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Galaxies, Volume 10, Issue 6 (December 2022) – 16 articles

Cover Story (view full-size image): As a subclass of AGN, low-luminosity AGN (LLAGN) systems have been categorized with a low (less than 1% of) Eddington black hole accretion rate. Very-long baseline interferometry (VLBI) observations of relativistic jets exhibited by some of the nearby LLAGN provide unique opportunities for exploring the property of central black holes and the origin of energetic electrons in jets. In our review, from the sub-parsec scale to the event horizon scale, we illustrate how the jet image features can be useful diagnostics of the underlying physics. Bridging the physics between the two scales is an important direction for both future VLBI observations and black hole jet physics. View this paper
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19 pages, 1265 KiB  
Article
On Low Hubble Expansion Rate from Planck Data Anomalies
by Abraão J. S. Capistrano, Luís A. Cabral, Carlos H. Coimbra-Araújo and José A. P. F. Marão
Galaxies 2022, 10(6), 118; https://doi.org/10.3390/galaxies10060118 - 19 Dec 2022
Viewed by 1753
Abstract
From the linear perturbations of Nash–Greene fluctuations of a background metric, we obtain profiles of Hubble function evolution H(z) and fσ8(z) measurements as compared with the ΛCDM results at intermediate redshifts [...] Read more.
From the linear perturbations of Nash–Greene fluctuations of a background metric, we obtain profiles of Hubble function evolution H(z) and fσ8(z) measurements as compared with the ΛCDM results at intermediate redshifts 0.1<z<1. For parameter estimation, we use joint data from Planck Cosmic Microwave Background (CMB) likelihoods of CMB temperature and polarization angular power spectra, Barionic Acoustic Oscillations (BAO) and local measurements of Hubble constant H0 from the Hubble Space Telescope (HST). We analyze the stability of the effective Newtonian constant Geff and its agreement with Big Bang Nucleosynthesis (BBN) constraints. We show that our results are highly compatible with the ΛCDM paradigm, rather extending the perspective for further studies on redshift-space galaxy clustering data. Moreover, we obtain the CMB TT angular spectra with the Integrated Sachs–Wolfe (ISW) effect, which is weakened on low-l scales. The resulting linear matter power spectrum P(k) profile is also compatible with ΛCDM results but somewhat degenerate with an early dark energy (DE) contribution. Finally, posing a dilemma to the solution of Hubble tension, our results indicate a low Hubble expansion rate suggesting possible anomalies in Planck data in consonance with the recent South Pole Telescope (SPT-3G) data. Full article
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6 pages, 299 KiB  
Technical Note
Progress on the Haystack Observatory Postprocessing System
by Daniel Hoak, John Barrett, Geoffrey Crew and Violet Pfeiffer
Galaxies 2022, 10(6), 119; https://doi.org/10.3390/galaxies10060119 - 17 Dec 2022
Cited by 1 | Viewed by 1296
Abstract
The Haystack Observatory Postprocessing System (HOPS) is a multipurpose tool for post-correlation calibration and data analysis in Very-Long Baseline Interferometry experiments. The requirements on stations, baselines, and bandwidth for the Next Generation Event Horizon Telescope (ngEHT) have motivated a significant refactoring of the [...] Read more.
The Haystack Observatory Postprocessing System (HOPS) is a multipurpose tool for post-correlation calibration and data analysis in Very-Long Baseline Interferometry experiments. The requirements on stations, baselines, and bandwidth for the Next Generation Event Horizon Telescope (ngEHT) have motivated a significant refactoring of the HOPS codebase. In this paper, we present the requirements, specifications, and design of HOPS 4.0 and the current state of the refactoring, and we discuss future work. Full article
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18 pages, 1244 KiB  
Article
Modeling Reconstructed Images of Jets Launched by SANE Super-Eddington Accretion Flows around SMBHs with the ngEHT
by Brandon Curd, Razieh Emami, Freek Roelofs and Richard Anantua
Galaxies 2022, 10(6), 117; https://doi.org/10.3390/galaxies10060117 - 13 Dec 2022
Cited by 5 | Viewed by 1495
Abstract
Tidal disruption events (TDEs) around supermassive black holes (SMBHs) are a potential laboratory to study super-Eddington accretion disks and sometimes result in powerful jets or outflows which may shine in the radio and sub-millimeter bands. In this work, we modeled the thermal synchrotron [...] Read more.
Tidal disruption events (TDEs) around supermassive black holes (SMBHs) are a potential laboratory to study super-Eddington accretion disks and sometimes result in powerful jets or outflows which may shine in the radio and sub-millimeter bands. In this work, we modeled the thermal synchrotron emission of jets by general relativistic radiation magneto-hydrodynamics (GRRMHD) simulations of a BH accretion disk/jet system which assumed the TDE resulted in a magnetized accretion disk around a BH accreting at ∼12–25 times the Eddington accretion rate. Through synthetic observations with the Next Generation Event Horizon Telescope (ngEHT) and an image reconstruction analysis, we demonstrate that TDE jets may provide compelling targets within the context of the models explored in this work. In particular, we found that jets launched by a SANE super-Eddington disk around a spin a*=0.9 reach the ngEHT detection threshold at large distances (up to 100 Mpc in this work). A two-temperature plasma in the jet or weaker jets, such as a spin a*=0 model, requires a much closer distance, as we demonstrate detection at 10 Mpc for limiting cases of a*=0,R=1 or a*=0.9,R=20. We also demonstrate that TDE jets may appear as superluminal sources if the BH is rapidly rotating and the jet is viewed nearly face on. Full article
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21 pages, 1955 KiB  
Review
Introduction to Electroweak Baryogenesis
by Radouane Gannouji
Galaxies 2022, 10(6), 116; https://doi.org/10.3390/galaxies10060116 - 12 Dec 2022
Viewed by 2454
Abstract
We present a pedagogical introduction to the electroweak baryogenesis. The review focuses principally on the sphaleron and baryon number (non)-conservation or chiral anomaly. All results are derived with details for a self-contained reading. Full article
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19 pages, 4676 KiB  
Article
Simulations for the Locking and Alignment Strategy of the DRMI Configuration of the Advanced Virgo Plus Detector
by Diego Bersanetti, Mattia Boldrini, Julia Casanueva Diaz, Andreas Freise, Riccardo Maggiore, Maddalena Mantovani and Michele Valentini
Galaxies 2022, 10(6), 115; https://doi.org/10.3390/galaxies10060115 - 09 Dec 2022
Cited by 1 | Viewed by 1252
Abstract
The Advanced Virgo Plus project aims to increase the sensitivity of the Virgo gravitational-wave detector, given the forthcoming O4 Observing Run. One of the major upgrades is the addition of the Signal Recycling Mirror in the optical layout. This additional mirror will provide [...] Read more.
The Advanced Virgo Plus project aims to increase the sensitivity of the Virgo gravitational-wave detector, given the forthcoming O4 Observing Run. One of the major upgrades is the addition of the Signal Recycling Mirror in the optical layout. This additional mirror will provide a broadband improvement to the sensitivity curve of the instrument, but poses significant challenges in the acquisition and operation of the detector’s working point. The process which brings the main optical components from the uncontrolled state to the final working point, which ensures the best detector sensitivity, is called lock acquisition: the lock acquisition is made by moving through increasingly more complex configurations toward the full control of all the interferometer’s longitudinal degrees of freedom. This paper will focus on the control of the Dual-Recycled Michelson Interferometer (DRMI, the central part of the Virgo interferometer), presenting a comprehensive study of the optical simulations used in the design and the commissioning of this configuration. Treated topics include: the characterization of optical fields, powers, and error signals for the controls; the development of a trigger logic to be used for the lock acquisition; the study of the alignment sensing and control system. The interdependence between the three items has also been studied. Moreover, the validity of the studied techniques will be assessed by a comparison with experimental data. Full article
(This article belongs to the Special Issue Research and Development for Gravitational Wave Detector)
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14 pages, 2414 KiB  
Article
Demonstration of Ultrawideband Polarimetry Using VLBI Exploration of Radio Astrometry (VERA)
by Yoshiaki Hagiwara, Kazuhiro Hada, Mieko Takamura, Tomoaki Oyama, Aya Yamauchi and Syunsaku Suzuki
Galaxies 2022, 10(6), 114; https://doi.org/10.3390/galaxies10060114 - 08 Dec 2022
Cited by 5 | Viewed by 1239
Abstract
We report on recent technical developments in the front- and back-ends for the four 20 m radio telescopes of the Japanese Very-Long-Baseline Interferometry (VLBI) project, VLBI Exploration of Radio Astrometry (VERA). We present a brief overview of a dual-circular polarization receiving and ultrawideband [...] Read more.
We report on recent technical developments in the front- and back-ends for the four 20 m radio telescopes of the Japanese Very-Long-Baseline Interferometry (VLBI) project, VLBI Exploration of Radio Astrometry (VERA). We present a brief overview of a dual-circular polarization receiving and ultrawideband (16 Giga bit s1) recording systems that were installed on each of the four telescopes operating at 22 and 43 GHz bands. The wider-band capability improves the sensitivity of VLBI observations for continuum emission, and the dual-polarization capability enables the study of magnetic fields in relativistic jets ejected from supermassive black holes in active galactic nuclei and in sites of star formation and around evolved stars.We present the linear polarization intensity maps of extragalactic sources at 22 and 43 GHz obtained from the most recent test observations to show the state of the art of the VERA polarimetric observations. At the end of this article, given the realization of VLBI polarimetry with VERA, we describe the future prospects for scientific aims and further technical developments. Full article
(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)
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25 pages, 66913 KiB  
Review
Overview of the Observing System and Initial Scientific Accomplishments of the East Asian VLBI Network (EAVN)
by Kazunori Akiyama, Juan-Carlos Algaba, Tao An, Keiichi Asada, Kitiyanee Asanok, Do-Young Byun, Thanapol Chanapote, Wen Chen, Zhong Chen, Xiaopeng Cheng, James O. Chibueze, Ilje Cho, Se-Hyung Cho, Hyun-Soo Chung, Lang Cui, Yuzhu Cui, Akihiro Doi, Jian Dong, Kenta Fujisawa, Wei Gou, Wen Guo, Kazuhiro Hada, Yoshiaki Hagiwara, Tomoya Hirota, Jeffrey A. Hodgson, Mareki Honma, Hiroshi Imai, Phrudth Jaroenjittichai, Wu Jiang, Yongbin Jiang, Yongchen Jiang, Takaaki Jike, Dong-Kyu Jung, Taehyun Jung, Noriyuki Kawaguchi, Dong-Jin Kim, Hyo-Ryoung Kim, Jaeheon Kim, Jeong-Sook Kim, Kee-Tae Kim, Soon-Wook Kim, Motoki Kino, Hideyuki Kobayashi, Shoko Koyama, Busaba H. Kramer, Jee-Won Lee, Jeong Ae Lee, Sang-Sung Lee, Sang Won Lee, Bin Li, Guanghui Li, Xiaofei Li, Zhixuan Li, Qinghui Liu, Xiang Liu, Ru-Sen Lu, Kazuhito Motogi, Masanori Nakamura, Kotaro Niinuma, Chungsik Oh, Hongjong Oh, Junghwan Oh, Se-Jin Oh, Tomoaki Oyama, Jongho Park, Saran Poshyachinda, Hyunwook Ro, Duk-Gyoo Roh, Wiphu Rujopakarn, Nobuyuki Sakai, Satoko Sawada-Satoh, Zhi-Qiang Shen, Katsunori M. Shibata, Bong Won Sohn, Boonrucksar Soonthornthum, Koichiro Sugiyama, Yunxia Sun, Mieko Takamura, Yoshihiro Tanabe, Fumie Tazaki, Sascha Trippe, Kiyoaki Wajima, Jinqing Wang, Na Wang, Shiqiang Wang, Xuezheng Wang, Bo Xia, Shuangjing Xu, Hao Yan, Wenjun Yang, Jae-Hwan Yeom, Kunwoo Yi, Sang-Oh Yi, Yoshinori Yonekura, Hasu Yoon, Linfeng Yu, Jianping Yuan, Youngjoo Yun, Bo Zhang, Hua Zhang, Yingkang Zhang, Guang-Yao Zhao, Rongbing Zhao, Weiye Zhong and on behalf of the East Asian VLBI Network Collaborationadd Show full author list remove Hide full author list
Galaxies 2022, 10(6), 113; https://doi.org/10.3390/galaxies10060113 - 07 Dec 2022
Cited by 5 | Viewed by 4619
Abstract
The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three [...] Read more.
The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 h in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the ’Global VLBI Alliance’ to provide an opportunity of VLBI observation with the longest baselines on the earth. Full article
(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)
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31 pages, 616 KiB  
Review
The Basics of Primordial Black Hole Formation and Abundance Estimation
by Chul-Moon Yoo
Galaxies 2022, 10(6), 112; https://doi.org/10.3390/galaxies10060112 - 06 Dec 2022
Cited by 11 | Viewed by 2225
Abstract
This paper is a biased review of the primordial black hole (PBH) formation and abundance estimation. We first review the three-zone model for PBH formation to help an intuitive understanding of the PBH formation process. Then, for more accurate analyses, we introduce necessary [...] Read more.
This paper is a biased review of the primordial black hole (PBH) formation and abundance estimation. We first review the three-zone model for PBH formation to help an intuitive understanding of the PBH formation process. Then, for more accurate analyses, we introduce necessary tools such as cosmological long-wavelength solutions, the definition of the mass and compaction function in a spherically symmetric spacetime and peak theory. Combining all these tools, we calculate the PBH mass spectrum for the case of the monochromatic curvature power spectrum as a demonstration. Full article
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17 pages, 3715 KiB  
Article
Measuring Photon Rings with the ngEHT
by Paul Tiede, Michael D. Johnson, Dominic W. Pesce, Daniel C. M. Palumbo, Dominic O. Chang and Peter Galison
Galaxies 2022, 10(6), 111; https://doi.org/10.3390/galaxies10060111 - 06 Dec 2022
Cited by 21 | Viewed by 1597
Abstract
General relativity predicts that images of optically thin accretion flows around black holes should generically have a “photon ring”, composed of a series of increasingly sharp subrings that correspond to increasingly strongly lensed emission near the black hole. Because the effects of lensing [...] Read more.
General relativity predicts that images of optically thin accretion flows around black holes should generically have a “photon ring”, composed of a series of increasingly sharp subrings that correspond to increasingly strongly lensed emission near the black hole. Because the effects of lensing are determined by the spacetime curvature, the photon ring provides a pathway to precise measurements of the black hole properties and tests of the Kerr metric. We explore the prospects for detecting and measuring the photon ring using very long baseline interferometry (VLBI) with the Event Horizon Telescope (EHT) and the next-generation EHT (ngEHT). We present a series of tests using idealized self-fits to simple geometrical models and show that the EHT observations in 2017 and 2022 lack the angular resolution and sensitivity to detect the photon ring, while the improved coverage and angular resolution of ngEHT at 230 GHz and 345 GHz is sufficient for these models. We then analyze detection prospects using more realistic images from general relativistic magnetohydrodynamic simulations by applying “hybrid imaging”, which simultaneously models two components: a flexible raster image (to capture the direct emission) and a ring component. Using the Bayesian VLBI modeling package Comrade.jl, we show that the results of hybrid imaging must be interpreted with extreme caution for both photon ring detection and measurement—hybrid imaging readily produces false positives for a photon ring, and its ring measurements do not directly correspond to the properties of the photon ring. Full article
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38 pages, 6701 KiB  
Article
MHD Modeling of Mass Transfer Processes in Close Binary Stars
by Andrey Zhilkin, Andrey Sobolev and Dmitry Bisikalo
Galaxies 2022, 10(6), 110; https://doi.org/10.3390/galaxies10060110 - 05 Dec 2022
Viewed by 1385
Abstract
A three-dimensional numerical model has been developed to study the flow structure in close binary systems with a magnetic field. The model uses a system of equations of modified magnetic hydrodynamics, which allows describing all the main dynamic effects associated with the magnetic [...] Read more.
A three-dimensional numerical model has been developed to study the flow structure in close binary systems with a magnetic field. The model uses a system of equations of modified magnetic hydrodynamics, which allows describing all the main dynamic effects associated with the magnetic field. It takes into account the processes of radiation heating and cooling, heating due to current dissipation, as well as magnetic field diffusion. The model allows calculations in a wide range of magnetic field values. Comparison of the calculation results with observational data confirms the reliability and high efficiency of the model. The paper presents the calculation results of the flow structure in a typical intermediate polar. It is shown that an accretion disk is formed in such a binary system, which has the following characteristic features: “hot line”, tidal shock waves, precession density wave, magnetospheric region, and accretion columns. In this case, the magnetic field in the disk is predominantly toroidal. The paper also presents the results of calculations for typical polars. In such systems, instead of an accretion disk, a collimated stream of matter is formed, moving along the magnetic field lines to the magnetic poles of the white dwarf. It is shown that in synchronous polars, variations of the mass transfer rate lead to a change in the spatial configuration of the flow. In asynchronous polars, changes in the flow structure for different phases of the beat period are observed as well as the processes of switching the flow between the magnetic poles of the accretor. Numerical calculations of the asynchronous system are performed under the assumption of the dipole configuration of the magnetic field for different values of the dipole offset relative to the center of the white dwarf. The paper presents a method for estimating this offset from observational light curves. Full article
(This article belongs to the Special Issue New Insights on Binary Stars)
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10 pages, 1086 KiB  
Article
Expectations for Horizon-Scale Supermassive Black Hole Population Studies with the ngEHT
by Dominic W. Pesce, Daniel C. M. Palumbo, Angelo Ricarte, Avery E. Broderick, Michael D. Johnson, Neil M. Nagar, Priyamvada Natarajan and José L. Gómez
Galaxies 2022, 10(6), 109; https://doi.org/10.3390/galaxies10060109 - 02 Dec 2022
Cited by 8 | Viewed by 1583
Abstract
We present estimates for the number of supermassive black holes (SMBHs) for which the next-generation Event Horizon Telescope (ngEHT) can identify the black hole “shadow”, along with estimates for how many black hole masses and spins the ngEHT can expect to constrain using [...] Read more.
We present estimates for the number of supermassive black holes (SMBHs) for which the next-generation Event Horizon Telescope (ngEHT) can identify the black hole “shadow”, along with estimates for how many black hole masses and spins the ngEHT can expect to constrain using measurements of horizon-resolved emission structure. Building on prior theoretical studies of SMBH accretion flows and analyses carried out by the Event Horizon Telescope (EHT) collaboration, we construct a simple geometric model for the polarized emission structure around a black hole, and we associate parameters of this model with the three physical quantities of interest. We generate a large number of realistic synthetic ngEHT datasets across different assumed source sizes and flux densities, and we estimate the precision with which our defined proxies for physical parameters could be measured from these datasets. Under April weather conditions and using an observing frequency of 230 GHz, we predict that a “Phase 1” ngEHT can potentially measure ∼50 black hole masses, ∼30 black hole spins, and ∼7 black hole shadows across the entire sky. Full article
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20 pages, 834 KiB  
Article
Cosmological Model Tests with JWST
by Nikita Lovyagin, Alexander Raikov, Vladimir Yershov and Yuri Lovyagin
Galaxies 2022, 10(6), 108; https://doi.org/10.3390/galaxies10060108 - 01 Dec 2022
Cited by 10 | Viewed by 16515
Abstract
The James Webb Space Telescope (JWST), which has recently become operational, is capable of detecting objects at record-breaking redshifts, z15. This is a crucial advance for observational cosmology, as at these redshifts the differences between alternative cosmological models manifest themselves [...] Read more.
The James Webb Space Telescope (JWST), which has recently become operational, is capable of detecting objects at record-breaking redshifts, z15. This is a crucial advance for observational cosmology, as at these redshifts the differences between alternative cosmological models manifest themselves in the most obvious way. In recent years, some observational hints have emerged indicating that the Standard Cosmological Model could require correcting. One of these hints is related to the discovery of remote galaxies whose redshifts correspond to the very young Universe (less than one billion years after the Big Bang) but which are similar to nearby galaxies. The issue is that such galaxies in the early Universe do not have enough time to evolve into something similar to the late-Universe galaxies. JWST observations of high-redshift objects are expected to shed light on the origin of this issue. Here we provide results on performing the “angular diameter—redshift” cosmological test for the first JWST observation data. We compare this result with predictions of the standard ΛCDM cosmological model and some static cosmological models, including Zwicky’s “tired-light” model. The latter is currently assumed to be ruled out by observations. We challenge this assumption and show that a static model can provide a natural and straightforward way of solving the puzzle of the well-evolved galaxies and better agreements with the results of the JWST “angular diameter—redshift” test at high redshifts than the correcting evolution model within the ΛCDM framework. We discuss several cosmological tests that will be important for further research on the possibility of revising the expanding Universe paradigm. Full article
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14 pages, 29582 KiB  
Article
Relativistic Signatures of Flux Eruption Events near Black Holes
by Zachary Gelles, Koushik Chatterjee, Michael Johnson, Bart Ripperda and Matthew Liska
Galaxies 2022, 10(6), 107; https://doi.org/10.3390/galaxies10060107 - 24 Nov 2022
Cited by 7 | Viewed by 1500
Abstract
Images of supermassive black holes produced using very long baseline interferometry provide a pathway to directly observing effects of a highly curved spacetime, such as a bright “photon ring” that arises from strongly lensed emission. In addition, the emission near supermassive black holes [...] Read more.
Images of supermassive black holes produced using very long baseline interferometry provide a pathway to directly observing effects of a highly curved spacetime, such as a bright “photon ring” that arises from strongly lensed emission. In addition, the emission near supermassive black holes is highly variable, with bright high-energy flares regularly observed. We demonstrate that intrinsic variability can introduce prominent associated changes in the relative brightness of the photon ring. We analyze both semianalytic toy models and GRMHD simulations with magnetic flux eruption events, showing that they each exhibit a characteristic “loop” in the space of relative photon ring brightness versus total flux density. For black holes viewed at high inclination, the relative photon ring brightness can change by an order of magnitude, even with variations in total flux density that are comparatively mild. We show that gravitational lensing, Doppler boosting, and magnetic field structure all significantly affect this feature, and we discuss the prospects for observing it in observations of M87 and Sgr A with the next-generation Event Horizon Telescope. Full article
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20 pages, 2457 KiB  
Article
Hidden Spectra Treasures in the Foster Archive: A Pilot Study of the Be Stars α Eri, α Col, ω Car and η Cen
by Catalina Arcos, Leonardo Vanzi, Nikolaus Vogt, Stefano Garcia, Virginia Ortiz and Ester Acuña
Galaxies 2022, 10(6), 106; https://doi.org/10.3390/galaxies10060106 - 22 Nov 2022
Viewed by 1318
Abstract
We present the archive of spectroscopic photographic plates of the Universidad Católica historic observatory Manuel Foster. The archive includes more than 4800 plates covering the period of time from 1928 to 1991. The spectra present in the archive are mostly those of bright [...] Read more.
We present the archive of spectroscopic photographic plates of the Universidad Católica historic observatory Manuel Foster. The archive includes more than 4800 plates covering the period of time from 1928 to 1991. The spectra present in the archive are mostly those of bright variable or binary stars observed at different epochs. We developed a method of digitalization and data processing for the plates and verified it through the analysis of a selected sample of spectra. As an example of the potential relevance of this Foster archive we studied the variation of helium, Hβ and Hγ spectral lines over time (1980−1991), complementing with high resolution spectroscopic data from the “Be Star Observation Survey” (2012−2015), of four Be stars mainly, α Eri, α Col, ω Car and η Cen. The spectra of these stars show evidence of a circumstellar gas disk present in both periods of time. From the spectroscopic analysis, we found these stars are variable in helium and this variability presents an opposite behavior with the variability observed in the EW of the Hβ line profile. This archive represents a unique source of data from past that is available for the use of the community. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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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 1889
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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21 pages, 1100 KiB  
Review
Modeling Nearby Low-Luminosity Active-Galactic-Nucleus Jet Images at All VLBI Scales
by Hung-Yi Pu, Keiichi Asada and Masanori Nakamura
Galaxies 2022, 10(6), 104; https://doi.org/10.3390/galaxies10060104 - 25 Oct 2022
Viewed by 1551
Abstract
Relativistic jets from nearby low-luminosity active-galactic-nuclei (LLAGN) were observed by Very-Long Baseline Interferometry (VLBI) across many orders of magnitude in space, from milliparsec to sub-parsec scales, and from the jet base in the vicinity of black holes to the jet collimation and acceleration [...] Read more.
Relativistic jets from nearby low-luminosity active-galactic-nuclei (LLAGN) were observed by Very-Long Baseline Interferometry (VLBI) across many orders of magnitude in space, from milliparsec to sub-parsec scales, and from the jet base in the vicinity of black holes to the jet collimation and acceleration regions. With the improved resolution for VLBI observations, resolved VLBI jet morphologies provide valuable opportunities for testing and constraining black hole jet physics. In this review, we summarize and discuss the current progress of modeling nearby LLAGN jet images from horizon scales to large scales, including the construction of jet models and the assumed emission details. Illustrative examples for jet image modeling are also given to demonstrate how jet image features may vary with the underlying physics. Full article
(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)
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