Galaxies

5 pages, 10077 KiB

Open AccessEditorial

Present and Future of Gravitational Wave Astronomy

by Gabriele Vajente

Galaxies 2022, 10(4), 91; https://doi.org/10.3390/galaxies10040091 - 19 Aug 2022

Cited by 2 | Viewed by 1692

Gravitational waves (GW) are propagating perturbations of the space-time metric, generated by time-varying mass distributions [...] Full article

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

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22 pages, 1362 KiB

Open AccessReview

Cosmic Explorer: A Next-Generation Ground-Based Gravitational-Wave Observatory

by Evan D. Hall

Galaxies 2022, 10(4), 90; https://doi.org/10.3390/galaxies10040090 - 18 Aug 2022

Cited by 18 | Viewed by 3054

Abstract

Cosmic Explorer is a concept for a new laser interferometric observatory in the United States to extend ground-based gravitational-wave astrophysics into the coming decades. Aiming to begin operation in the 2030s, Cosmic Explorer will extend current and future detector technologies to a 40 [...] Read more.

Cosmic Explorer is a concept for a new laser interferometric observatory in the United States to extend ground-based gravitational-wave astrophysics into the coming decades. Aiming to begin operation in the 2030s, Cosmic Explorer will extend current and future detector technologies to a 40 km interferometric baseline—ten times larger than the LIGO observatories. Operating as part of a global gravitational-wave observatory network, Cosmic Explorer will have a cosmological reach, detecting black holes and neutron stars back to the times of earliest star formation. It will observe nearby binary collisions with enough precision to reveal details of the dynamics of the ultradense matter in neutron stars and to test the general-relativistic model of black holes. Full article

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

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9 pages, 2205 KiB

Open AccessEssay

Jets, Disks and Winds from Spinning Black Holes: Nature or Nurture?

by Roger Blandford and Noémie Globus

Galaxies 2022, 10(4), 89; https://doi.org/10.3390/galaxies10040089 - 11 Aug 2022

Cited by 3 | Viewed by 1886

Abstract

A brief summary is given of an alternative interpretation of the Event Horizon Telescope observations of the massive black hole in the nucleus of the nearby galaxy M87. It is proposed that the flow is primarily powered by the black hole rotation, not [...] Read more.

A brief summary is given of an alternative interpretation of the Event Horizon Telescope observations of the massive black hole in the nucleus of the nearby galaxy M87. It is proposed that the flow is primarily powered by the black hole rotation, not the release of gravitational energy by the infalling gas. Consequently, the observed millimetre emission is produced by an “ergomagnetosphere” that connects the black hole horizon to an “ejection disk” from which most of the gas supplied at a remote “magnetopause” is lost through a magnetocentrifugal wind. It is argued that the boundary conditions at high latitude on the magnetopause play a crucial role in the collimation of the relativistic jets. The application of these ideas to other types of source is briefly discussed. 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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10 pages, 1381 KiB

Open AccessArticle

Impact of Economic Constraints on the Projected Timeframe for Human-Crewed Deep Space Exploration

by Philip E. Rosen, Dan Zhang, Jonathan H. Jiang, Leopold Van Ijzendoorn, Kristen A. Fahy and Zong-Hong Zhu

Galaxies 2022, 10(4), 88; https://doi.org/10.3390/galaxies10040088 - 11 Aug 2022

Cited by 1 | Viewed by 2529

Abstract

Deep space exploration offers the most profound opportunity for the expansion of humanity and our understanding of the universe but remains extremely challenging. Progress will continue to be paced by uncrewed missions followed up by crewed missions to ever further destinations. Major space [...] Read more.

Deep space exploration offers the most profound opportunity for the expansion of humanity and our understanding of the universe but remains extremely challenging. Progress will continue to be paced by uncrewed missions followed up by crewed missions to ever further destinations. Major space powers continue to invest in crewed deep space exploration as an important national strategy. An improved model based on previous work is developed, which projects the earliest possible launch dates for human-crewed missions from cis-lunar space to selected destinations in the Solar System and beyond based on NASA’s historic budget trend and overall development trends of deep space exploration research. The purpose of the analysis is to provide a projected timeframe for crewed missions beyond Mars. Our findings suggest the first human missions from a spacefaring nation or international collaboration to the asteroid belt and Jovian system could be scheduled as soon as ~2071 to ~2087 and ~2101 to ~2121, respectively, while a launch to the Saturn system may occur by the year ~2132, with an uncertainty window of ~2129 to ~2153. Full article

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12 pages, 317 KiB

Open AccessReview

The Polarized Emission of AGN at Millimeter Wavelengths as Seen by POLAMI

by Iván Agudo and Clemens Thum

Galaxies 2022, 10(4), 87; https://doi.org/10.3390/galaxies10040087 - 04 Aug 2022

Cited by 2 | Viewed by 1476

Abstract

We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at [...] Read more.

We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at 3 mm and that the linear polarization is also more variable than the total flux. The linear polarization angle is highly variable in all sources with excursions > 180°; and for the case of the polarization angle, also the 1 mm variations appear to be faster than those at 3 mm. These results are fully compatible with recent multi-zone turbulent jet models, and they definitively rule out the popular single-zone models for blazars. They also further confirm that the short-wavelength (inner) emitting regions have better ordered magnetic fields than the long-wavelength ones (further downstream). Moreover, the POLAMI program has shown statistical evidence that, for most of the monitored sources, circular polarization emission is displayed the majority of the time. The circular polarization detection rate and the maximum degree of circular polarization found are comparable with previous surveys at much longer wavelengths, thus, opening a new window for circular polarization and jet composition studies in the mm range. The process generating circular polarization must not be strongly wavelength-dependent. The widespread presence of circular polarization in the POLAMI sample is likely due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jets. The peculiar behavior of circular polarization in 3C 66A, which we consider a hallmark of circular polarization generation by Faraday conversion in helical fields, is discussed. Full article

(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)

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14 pages, 1574 KiB

Open AccessArticle

Determination of the Stray Light-Induced Noise from the Baffle in the Cryogenic Trapping Area of Advanced Virgo in O5

by Alba Romero-Rodríguez, Mario Martínez, Lluïsa M. Mir and Hiroaki Yamamoto

Galaxies 2022, 10(4), 86; https://doi.org/10.3390/galaxies10040086 - 02 Aug 2022

Cited by 3 | Viewed by 1571

Abstract

As part of the Advanced Virgo upgrade for the O5 observation run in 2026, a more powerful laser and larger end mirrors in the Fabry–Perot cavities will be installed. The new optical configuration will increase the laser beam waist in the cryogenic trapping [...] Read more.

As part of the Advanced Virgo upgrade for the O5 observation run in 2026, a more powerful laser and larger end mirrors in the Fabry–Perot cavities will be installed. The new optical configuration will increase the laser beam waist in the cryogenic trapping area close to the end towers. This could require enlarging the apertures in the vacuum pipe, now dictated by the presence of a baffle, in order to avoid beam clipping and noise due to coupling with baffle vibrations, potentially leading to a significant background to the gravitational-wave signals. This is a delicate operation that would require displacing the cryotrap and the end tower. In this study, we compute the expected back-scattering from the existing cryogenic trap baffle and compare it to the expected Virgo sensitivity in O5 to determine whether the existing hardware configuration constitutes a threat for the future performance of the detector. Full article

(This article belongs to the Special Issue Research and Development for Gravitational Wave Detector)

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23 pages, 1798 KiB

Open AccessReview

Open-Source Radiative Modeling Tools for Extragalactic VHE Gamma-ray Sources

by Cosimo Nigro and Andrea Tramacere

Galaxies 2022, 10(4), 85; https://doi.org/10.3390/galaxies10040085 - 31 Jul 2022

Viewed by 2035

Abstract

In this review, we discuss various open-source software for modeling the broadband emission of extragalactic sources from radio up to the highest gamma-ray energies. As we provide an overview of the different tools available, we discuss the physical processes that such tools implement [...] Read more.

In this review, we discuss various open-source software for modeling the broadband emission of extragalactic sources from radio up to the highest gamma-ray energies. As we provide an overview of the different tools available, we discuss the physical processes that such tools implement and detail the computations they can perform. We also examine their conformity with modern good software practices. After considering the currently available software as a first generation of open-source modeling tools, we outline some desirable characteristics for the next generation. Full article

(This article belongs to the Special Issue Extragalactic TeV Astronomy)

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14 pages, 482 KiB

Open AccessArticle

Ultra-High-Energy Particles at the Border of Kerr Black Holes Triggered by Magnetocentrifugal Winds

by Carlos H. Coimbra-Araújo and Rita C. dos Anjos

Galaxies 2022, 10(4), 84; https://doi.org/10.3390/galaxies10040084 - 26 Jul 2022

Cited by 1 | Viewed by 1709

Abstract

The source, origin, and acceleration mechanisms of ultra-high-energy cosmic rays (UHECR) (

E > 10^{20}

eV, beyond the GZK limit) remain uncertain and unclear. The main explanations are associated with particular mechanisms, such as the Fermi mechanism, in which charged particles could [...] Read more.

The source, origin, and acceleration mechanisms of ultra-high-energy cosmic rays (UHECR) (

E > 10^{20}

eV, beyond the GZK limit) remain uncertain and unclear. The main explanations are associated with particular mechanisms, such as the Fermi mechanism, in which charged particles could be accelerated by clouds of magnetized gas moving within our Galaxy, or by the magnetic reconnection of field lines at, e.g., the core of high-energy astrophysical sources, where the topology of the magnetic field is rearranged and magnetic energy is converted into kinetic energy. However, the recent observation of extragalactic neutrinos may suggest that the source of UHECRs is likely an extragalactic supermassive black hole. In the present work, we propose that charged particles can be accelerated to ultrahigh energies in marginally bound orbits near extreme rotating black holes and could be triggered by collisions of magnetocentrifugal winds; the accretion disk surrounding the black hole would provide such winds. The ultra-high-energy process is governed by the frame-dragging effects of the black hole spacetime. 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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5 pages, 256 KiB

Open AccessArticle

Sensitivity of Cross-Correlation Studies by Using X_max Information

by Cynthia V. Ventura, Rogerio M. de Almeida, Rita C. Dos Anjos and Jaziel G. Coelho

Galaxies 2022, 10(4), 83; https://doi.org/10.3390/galaxies10040083 - 12 Jul 2022

Viewed by 1395

Abstract

The investigation about cosmic-ray sources through the study of the arrival direction of the particles is completely dependent on primary mass compositions. This is due to heavier particles having an average larger deflection during propagation in galactic and extragalactic magnetic fields. Recent result [...] Read more.

The investigation about cosmic-ray sources through the study of the arrival direction of the particles is completely dependent on primary mass compositions. This is due to heavier particles having an average larger deflection during propagation in galactic and extragalactic magnetic fields. Recent result from the Auger Collaboration made it possible to obtain information on the chemical composition of the cosmic rays on an event-by-event basis in addition to data from the surface detector. Here, we have investigated the gain in detection efficiency of standard cross-correlation using the Landy–Szalay estimator. Our sample contained only events with the highest

X_{\max}

values (corresponding to the lightest charged particles, for which the deflections in magnetic fields are smaller on average). We obtained results showing that that gain depends on the proton fraction ranging from a factor of ∼2 to ∼4. 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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2 pages, 159 KiB

Open AccessEditorial

Preface of “Particles and Fields in Black Hole Environment”

by Bobomuat Ahmedov, Ahmadjon Abdujabbarov and Wenbiao Han

Galaxies 2022, 10(4), 82; https://doi.org/10.3390/galaxies10040082 - 09 Jul 2022

Viewed by 1158

Abstract

This Special Issue is dedicated to the memory of the physicist Professor James Maxwell Bardeen (9 May 1939–20 June 2022), who is well known for his work on general relativity, particularly his role in formulating the laws of black hole mechanics, the pioneering [...] Read more.

This Special Issue is dedicated to the memory of the physicist Professor James Maxwell Bardeen (9 May 1939–20 June 2022), who is well known for his work on general relativity, particularly his role in formulating the laws of black hole mechanics, the pioneering idea on black hole shadow observation and the Bardeen solution to gravitational field equations [...] Full article

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

3 pages, 179 KiB

Open AccessEditorial

Preface of “Asymmetric Planetary Nebulae 8e”

by Martin A. Guerrero, Quentin A. Parker and Noam Soker

Galaxies 2022, 10(4), 81; https://doi.org/10.3390/galaxies10040081 - 08 Jul 2022

Viewed by 1109

Abstract

Planetary nebulae (PNe) are the progeny of low- and intermediate-mass stars, at the exact time in their late evolution when they eject their hydrogen-rich envelopes and start their transition towards white dwarfs [...] Full article

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

7 pages, 578 KiB

Open AccessArticle

Direct Imaging of the Cosmic Battery in M87*? Not Yet

by Ioannis Contopoulos, Ioannis Myserlis, Demosthenes Kazanas and Antonios Nathanail

Galaxies 2022, 10(4), 80; https://doi.org/10.3390/galaxies10040080 - 29 Jun 2022

Cited by 2 | Viewed by 1620

Abstract

One of the fundamental conclusions of the Cosmic Battery (a means for producing poloidal magnetic flux in the vicinity of a black hole via the Poynting-Robertson effect on the accretion disk) is that it determines the direction of the axial magnetic field: near [...] Read more.

One of the fundamental conclusions of the Cosmic Battery (a means for producing poloidal magnetic flux in the vicinity of a black hole via the Poynting-Robertson effect on the accretion disk) is that it determines the direction of the axial magnetic field: near the black hole it is parallel to the angular velocity

Ω

of the surrounding disk, while further away it is anti-parallel to

Ω

. The EHT polarization observations of M87

^{*}

where the innermost accretion disk is observed almost face-on thus offer an ideal opportunity to study the action of the Cosmic Battery, by deciding whether the field geometry is consistent with its premises. Unfortunately, such a determination is difficult at the moment due to the lack of reliable Faraday Rotation Measure (RM) maps of M87* at event horizon scales. Furthermore, in agreement with recent General Relativistic Magnetohydrodynamic (GRMHD) numerical simulations, if the inner accretion disk is highly turbulent, one would expect the RM to flip sign on dynamical time scales. While such RM observations are paramount for the determination of the field geometry in confirmation or refutation of the Cosmic Battery, this may have to wait for long term monitoring at event horizon scales and perhaps the synergy of lower resolution RM observations. 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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11 pages, 1128 KiB

Open AccessArticle

Temperature Effects on Core g-Modes of Neutron Stars

by Nicholas Lozano, Vinh Tran and Prashanth Jaikumar

Galaxies 2022, 10(4), 79; https://doi.org/10.3390/galaxies10040079 - 28 Jun 2022

Cited by 4 | Viewed by 1604

Abstract

Neutron stars provide a unique physical laboratory in which to study the properties of matter at high density and temperature. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be [...] Read more.

Neutron stars provide a unique physical laboratory in which to study the properties of matter at high density and temperature. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be excited by tidal forces and couple to gravitational waves. We extend prior results for the g-mode spectrum of cold neutron star matter to high temperatures that are expected to be achieved in neutron star mergers using a parameterization for finite-temperature effects on equations of state recently proposed by Raithel, Özel and Psaltis. We find that the g-modes of canonical mass neutron stars (≈1.4

M_{⊙}

) are suppressed at high temperatures, and core g-modes are supported only in the most massive (≥2

M_{⊙}

) of hot neutron stars. Full article

(This article belongs to the Special Issue Neutron Stars and Hadrons in the Era of Gravitational Wave Astrophysics)

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22 pages, 4351 KiB

Open AccessArticle

Two Classes of Gamma-ray Bursts Distinguished within the First Second of Their Prompt Emission

by Lána Salmon, Lorraine Hanlon and Antonio Martin-Carrillo

Galaxies 2022, 10(4), 78; https://doi.org/10.3390/galaxies10040078 - 26 Jun 2022

Cited by 5 | Viewed by 2915

Abstract

Studies of Gamma-Ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes, beyond the short/hard and long/soft prototypes, using model-dependent methods. In this paper, a model-independent approach was used to analyse the gamma-ray light curves of large [...] Read more.

Studies of Gamma-Ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes, beyond the short/hard and long/soft prototypes, using model-dependent methods. In this paper, a model-independent approach was used to analyse the gamma-ray light curves of large samples of GRBs detected by BATSE, Swift/BAT and Fermi/GBM. All the features were extracted from the GRB time profiles in four energy bands using the Stationary Wavelet Transform and Principal Component Analysis. t-distributed Stochastic Neighbourhood Embedding (t-SNE) visualisation of the features revealed two distinct groups of Swift/BAT bursts using the T₁₀₀ interval with 64 ms resolution data. When the same analysis was applied to 4 ms resolution data, two groups were seen to emerge within the first second (T₁) post-trigger. These two groups primarily consisted of short/hard (Group 1) and long/soft (Group 2) bursts, and were 95% consistent with the groups identified using the T₁₀₀ 64 ms resolution data. Kilonova candidates, arising from compact object mergers, were found to belong to Group 1, while those events with associated supernovae fell into Group 2. Differences in cumulative counts between the two groups in the first second, and in the minimum variability timescale, identifiable only with the 4 ms resolution data, may account for this result. Short GRBs have particular significance for multi-messenger science as a distinctive EM signature of a binary merger, which may be discovered by its gravitational wave emissions. Incorporating the T₁ interval into classification algorithms may support the rapid classification of GRBs, allowing for an improved prioritisation of targets for follow-up observations. Full article

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

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17 pages, 766 KiB

Open AccessArticle

Two Dimensional Clustering of Swift/BAT and Fermi/GBM Gamma-ray Bursts

by Lána Salmon, Lorraine Hanlon and Antonio Martin-Carrillo

Galaxies 2022, 10(4), 77; https://doi.org/10.3390/galaxies10040077 - 25 Jun 2022

Cited by 4 | Viewed by 2176

Abstract

Studies of Gamma-ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes beyond the short-hard (merger) and long-soft (collapsar) prototypes. Several clustering analyses of the duration-hardness plane identified a third, intermediate duration, class. In this work, Gaussian [...] Read more.

Studies of Gamma-ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes beyond the short-hard (merger) and long-soft (collapsar) prototypes. Several clustering analyses of the duration-hardness plane identified a third, intermediate duration, class. In this work, Gaussian Mixture Model-based (GMM) clustering is applied to the Swift/BAT and Fermi/GBM samples of GRBs. The results obtained by the hierarchical combination of Gaussian components (or clusters) based on an entropy criterion are presented. This method counteracts possible overfitting arising from the application of Gaussian models to non-Gaussian underlying data. While the initial GMM clustering of the hardness-duration plane identifies three components (short/intermediate/long) for the Swift/BAT and Fermi/GBM samples, only two components (short/long) remain once the entropy criterion is applied. The analysis presented here suggests that the intermediate duration class may be the result of overfitting, rather than evidence of a distinct underlying population. Full article

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

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64 pages, 3915 KiB

Open AccessReview

Compact Binary Coalescences: Astrophysical Processes and Lessons Learned

by Mario Spera, Alessandro Alberto Trani and Mattia Mencagli

Galaxies 2022, 10(4), 76; https://doi.org/10.3390/galaxies10040076 - 25 Jun 2022

Cited by 25 | Viewed by 3128

Abstract

On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the [...] Read more.

On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the beginning of an entirely new way to investigate the Universe. The latest gravitational-wave catalog by LIGO, Virgo and KAGRA brings the total number of gravitational-wave events to 90, and the count is expected to significantly increase in the next years, when additional ground-based and space-born interferometers will be operational. From the theoretical point of view, we have only fuzzy ideas about where the detected events came from, and the answers to most of the five Ws and How for the astrophysics of compact binary coalescences are still unknown. In this work, we review our current knowledge and uncertainties on the astrophysical processes behind merging compact-object binaries. Furthermore, we discuss the astrophysical lessons learned through the latest gravitational-wave detections, paying specific attention to the theoretical challenges coming from exceptional events (e.g., GW190521 and GW190814). Full article

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

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Galaxies, Volume 10, Issue 4 (August 2022) – 16 articles

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