Galaxies

28 pages, 2221 KiB

Open AccessReview

Determination of the Cosmic-Ray Chemical Composition: Open Issues and Prospects

by Alberto Daniel Supanitsky

Galaxies 2022, 10(3), 75; https://doi.org/10.3390/galaxies10030075 - 17 Jun 2022

Cited by 4 | Viewed by 2759

Cosmic rays are relativistic particles that come to the Earth from outer space. Despite a great effort made in both experimental and theoretical research, their origin is still unknown. One of the main keys to understand their nature is the determination of its [...] Read more.

Cosmic rays are relativistic particles that come to the Earth from outer space. Despite a great effort made in both experimental and theoretical research, their origin is still unknown. One of the main keys to understand their nature is the determination of its chemical composition as a function of primary energy. In this paper, we review the measurements of the mass composition above

10^{15}

eV. We first summarize the main aspects of air shower physics that are relevant in composition analyses. We discuss the composition measurements made by using optical, radio, and surface detectors and the limitations imposed by current high-energy hadronic interaction models that are used to interpret the experimental data. We also review the photons and neutrinos searches conducted in different experiments, which, in addition to being important to understand the nature of cosmic rays, can provide relevant information related to the abundance of heavy or light elements in the flux at the highest energies. Finally, we summarize the future composition measurements that are currently being planned or under development. Full article

(This article belongs to the Special Issue Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations. Quo Vadis?)

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34 pages, 2096 KiB

Open AccessReview

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 2655

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

(0.1

–

1)

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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9 pages, 265 KiB

Open AccessArticle

Accretion of Galaxies around Supermassive Black Holes and a Theoretical Model of the Tully-Fisher and M-Sigma Relations

by Nick Gorkavyi

Galaxies 2022, 10(3), 73; https://doi.org/10.3390/galaxies10030073 - 27 May 2022

Viewed by 2484

Abstract

The observed Tully-Fisher and Faber-Jackson laws between the baryonic mass of galaxies and the velocity of motion of stars at the edge of galaxies are explained within the framework of the model of accretion of galaxies around supermassive black holes (SMBH). The accretion [...] Read more.

The observed Tully-Fisher and Faber-Jackson laws between the baryonic mass of galaxies and the velocity of motion of stars at the edge of galaxies are explained within the framework of the model of accretion of galaxies around supermassive black holes (SMBH). The accretion model can also explain the M-sigma relation between the mass of a supermassive black hole and the velocity of stars in the bulge. The difference in the mechanisms of origin of elliptical galaxies with low angular momentum and disk galaxies with high angular momentum can be associated with 3D and 2D accretion. Full article

(This article belongs to the Special Issue Particles and Fields in Black Hole Environment)

36 pages, 1466 KiB

Open AccessReview

Status and Perspectives of Continuous Gravitational Wave Searches

by Ornella Juliana Piccinni

Galaxies 2022, 10(3), 72; https://doi.org/10.3390/galaxies10030072 - 25 May 2022

Cited by 27 | Viewed by 3363

Abstract

The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had [...] Read more.

The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had a significant impact in many fields of science: astrophysics, cosmology, nuclear physics and fundamental physics. However, compact binary coalescence signals are not the only type of gravitational waves potentially detectable by LIGO, Virgo, and KAGRA. An interesting family of still undetected signals, and the ones that are considered in this review, are the so-called continuous waves, paradigmatically exemplified by the gravitational radiation emitted by galactic, fast-spinning isolated neutron stars with a certain degree of asymmetry in their mass distribution. In this work, I will review the status and the latest results from the analyses of advanced detector data. Full article

(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)

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11 pages, 313 KiB

Open AccessArticle

Anthropic Principle and the Hubble-Lemaître Constant

by Michal Křížek and Lawrence Somer

Galaxies 2022, 10(3), 71; https://doi.org/10.3390/galaxies10030071 - 24 May 2022

Cited by 1 | Viewed by 1939

Abstract

According to the weak formulation of the anthropic principle, all fundamental physical constants have just such values that they enabled the origin of life. In this survey paper, we demonstrate also that the current value of the Hubble–Lemaître constant essentially contributed to the [...] Read more.

According to the weak formulation of the anthropic principle, all fundamental physical constants have just such values that they enabled the origin of life. In this survey paper, we demonstrate also that the current value of the Hubble–Lemaître constant essentially contributed to the existence of humankind. Life on Earth has existed continually for at least 3.5 Gyr, and this requires very stable conditions during this quite long time interval. Nevertheless, as the luminosity of the Sun increases, Earth has receded from the Sun by an appropriate speed such that it received an almost constant solar flux during the last 3.5 Gyr. We introduce several other examples illustrating that the solar system and also our galaxy expand by a speed comparable to the Hubble–Lemaître constant. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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10 pages, 682 KiB

Open AccessArticle

Gravitational Waves from a Core-Collapse Supernova: Perspectives with Detectors in the Late 2020s and Early 2030s

by Marek Szczepańczyk and Michele Zanolin

Galaxies 2022, 10(3), 70; https://doi.org/10.3390/galaxies10030070 - 23 May 2022

Cited by 6 | Viewed by 2063

Abstract

We studied the detectability and reconstruction of gravitational waves from core-collapse supernova multidimensional models using simulated data from detectors predicted to operate in the late 2020s and early 2030s. We found that the detection range will improve by a factor of around two [...] Read more.

We studied the detectability and reconstruction of gravitational waves from core-collapse supernova multidimensional models using simulated data from detectors predicted to operate in the late 2020s and early 2030s. We found that the detection range will improve by a factor of around two with respect to the second-generation gravitational-wave detectors, and the sky localization will significantly improve. We analyzed the reconstruction accuracy for the lower frequency and higher frequency portion of supernova signals with a 250 Hz cutoff. Since the waveform’s peak frequencies are usually at high frequencies, the gravitational-wave signals in this frequency band were reconstructed more accurately. Full article

(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)

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20 pages, 984 KiB

Open AccessArticle

A Mass Dependent Density Profile from Dwarfs to Clusters

by Antonino Del Popolo and Morgan Le Delliou

Galaxies 2022, 10(3), 69; https://doi.org/10.3390/galaxies10030069 - 18 May 2022

Viewed by 1814

Abstract

In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was [...] Read more.

In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was obtained using the NIHAO set of simulations. In our case, we used simulations based on a model of ours. Following Freundlich et al. 2020, we obtained the dependence from baryon physics of the two shape parameters, obtaining in this way a mass dependent Dekel–Zhao profile describing the dark matter profiles from galaxies to clusters of galaxies. The extension to the Dekel–Zhao mass dependent profile to clusters of galaxies is the main result of the paper. In the paper, we show how the Dekel–Zhao mass dependent profile gives a good description of the density profiles of galaxies, already shown by Freundlich et al. 2020, but also to a set of clusters of galaxies. Full article

(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)

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13 pages, 2632 KiB

Open AccessArticle

Avoiding the Great Filter: Predicting the Timeline for Humanity to Reach Kardashev Type I Civilization

by Jonathan H. Jiang, Fuyang Feng, Philip E. Rosen, Kristen A. Fahy, Prithwis Das, Piotr Obacz, Antong Zhang and Zong-Hong Zhu

Galaxies 2022, 10(3), 68; https://doi.org/10.3390/galaxies10030068 - 12 May 2022

Cited by 2 | Viewed by 11676

Abstract

The level of technological development of any civilization can be gauged in large part by the amount of energy it produces for its use, but also encompasses that civilization’s stewardship of its home world. Following the Kardashev definition, a Type I civilization is [...] Read more.

The level of technological development of any civilization can be gauged in large part by the amount of energy it produces for its use, but also encompasses that civilization’s stewardship of its home world. Following the Kardashev definition, a Type I civilization is able to store and use all the energy available on its planet. In this study, we develop a model based on Carl Sagan’s K formula, and use this model to analyze the consumption and energy supply of the three most important energy sources: fossil fuels (e.g., coal, oil, natural gas, crude, NGL, and feedstocks), nuclear energy, and renewable energy. We also consider environmental limitations suggested by the United Nations Framework Convention on Climate Change, the International Energy Agency, and those specific to our calculations, to predict when humanity will reach the level of a Kardashev Scale Type I civilization. Our findings suggest that the best estimate for our civilization to attain Type I status is within the common calendar year range of 2333 to 2404. Full article

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24 pages, 2624 KiB

Open AccessReview

The Detection of GRBs at VHE: A Challenge Lasting for More than Two Decades, What Is Next?

by Alessio Berti and Alessandro Carosi

Galaxies 2022, 10(3), 67; https://doi.org/10.3390/galaxies10030067 - 10 May 2022

Cited by 6 | Viewed by 2318

Abstract

Unveiling the mystery of gamma-ray bursts (GRBs) has been the target of many multi-waveband observational and theoretical efforts during the last decades. The results collected by current and past space-based instruments have provided important insights into the mechanisms at the origin of their [...] Read more.

Unveiling the mystery of gamma-ray bursts (GRBs) has been the target of many multi-waveband observational and theoretical efforts during the last decades. The results collected by current and past space-based instruments have provided important insights into the mechanisms at the origin of their prompt and afterglow phases. On the other hand, many questions, such as the the origin of the multi-GeV signal observed in a large number of events, remained unanswered. Within this framework, the first firm detections of a very-high-energy (VHE,

E ≳ 100

GeV) emission component by MAGIC and H.E.S.S. collaborations represented an important, long-awaited result for the VHE astrophysics community. However, while such discoveries opened a new era in the study of GRBs, they also provided an unexpected complexity due to the differences between the phenomenology of the observed events. This revealed that we still have an incomplete comprehension of GRB physics. In the nearby future, observations by the Cherenkov Telescope Array Observatory (CTAO), with unprecedented sensitivity in the VHE band, will have a key role in the study of these enigmatic objects and their interactions with the surrounding environment. In this review we will cover the recent GRB history, highlighting the efforts of follow-up campaigns by the VHE community that led to the first VHE GRB detection, and outlining what we can expect from future facilities in the next decades. Full article

(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)

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65 pages, 11248 KiB

Open AccessArticle

Gamma-Ray Bursts Afterglow Physics and the VHE Domain

by Davide Miceli and Lara Nava

Galaxies 2022, 10(3), 66; https://doi.org/10.3390/galaxies10030066 - 05 May 2022

Cited by 19 | Viewed by 3582

Abstract

Afterglow radiation in gamma-ray bursts (GRB), extending from the radio band to GeV energies, is produced as a result of the interaction between the relativistic jet and the ambient medium. Although in general the origin of the emission is robustly identified as synchrotron [...] Read more.

Afterglow radiation in gamma-ray bursts (GRB), extending from the radio band to GeV energies, is produced as a result of the interaction between the relativistic jet and the ambient medium. Although in general the origin of the emission is robustly identified as synchrotron radiation from the shock-accelerated electrons, many aspects remain poorly constrained, such as the role of inverse Compton emission, the particle acceleration mechanism, the properties of the environment and of the GRB jet itself. The extension of the afterglow emission into the TeV band has been discussed and theorized for years, but has eluded for a long time the observations. Recently, the Cherenkov telescopes, MAGIC and H.E.S.S., have unequivocally proven that afterglow radiation is also produced above 100 GeV, up to at least a few TeV. The accessibility of the TeV spectral window will largely improve with the upcoming facility CTA (the Cherenkov Telescope Array). In this review article, we first revise the current model for afterglow emission in GRBs, its limitations and open issues. Then, we describe the recent detections of very high energy emission from GRBs and the origin of this radiation. Implications on the understanding of afterglow radiation and constraints on the physics of the involved processes will be deeply investigated, demonstrating how future observations, especially by the CTA Observatory, are expected to give a key contribution in improving our comprehension of such elusive sources. Full article

(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)

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36 pages, 53972 KiB

Open AccessArticle

Research Facilities for Europe’s Next Generation Gravitational-Wave Detector Einstein Telescope

by Sibilla Di Pace, Valentina Mangano, Lorenzo Pierini, Amirsajjad Rezaei, Jan-Simon Hennig, Margot Hennig, Daniela Pascucci, Annalisa Allocca, Iara Tosta e Melo, Vishnu G. Nair, Philippe Orban, Ameer Sider, Shahar Shani-Kadmiel and Joris van Heijningen

Galaxies 2022, 10(3), 65; https://doi.org/10.3390/galaxies10030065 - 28 Apr 2022

Cited by 15 | Viewed by 4502

Abstract

The Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy) [...] Read more.

The Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy) and E-TEST in Liége (Belgium) and its surroundings. The ARC pursues the investigation of a large cryostat, equipped with dedicated low-vibration cooling lines, to test full-scale cryogenic payloads. The installation will be gradual and interlaced with the payload development. ETpathfinder aims to provide a low-noise facility that allows the testing of full interferometer configurations and the interplay of their subsystems in an ET-like environment. ETpathfinder will focus amongst others on cryogenic technologies, silicon mirrors, lasers and optics at 1550 and 2090 nm and advanced quantum noise reduction schemes. The SarGrav laboratory has a surface lab and an underground operation. On the surface, the Archimedes experiment investigates the interaction of vacuum fluctuations with gravity and is developing (tilt) sensor technology for the Einstein Telescope. In an underground laboratory, seismic characterisation campaigns are undertaken for the Sardinian site characterisation. Lastly, the Einstein Telecope Euregio meuse-rhine Site & Technology (E-TEST) is a single cryogenic suspension of an ET-sized silicon mirror. Additionally, E-TEST investigates the Belgian–Dutch–German border region that is the other candidate site for Einstein Telescope using boreholes and seismic arrays and hydrogeological characterisation. In this article, we describe the Einstein Telescope, the low-frequency part of its science case and the four research facilities. Full article

(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)

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3 pages, 199 KiB

Open AccessEditorial

Global Understanding of Accretion and Ejection around Black Holes

by Santanu Mondal

Galaxies 2022, 10(3), 64; https://doi.org/10.3390/galaxies10030064 - 27 Apr 2022

Viewed by 1590

Abstract

Accretion and ejection around compact objects, mainly around black holes, both in low mass, and supermassive, is rich and has been studied exhaustively [...] Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

26 pages, 5912 KiB

Open AccessArticle

The Current Status and Future Prospects of KAGRA, the Large-Scale Cryogenic Gravitational Wave Telescope Built in the Kamioka Underground

by Homare Abe, Tomotada Akutsu, Masaki Ando, Akito Araya, Naoki Aritomi, Hideki Asada, Yoichi Aso, Sangwook Bae, Rishabh Bajpai, Kipp Cannon, Zhoujian Cao, Eleonora Capocasa, Man Leong Chan, Dan Chen, Yi-Ru Chen, Marc Eisenmann, Raffaele Flaminio, Heather K. Fong, Yuta Fujikawa, Yuya Fujimoto, I. Putu Wira Hadiputrawan, Sadakazu Haino, Wenbiao Han, Kazuhiro Hayama, Yoshiaki Himemoto, Naoatsu Hirata, Chiaki Hirose, Tsung-Chieh Ho, Bin-Hua Hsieh, He-Feng Hsieh, Chia-Hsuan Hsiung, Hsiang-Yu Huang, Panwei Huang, Yao-Chin Huang, Yun-Jing Huang, David C. Y. Hui, Kohei Inayoshi, Yuki Inoue, Yousuke Itoh, Pil-Jong Jung, Takaaki Kajita, Masahiro Kamiizumi, Nobuyuki Kanda, Takashi Kato, Chunglee Kim, Jaewan Kim, Young-Min Kim, Yuichiro Kobayashi, Kazunori Kohri, Keiko Kokeyama, Albert K. H. Kong, Naoki Koyama, Chihiro Kozakai, Jun’ya Kume, Sachiko Kuroyanagi, Kyujin Kwak, Eunsub Lee, Hyung Won Lee, Ray-Kuang Lee, Matteo Leonardi, Kwan-Lok Li, Pengbo Li, Lupin Chun-Che Lin, Chun-Yu Lin, En-Tzu Lin, Hong-Lin Lin, Guo-Chin Liu, Ling-Wei Luo, Miftahul Ma’arif, Yuta Michimura, Norikatsu Mio, Osamu Miyakawa, Kouseki Miyo, Shinji Miyoki, Nozomi Morisue, Kouji Nakamura, Hiroyuki Nakano, Masayuki Nakano, Tatsuya Narikawa, Lan Nguyen Quynh, Takumi Nishimoto, Atsushi Nishizawa, Yoshihisa Obayashi, Kwangmin Oh, Masatake Ohashi, Tomoya Ohashi, Masashi Ohkawa, Yoshihiro Okutani, Ken-ichi Oohara, Shoichi Oshino, Kuo-Chuan Pan, Alessandro Parisi, June Gyu Park, Fabián E. Peña Arellano, Surojit Saha, Kazuki Sakai, Takahiro Sawada, Yuichiro Sekiguchi, Lijing Shao, Yutaka Shikano, Hirotaka Shimizu, Katsuhiko Shimode, Hisaaki Shinkai, Ayaka Shoda, Kentaro Somiya, Inhyeok Song, Ryosuke Sugimoto, Jishnu Suresh, Takamasa Suzuki, Takanori Suzuki, Toshikazu Suzuki, Hideyuki Tagoshi, Hirotaka Takahashi, Ryutaro Takahashi, Hiroki Takeda, Mei Takeda, Atsushi Taruya, Takayuki Tomaru, Tomonobu Tomura, Lucia Trozzo, Terrence T. L. Tsang, Satoshi Tsuchida, Takuya Tsutsui, Darkhan Tuyenbayev, Nami Uchikata, Takashi Uchiyama, Tomoyuki Uehara, Koh Ueno, Takafumi Ushiba, Maurice H. P. M. van Putten, Tatsuki Washimi, Chien-Ming Wu, Hsun-Chung Wu, Tomohiro Yamada, Kazuhiro Yamamoto, Takahiro Yamamoto, Ryo Yamazaki, Shu-Wei Yeh, Jun’ichi Yokoyama, Takaaki Yokozawa, Hirotaka Yuzurihara, Simon Zeidler and Yuhang Zhao Show full author list Hide full author list

Galaxies 2022, 10(3), 63; https://doi.org/10.3390/galaxies10030063 - 26 Apr 2022

Cited by 18 | Viewed by 5647

Abstract

KAGRA is a gravitational-wave (GW) detector constructed in Japan with two unique key features: It was constructed underground, and the test-mass mirrors are cooled to cryogenic temperatures. These features are not included in other kilometer-scale detectors but will be adopted in future detectors [...] Read more.

KAGRA is a gravitational-wave (GW) detector constructed in Japan with two unique key features: It was constructed underground, and the test-mass mirrors are cooled to cryogenic temperatures. These features are not included in other kilometer-scale detectors but will be adopted in future detectors such as the Einstein Telescope. KAGRA performed its first joint observation run with GEO600 in 2020. In this observation, the sensitivity of KAGRA to GWs was inferior to that of other kilometer-scale detectors such as LIGO and Virgo. However, further upgrades to the detector are ongoing to reach the sensitivity for detecting GWs in the next observation run, which is scheduled for 2022. In this article, the current situation, sensitivity, and future perspectives are reviewed. Full article

(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)

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10 pages, 1947 KiB

Open AccessArticle

Understanding High-Energy (UV and X-ray) Emission from AGB Stars—Episodic Accretion in Binary Systems

by Raghvendra Sahai, Jorge Sanz-Forcada, Martin Guerrero, Roberto Ortiz and Carmen Sanchez Contreras

Galaxies 2022, 10(3), 62; https://doi.org/10.3390/galaxies10030062 - 25 Apr 2022

Cited by 2 | Viewed by 1963

Abstract

X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratio

R_{f u v / n u v}

\overset{>}{\sim}

0.2 (fuvAGB stars) have variable X-ray emission characterized by very high temperatures (Tx∼35–160 [...] Read more.

X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratio

R_{f u v / n u v}

\overset{>}{\sim}

0.2 (fuvAGB stars) have variable X-ray emission characterized by very high temperatures (Tx∼35–160 MK) and luminosities (Lx∼0.002–0.2

L_{⊙}

), indicating the presence of accretion associated with a close binary companion. However, the UV-emitting AGB star population is dominated by objects with

R_{f u v / n u v}

≲ 0.06 (nuvAGB stars), and we do not know whether the UV emission from these is intrinsic to the AGB star or extrinsic (i.e., due to binarity). In order to help distinguish between intrinsic and extrinsic models of the puzzling high-energy emission of cool AGB stars, we report results from two studies—(i) XMM-Newton X-observations of two nuvAGB stars, and (ii) simple chromosphere modeling. In study (i), we detect the one which has the lower FUV/NUV ratio, with a total Lx = 0.00027

L_{⊙}

, and a spectrum best fitted with a dominant component at Tx∼10 MK, most likely coronal emission from a main-sequence companion. Therefore, a significant fraction of nuvAGB stars may also be binaries with active, but weak accretion. Study (ii) shows that chromospheres with temperatures of ∼10,000 K can produce

R_{f u v / n u v} ≲ 0.06

; higher ratios require hotter gas, implying active accretion. Full article

(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)

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27 pages, 2685 KiB

Open AccessReview

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 3217

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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12 pages, 2764 KiB

Open AccessArticle

Breakthrough Multi-Messenger Astrophysics with the THESEUS Space Mission

by Giulia Stratta, Lorenzo Amati, Marica Branchesi, Riccardo Ciolfi, Nial Tanvir, Enrico Bozzo, Diego Götz, Paul O’Brien and Andrea Santangelo

Galaxies 2022, 10(3), 60; https://doi.org/10.3390/galaxies10030060 - 21 Apr 2022

Cited by 4 | Viewed by 2232

Abstract

The mission concept THESEUS (Transient High Energy Sky and Early Universe Surveyor) aims at exploiting Gamma-Ray Bursts (GRB) to explore the early Universe, as well as becoming a cornerstone of multi-messenger and time-domain astrophysics. To achieve these goals, a key feature is the [...] Read more.

The mission concept THESEUS (Transient High Energy Sky and Early Universe Surveyor) aims at exploiting Gamma-Ray Bursts (GRB) to explore the early Universe, as well as becoming a cornerstone of multi-messenger and time-domain astrophysics. To achieve these goals, a key feature is the capability to survey the soft X-ray transient sky and to detect the faint and soft GRB population so far poorly explored. Among the expected transients there will be high-redshift GRBs, nearby low-luminosity, X-ray Flashes and short GRBs. Our understanding of the physics governing the GRB prompt emission will benefit from the 0.3 keV–10 MeV simultaneous observations for an unprecedented large number of hundreds of events per year. In particular the mission will provide the identification, accurate sky localisation and characterization of electromagnetic counterparts to sources of gravitational wave and neutrino sources, which will be routinely detected during the 2030s by the upgraded second generation and third generation Gravitational Wave (GW) interferometers and next generation neutrino detectors. Full article

(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)

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