (CTA) and new proposed facilities such as the Southern Wide-Field Gamma-ray Observatory (SWGO), we wish to propose a reflection on some fundamental astrophysics topics that have dominated the field of very high-energy (VHE) gamma-ray astrophysics, presenting the principal results achieved, and highlighting the main questions that are at the forefront of future investigation by the new instruments and observatories.

Until little over a decade ago, gamma-ray astrophysics was a topic dominated by space-based instrumentation, which were revolutionised by the achievements of the Fermi satellite, and in particular its Large Array Telescope (LAT). Concomitantly with the Fermi operations, ground-based experiments led by H.E.S.S., MAGIC and VERITAS pushed the field beyond the GeV domain, consolidating the new observational window of TeV  Astrophysics. Today, very-high energy gamma-ray astronomy is a mature field, and its results cover almost every topic of modern astrophysics, from star-formation and compact objects to active galaxies and gamma-ray bursts. It is also right at the center of the so-called multi-messenger astrophysics. Thanks to the extreme photon energies involved, VHE gamma-ray astronomy is a relevant tool to probe a number of fundamental physics questions, from dark-matter and photon propagation issues, to cosmology and cosmic-ray physics. 

The improvement that CTA will represent in terms of sensitivity in the VHE domain, plus the fact that it will be the first open observatory in the field, mean that gamma-ray astronomy is about to enter a new era of development, advancing new research questions and expanding its reach to new topics and communities in astrophysics.

Within this very special context, which marks an important transition moment in the field, the online journal Universe is preparing a special volume to which contributions are now welcome.We aim to follow an original approach, which we hope will result in a valuable reference for specialists and young researchers from the field alike, as well as being of interest to the astrophysics community at large. We are looking for contributions that propose a discussion of relevant results and scientific achievements of the field, as well as its future expectations in tackling important astrophysics questions. 

The focus of the issue is on topics which bear relevance to the field of VHE or ground-based gamma-ray astronomy in general, but we wish to keep flexible with definitions in order to put an emphasis in the astrophysics, and so discussions can extrapolate to the GeV or space-based domain and other ground-based techniques beyond the Atmospheric Cherenkov observatories.

A number of invited contributions, covering a range of topics are being commissioned, and will be updated below.

Contributed papers are welcome and submissions are open until the 28 February 2021. 

Interested authors are welcome to write the Guest Editors directly for further information or discussion of topics and ideas for submission. 

With Best Regards

Dr. Ulisses Barres de Almeida
Prof. Michele Doro
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

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Keywords

  • high-energy gamma-ray astrophysics
  • relativistic astrophysics
  • astroparticle physics

Published Papers (20 papers)

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Editorial

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3 pages, 176 KiB  
Editorial
Editorial to the Special Issue: “High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations”
Universe 2022, 8(8), 389; https://doi.org/10.3390/universe8080389 - 22 Jul 2022
Viewed by 864
Abstract
Gamma-ray astronomy is the observational science that studies the cosmos in the last unexplored electromagnetic window, namely, above the megaelectronvolt (EeV = 106 eV) (MeV) [...] Full article

Research

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17 pages, 3547 KiB  
Article
35 Years of Ground-Based Gamma-ray Astronomy
Universe 2021, 7(11), 432; https://doi.org/10.3390/universe7110432 - 12 Nov 2021
Cited by 4 | Viewed by 2313
Abstract
This paper provides a brief, personal account of the development of ground-based gamma-ray astronomy, primarily over the last 35 years, with some digressions into the earlier history of the field. Ideas related to the imaging of Cherenkov events and the potential for the [...] Read more.
This paper provides a brief, personal account of the development of ground-based gamma-ray astronomy, primarily over the last 35 years, with some digressions into the earlier history of the field. Ideas related to the imaging of Cherenkov events and the potential for the use of arrays were in existence for some time before the technical expertise required for their exploitation emerged. There has been occasional controversy, great creativity and some heroic determination—all of it part of establishing a new window into the universe. Full article
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32 pages, 10297 KiB  
Article
The Making of Catalogues of Very-High-Energy γ-ray Sources
Universe 2021, 7(11), 421; https://doi.org/10.3390/universe7110421 - 05 Nov 2021
Cited by 5 | Viewed by 1577
Abstract
Thirty years after the discovery of the first very-high-energy γ-ray source by the Whipple telescope, the field experienced a revolution mainly driven by the third generation of imaging atmospheric Cherenkov telescopes (IACTs). The combined use of large mirrors and the invention of [...] Read more.
Thirty years after the discovery of the first very-high-energy γ-ray source by the Whipple telescope, the field experienced a revolution mainly driven by the third generation of imaging atmospheric Cherenkov telescopes (IACTs). The combined use of large mirrors and the invention of the imaging technique at the Whipple telescope, stereoscopic observations, developed by the HEGRA array and the fine-grained camera, pioneered by the CAT telescope, led to a jump by a factor of more than ten in sensitivity. The advent of advanced analysis techniques led to a vast improvement in background rejection, as well as in angular and energy resolutions. Recent instruments already have to deal with a very large amount of data (petabytes), containing a large number of sources often very extended (at least within the Galactic plane) and overlapping each other, and the situation will become even more dramatic with future instruments. The first large catalogues of sources have emerged during the last decade, which required numerous, dedicated observations and developments, but also made the first population studies possible. This paper is an attempt to summarize the evolution of the field towards the building up of the source catalogues, to describe the first population studies already made possible, and to give some perspectives in the context of the upcoming, new generation of instruments. Full article
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25 pages, 1196 KiB  
Article
Photon–Photon Interactions and the Opacity of the Universe in Gamma Rays
Universe 2021, 7(5), 146; https://doi.org/10.3390/universe7050146 - 14 May 2021
Cited by 13 | Viewed by 2785
Abstract
We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of [...] Read more.
We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of physical science. Extensive analysis of the currently available information appears to us as revealing a global coherence among the astrophysical, cosmological, and fundamental physics data, or, at least, no evident need so far of substantial modification of our present understanding. Deeper data from future experiments will verify to what extent and in which directions this conclusion should be modified. Full article
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29 pages, 4295 KiB  
Article
INTEGRAL View of TeV Sources: A Legacy for the CTA Project
Universe 2021, 7(5), 135; https://doi.org/10.3390/universe7050135 - 07 May 2021
Cited by 2 | Viewed by 2416
Abstract
Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions [...] Read more.
Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions of the Universe where relativistic acceleration processes are in place. In particular, simultaneous and/or combined observations with γ-ray satellites and ground based high-energy telescopes, have clarified the scenario of the mechanisms responsible for high energy photon emission by leptonic and hadronic accelerated particles in the presence of magnetic fields. Specifically, the European Space Agency INTEGRAL soft γ-ray observatory has detected more than 1000 sources in the soft γ-ray band, providing accurate positions, light curves and time resolved spectral data for them. Space observations with Fermi-LAT and observations that were carried out from the ground with H.E.S.S., MAGIC, VERITAS, and other telescopes sensitive in the GeV-TeV domain have, at the same time, provided evidence that a substantial fraction of the cosmic sources detected are emitting in the keV to TeV band via Synchrotron-Inverse Compton processes, in particular from stellar galactic BH systems as well as from distant black holes. In this work, employing a spatial cross correlation technique, we compare the INTEGRAL/IBIS and TeV all-sky data in search of secure or likely associations. Although this analysis is based on a subset of the INTEGRAL all-sky observations (1000 orbits), we find that there is a significant correlation: 39 objects (∼20% of the VHE γ-ray catalogue) show emission in both soft γ-ray and TeV wavebands. The full INTEGRAL database, now comprising almost 19 years of public data available, will represent an important legacy that will be useful for the Cherenkov Telescope Array (CTA) and other ground based large projects. Full article
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Review

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24 pages, 2830 KiB  
Review
Technological Novelties of Ground-Based Very High Energy Gamma-Ray Astrophysics with the Imaging Atmospheric Cherenkov Telescopes
Universe 2022, 8(4), 219; https://doi.org/10.3390/universe8040219 - 29 Mar 2022
Cited by 2 | Viewed by 2356
Abstract
In the past three decades, the ground-based technique of imaging atmospheric Cherenkov telescopes has established itself as a powerful discipline in science. Approximately 250 sources of very high gamma rays of both galactic and extra-galactic origin have been discovered largely due to this [...] Read more.
In the past three decades, the ground-based technique of imaging atmospheric Cherenkov telescopes has established itself as a powerful discipline in science. Approximately 250 sources of very high gamma rays of both galactic and extra-galactic origin have been discovered largely due to this technique. The study of these sources is providing clues to many basic questions in astrophysics, astro-particle physics, physics of cosmic rays and cosmology. The currently operational generation of telescopes offer a solid performance. Further improvements of this technique led to the next-generation large instrument known as the Cherenkov Telescope Array. In its final configuration, the sensitivity of CTA will be several times higher than that of the currently best instruments VERITAS, H.E.S.S., and MAGIC. This article is devoted to outlining the technological developments that shaped this technique and led to today’s success. Full article
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25 pages, 4683 KiB  
Review
Statistical Tools for Imaging Atmospheric Cherenkov Telescopes
Universe 2022, 8(2), 90; https://doi.org/10.3390/universe8020090 - 29 Jan 2022
Cited by 3 | Viewed by 1939
Abstract
The development of Imaging Atmospheric Cherenkov Telescopes (IACTs) unveiled the sky in the teraelectronvolt regime, initiating the so-called “TeV revolution”, at the beginning of the new millennium. This revolution was also facilitated by the implementation and adaptation of statistical tools for analyzing the [...] Read more.
The development of Imaging Atmospheric Cherenkov Telescopes (IACTs) unveiled the sky in the teraelectronvolt regime, initiating the so-called “TeV revolution”, at the beginning of the new millennium. This revolution was also facilitated by the implementation and adaptation of statistical tools for analyzing the shower images collected by these telescopes and inferring the properties of the astrophysical sources that produce such events. Image reconstruction techniques, background discrimination, and signal-detection analyses are just a few of the pioneering studies applied in recent decades in the analysis of IACTs data. This (succinct) review has the intent of summarizing the most common statistical tools that are used for analyzing data collected with IACTs, focusing on their application in the full analysis chain, including references to existing literature for a deeper examination. Full article
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31 pages, 6950 KiB  
Review
Gamma-ray Bursts at the Highest Energies
Universe 2021, 7(12), 503; https://doi.org/10.3390/universe7120503 - 17 Dec 2021
Cited by 7 | Viewed by 2018
Abstract
Emission from Gamma-ray bursts is thought to be powered mainly by synchrotron radiation from energetic electrons. The same electrons might scatter these synchrotron seed photons to higher (>10 GeV) energies, building a distinct spectral component (synchrotron self-Compton, SSC). This process is expected to [...] Read more.
Emission from Gamma-ray bursts is thought to be powered mainly by synchrotron radiation from energetic electrons. The same electrons might scatter these synchrotron seed photons to higher (>10 GeV) energies, building a distinct spectral component (synchrotron self-Compton, SSC). This process is expected to take place, but its relevance (e.g., the ratio between the SSC and synchrotron emitted power) is difficult to predict on the basis of current knowledge of physical conditions at GRB emission sites. Very high-energy radiation in GRBs can be produced also by other mechanisms, such as synchrotron itself (if PeV electrons are produced at the source), inverse Compton on external seed photons, and hadronic processes. Recently, after years of efforts, very high-energy radiation has been finally detected from at least four confirmed long GRBs by the Cherenkov telescopes H.E.S.S. and MAGIC. In all four cases, the emission has been recorded during the afterglow phase, well after the end of the prompt emission. In this work, I give an overview, accessible also to non-experts of the field, of the recent detections, theoretical implications, and future challenges, with a special focus on why very high-energy observations are relevant for our understanding of Gamma-ray bursts and which long-standing questions can be finally answered with the help of these observations. Full article
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13 pages, 4124 KiB  
Review
Astrophysical Neutrinos and Blazars
Universe 2021, 7(12), 492; https://doi.org/10.3390/universe7120492 - 13 Dec 2021
Cited by 18 | Viewed by 2013
Abstract
We review and discuss recent results on the search for correlations between astrophysical neutrinos and γ-ray-detected sources, with many extragalactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and [...] Read more.
We review and discuss recent results on the search for correlations between astrophysical neutrinos and γ-ray-detected sources, with many extragalactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and all the multi-frequency data available. Through the study of similarities and differences in these sources we conclude that blazars come in two distinct flavours: LBLs and IHBLs (low-energy-peaked and intermediate-high-energy-peaked objects). These are distinguished by widely different properties such as the overall spectral energy distribution shape, jet speed, cosmological evolution, broad-band spectral variability, and optical polarisation properties. Although blazars of all types have been proposed as neutrino sources, evidence is accumulating in favour of IHBLs being the counterparts of astrophysical neutrinos. If this is indeed the case, we argue that the peculiar observational properties of IHBLs may be indirectly related to proton acceleration to very high energies. Full article
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24 pages, 6429 KiB  
Review
The Crab Pulsar and Nebula as Seen in Gamma-Rays
Universe 2021, 7(11), 448; https://doi.org/10.3390/universe7110448 - 19 Nov 2021
Cited by 17 | Viewed by 2200
Abstract
Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast [...] Read more.
Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article, we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator. Full article
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21 pages, 343 KiB  
Review
High-Energy Alerts in the Multi-Messenger Era
Universe 2021, 7(11), 393; https://doi.org/10.3390/universe7110393 - 20 Oct 2021
Cited by 5 | Viewed by 1280
Abstract
The observation of electromagnetic counterparts to both high energy neutrinos and gravitational waves marked the beginning of a new era in astrophysics. The multi-messenger approach allows us to gain new insights into the most energetic events in the Universe such as gamma-ray bursts, [...] Read more.
The observation of electromagnetic counterparts to both high energy neutrinos and gravitational waves marked the beginning of a new era in astrophysics. The multi-messenger approach allows us to gain new insights into the most energetic events in the Universe such as gamma-ray bursts, supernovas, and black hole mergers. Real-time multi-messenger alerts are the key component of the observational strategies to unravel the transient signals expected from astrophysical sources. Focusing on the high-energy regime, we present a historical perspective of multi-messenger observations, the detectors and observational techniques used to study them, the status of the multi-messenger alerts and the most significant results, together with an overview of the future prospects in the field. Full article
14 pages, 1162 KiB  
Review
Evolution of Data Formats in Very-High-Energy Gamma-Ray Astronomy
Universe 2021, 7(10), 374; https://doi.org/10.3390/universe7100374 - 08 Oct 2021
Cited by 9 | Viewed by 1982
Abstract
Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent [...] Read more.
Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent of the next generation of telescopes and their operation as observatories open to the astronomical community, along with a generally increasing demand for open science, confront gamma-ray astronomers with the challenge of sharing their data and analysis tools. As a consequence, in the last few years, the development of open-source science tools has progressed in parallel with the endeavour to define a standardised data format for astronomical gamma-ray data. The latter constitutes the main topic of this review. Common data specifications provide equally important benefits to the current and future generation of gamma-ray instruments: they allow the data from different instruments, including legacy data from decommissioned telescopes, to be easily combined and analysed within the same software framework. In addition, standardised data accessible to the public, and analysable with open-source software, grant fully-reproducible results. In this article, we provide an overview of the evolution of the data format for gamma-ray astronomical data, focusing on its progression from private and diverse specifications to prototypical open and standardised ones. The latter have already been successfully employed in a number of publications paving the way to the analysis of data from the next generation of gamma-ray instruments, and to an open and reproducible way of conducting gamma-ray astronomy. Full article
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44 pages, 1332 KiB  
Review
Probing Quantum Gravity with Imaging Atmospheric Cherenkov Telescopes
Universe 2021, 7(9), 345; https://doi.org/10.3390/universe7090345 - 14 Sep 2021
Cited by 13 | Viewed by 2117
Abstract
High energy photons from astrophysical sources are unique probes for some predictions of candidate theories of Quantum Gravity (QG). In particular, Imaging atmospheric Cherenkov telescope (IACTs) are instruments optimised for astronomical observations in the energy range spanning from a few tens of GeV [...] Read more.
High energy photons from astrophysical sources are unique probes for some predictions of candidate theories of Quantum Gravity (QG). In particular, Imaging atmospheric Cherenkov telescope (IACTs) are instruments optimised for astronomical observations in the energy range spanning from a few tens of GeV to ∼100 TeV, which makes them excellent instruments to search for effects of QG. In this article, we will review QG effects which can be tested with IACTs, most notably the Lorentz invariance violation (LIV) and its consequences. It is often represented and modelled with photon dispersion relation modified by introducing energy-dependent terms. We will describe the analysis methods employed in the different studies, allowing for careful discussion and comparison of the results obtained with IACTs for more than two decades. Loosely following historical development of the field, we will observe how the analysis methods were refined and improved over time, and analyse why some studies were more sensitive than others. Finally, we will discuss the future of the field, presenting ideas for improving the analysis sensitivity and directions in which the research could develop. Full article
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9 pages, 1592 KiB  
Review
EAS Arrays at High Altitudes Start the Era of UHE γ-ray Astronomy
Universe 2021, 7(9), 339; https://doi.org/10.3390/universe7090339 - 09 Sep 2021
Cited by 4 | Viewed by 1468
Abstract
The evolution of extensive air shower detection as a technique for γ-ray astronomical instrumentation for the last three decades is reviewed. The first discoveries of galactic PeVatrons by the Large High Altitude Air Shower Observatory demonstrate the importance of this technique in [...] Read more.
The evolution of extensive air shower detection as a technique for γ-ray astronomical instrumentation for the last three decades is reviewed. The first discoveries of galactic PeVatrons by the Large High Altitude Air Shower Observatory demonstrate the importance of this technique in ultra-high energy γ-ray astronomy. Utilizing this technique, the origins of high energy cosmic rays may be discovered in the near future. Full article
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19 pages, 378 KiB  
Review
The Hunt for Pevatrons: The Case of Supernova Remnants
Universe 2021, 7(9), 324; https://doi.org/10.3390/universe7090324 - 31 Aug 2021
Cited by 42 | Viewed by 1886
Abstract
The search for Galactic pevatrons is now a well-identified key science project of all instruments operating in the very-high-energy domain. Indeed, in this energy range, the detection of gamma rays clearly indicates that efficient particle acceleration is taking place, and observations can thus [...] Read more.
The search for Galactic pevatrons is now a well-identified key science project of all instruments operating in the very-high-energy domain. Indeed, in this energy range, the detection of gamma rays clearly indicates that efficient particle acceleration is taking place, and observations can thus help identify which astrophysical sources can energize particles up to the ~PeV range, thus being pevatrons. In the search for the origin of Galactic cosmic rays (CRs), the PeV range is an important milestone, since the sources of Galactic CRs are expected to accelerate PeV particles. This is how the central scientific goal that is ’solving the mystery of the origin of CRs’ has often been distorted into ’finding (a) pevatron(s)’. Since supernova remnants (SNRs) are often cited as the most likely candidates for the origin of CRs, ’finding (a) pevatron(s)’ has often become ’confirming that SNRs are pevatrons’. Pleasingly, the first detection(s) of pevatron(s) were not associated to SNRs. Moreover, all clearly detected SNRs have yet revealed to not be pevatrons, and the detection from VHE gamma rays from regions unassociated with SNRs, are reminding us that other astrophysical sites might well be pevatrons. This short review aims at highlighting a few important results on the search for Galactic pevatrons. Full article
48 pages, 2527 KiB  
Review
The Gamma-ray Window to Intergalactic Magnetism
Universe 2021, 7(7), 223; https://doi.org/10.3390/universe7070223 - 02 Jul 2021
Cited by 51 | Viewed by 2916
Abstract
One of the most promising ways to probe intergalactic magnetic fields (IGMFs) is through gamma rays produced in electromagnetic cascades initiated by high-energy gamma rays or cosmic rays in the intergalactic space. Because the charged component of the cascade is sensitive to magnetic [...] Read more.
One of the most promising ways to probe intergalactic magnetic fields (IGMFs) is through gamma rays produced in electromagnetic cascades initiated by high-energy gamma rays or cosmic rays in the intergalactic space. Because the charged component of the cascade is sensitive to magnetic fields, gamma-ray observations of distant objects such as blazars can be used to constrain IGMF properties. Ground-based and space-borne gamma-ray telescopes deliver spectral, temporal, and angular information of high-energy gamma-ray sources, which carries imprints of the intervening magnetic fields. This provides insights into the nature of the processes that led to the creation of the first magnetic fields and into the phenomena that impacted their evolution. Here we provide a detailed description of how gamma-ray observations can be used to probe cosmic magnetism. We review the current status of this topic and discuss the prospects for measuring IGMFs with the next generation of gamma-ray observatories. Full article
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21 pages, 3382 KiB  
Review
Axion-like Particle Searches with IACTs
Universe 2021, 7(6), 185; https://doi.org/10.3390/universe7060185 - 05 Jun 2021
Cited by 6 | Viewed by 2637 | Correction
Abstract
The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from [...] Read more.
The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from astrophysical sources, at distances where the strong absorption by the intergalactic medium should make the signal very dim. In this latter condition, which is the focus of this review, a possible explanation is that TeV photons convert to ALPs in the presence of strong and/or extended magnetic fields, such as those in the core of galaxy clusters or around compact objects, or even those in the intergalactic space. This mixing affects the observed γ-ray spectrum of distant sources, either by signal recovery or the production of irregularities in the spectrum, called ‘wiggles’, according to the specific microscopic realization of the ALP and the ambient magnetic field at the source, and in the Milky Way, where ALPs may be converted back to γ rays. ALPs are also proposed as candidate particles for the Dark Matter. Imaging Atmospheric Cherenkov telescopes (IACTs) have the potential to detect the imprint of ALPs in the TeV spectrum from several classes of sources. In this contribution, we present the ALP case and review the past decade of searches for ALPs with this class of instruments. Full article
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59 pages, 3140 KiB  
Review
Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies
Universe 2021, 7(5), 141; https://doi.org/10.3390/universe7050141 - 11 May 2021
Cited by 32 | Viewed by 3432
Abstract
Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in [...] Read more.
Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress. Full article
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22 pages, 10048 KiB  
Review
20 Years of Indian Gamma Ray Astronomy Using Imaging Cherenkov Telescopes and Road Ahead
Universe 2021, 7(4), 96; https://doi.org/10.3390/universe7040096 - 10 Apr 2021
Cited by 10 | Viewed by 2144
Abstract
The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has [...] Read more.
The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has immensely contributed to this field with the discovery of more than 220 γ-ray sources in the Universe. This has greatly improved our understanding of the various astrophysical processes involved in the non-thermal emission at energies above 100 GeV. In this paper, we summarize the important results achieved by the Indian γ-ray astronomers from the GeV-TeV observations using imaging Cherenkov telescopes over the last two decades. We mainly emphasize the results obtained from the observations of active galactic nuclei with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) telescope, which has been operational since 1997 at Mount Abu, India. We also discuss the future plans of the Indian γ-ray astronomy program with special focus on the scientific objectives of the recently installed 21 m diameter MACE (Major Atmospheric Cherenkov Experiment) telescope at Hanle, India. Full article
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Other

1 pages, 179 KiB  
Correction
Correction: Batković et al. Axion-like Particle Searches with IACTs. Universe 2021, 7, 185
Universe 2022, 8(2), 74; https://doi.org/10.3390/universe8020074 - 27 Jan 2022
Cited by 1 | Viewed by 1321
Abstract
The authors wish to make the following corrections to their paper [...] Full article
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