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Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics,...
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Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations. Quo Vadis?

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

Deadline for manuscript submissions: closed (1 November 2022) | Viewed by 19634

Special Issue Editors

Prof. Dr. Jaziel Goulart Coelho

E-Mail Website
Guest Editor
Núcleo de Astrofísica e Cosmologia (Cosmo-ufes) & Departamento de Física, Universidade Federal do Espírito Santo (UFES), Av. Fernando Ferrari, 514, Vitória 29065-910, ES, Brazil
Interests: compact objects; gravitational waves; relativistic astrophysics; nuclear astrophysics
Prof. Dr. Rita C. Anjos

E-Mail Website
Guest Editor
Department of Engineering and Exact Sciences – Sector Palotina (DEE-SP), Federal University of Paraná (UFPR) Rua Pioneiro, 2153, Jardim Dallas, 85950-000 Palotina-PR, Brazil
Interests: high-energy astrophysics; cosmic rays; neutrinos; multimessenger

Special Issue Information

Dear Colleagues,

We have recently inaugurated the era of multi-messenger astronomy with pioneering experiments, theoretical interpretation, and models. We have advanced many questions regarding gravitational waves, compact objects, and high-energy cosmic ray physics.

There are reasons to believe that the 21st century will be the best ever for astrophysics: gravitational waves produced by compact objects (such as supermassive black holes (BHs), BH/neutron star (NS) mergers, gamma-ray bursts, and white dwarf inspirals) have unveiled a new area of astronomy. Framed by this background, compact stars represent unique astrophysical laboratories for developing the theoretical understanding of microphysics (equation of state, particle composition, etc.) and the macrophysical structure of superdense matter in compact objects. In recent years, multi-messenger observations of these compact sources in binaries as well as in isolation, e.g., in X-rays (e.g., NICER, HXMT, XMM-Newton, Chandra, and Swift), gamma-rays (e.g., HESS, Fermi, VERITAS, and MAGIC), and radio facilities (e.g., LOFAR, FAST, CHIME, VLA, ALMA, etc.), particles (e.g., Pierre Auger Observatory and Telescope Array) together with synergic analyses of gravitational waves, and neutrinos have given rise to new observational windows and constraints on compact stars’ structure, on which this Special Issue welcomes discussions, including perspectives from forthcoming experimental facilities. We stress that several questions in high-energy astrophysics are also still a mystery. In the coming decades, we hope to further our understanding of the nature of ultra-high-energy cosmic rays (UHECRs); their composition; sources of UHECRs and GeV-TeV gamma-rays; the connections between galactic and extragalactic acceleration mechanisms; the generation, interactions and the propagation of CRs and multi-messengers; and the description of re-acceleration scenarios based on gamma ray observations. Furthermore, the future of astrophysics with the Cherenkov Telescope Array in operation, along with other high-energy experiments, will be a window of discovery and enhance understanding of the universe in extreme energy conditions.

The proposal in this Special Issue is to review the current state of the art and to explore the potential advances in high-density compact objects, high-energy astrophysics, and multi-messenger observations.

It is worth mentioning that 2022 marks 55 years since pulsars were discovered. In that time, we have found more than 2000 pulsars, and used them to test the general theory of relativity and to hunt for gravitational waves. In fact, pulsars have changed our understanding of the universe, and their true importance is still unfolding. Fifty-five years on, we have a much better (but still incomplete) understanding of these extreme objects. Additionally, in 2022, we celebrate 110 years since Victor Hess discovered cosmic rays with balloon flights; in these more than 100 years of study, we have advanced several high-energy questions to an unprecedented precision. The aim of this Special Issue is to bring together researchers from particle physics, nuclear theory and astrophysics, working on these topics from different but complementary viewpoints. This Special Issue welcomes theoretical, analytical and observational research on these astrophysics topics. Original research articles and reviews are encouraged. All submissions will be peer-reviewed before being accepted for publication. Your contributions will be much appreciated.

Prof. Dr. Jaziel Goulart Coelho
Prof. Dr. Rita C. Anjos
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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. Galaxies is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • discoveries in high-energy astrophysics
  • multi-messenger observations
  • compact objects
  • cosmic rays
  • gamma-rays
  • gravitational waves
  • relativistic astrophysics
  • neutrinos
  • acceleration mechanisms
  • sources

Published Papers (11 papers)

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Editorial

Jump to: Research, Review, Other

2 pages, 172 KiB  
Editorial
Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations: Quo Vadis?
by Jaziel G. Coelho and Rita. C. Dos Anjos
Galaxies 2023, 11(2), 48; https://doi.org/10.3390/galaxies11020048 - 22 Mar 2023
Viewed by 887
Abstract
The era of multi-messenger astronomy has recently be inaugurated with pioneering experiments, theoretical interpretation, and models [...] 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?)

Research

Jump to: Editorial, Review, Other

35 pages, 1622 KiB  
Article
An Overview of Compact Star Populations and Some of Its Open Problems
by Lucas M. de Sá, Antônio Bernardo, Riis R. A. Bachega, Livia S. Rocha, Pedro H. R. S. Moraes and Jorge E. Horvath
Galaxies 2023, 11(1), 19; https://doi.org/10.3390/galaxies11010019 - 18 Jan 2023
Cited by 5 | Viewed by 2031
Abstract
The study of compact object populations has come a long way since the determination of the mass of the Hulse–Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects [...] Read more.
The study of compact object populations has come a long way since the determination of the mass of the Hulse–Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects from binary mergers detected from gravitational-waves by the Ligo–Virgo–KAGRA Collaboration. With a growing understanding of the variety of systems that host these objects, their formation, evolution and frequency, we are now in a position to evaluate the statistical nature of these populations, their properties, parameter correlations and long-standing problems, such as the maximum mass of neutron stars and the black hole lower mass gap, to a reasonable level of statistical significance. Here, we give an overview of the evolution and current state of the field and point to some of its standing issues. We focus on Galactic black holes, and offer an updated catalog of 35 black hole masses and orbital parameters, as well as a standardized procedure for dealing with uncertainties. 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, 1422 KiB  
Article
A Method to Fit Phase Diagrams of Slow-Rotation Pulsars with Accretion Columns
by Rodrigo R. Silva, Rafael C. R. de Lima, Jaziel G. Coelho, Paulo E. Stecchini, Caroline M. de Liz and José C. N. de Araujo
Galaxies 2023, 11(1), 18; https://doi.org/10.3390/galaxies11010018 - 16 Jan 2023
Viewed by 1207
Abstract
We demonstrate a method to simulate a pulse profile of the emission of accretion columns in a neutron star. Given a set of parameters that characterize the star, e.g., mass and radius, and the positions and shapes of the accretion columns, the pulse [...] Read more.
We demonstrate a method to simulate a pulse profile of the emission of accretion columns in a neutron star. Given a set of parameters that characterize the star, e.g., mass and radius, and the positions and shapes of the accretion columns, the pulse profile can be calculated and compared with real data. Some characteristics of phase diagrams are shown considering an accretion column in the form of a cone trunk and also in the form of a cylinder. Furthermore, in our first approach we develop a combination between a genetic algorithm and a Bayesian sampling algorithm to constrain some variables. Finally, as an example, we apply the method to observed data of source 1A 0535+262. 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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23 pages, 3035 KiB  
Article
Spun-Up Rotation-Powered Magnetized White Dwarfs in Close Binaries as Possible Gamma-ray Sources: Signatures of Pulsed Modulation from AE Aquarii and AR Scorpii in Fermi-LAT Data
by Pieter J. Meintjes, Spencer T. Madzime, Quinton Kaplan and Hendrik J. van Heerden
Galaxies 2023, 11(1), 14; https://doi.org/10.3390/galaxies11010014 - 11 Jan 2023
Cited by 2 | Viewed by 1340
Abstract
In this paper, the possibility of periodic pulsar-like gamma-ray emission from the white dwarfs in AE Aquarii and AR Scorpii is investigated. We show that the white dwarf magnetospheres in AE Aquarii and AR Scorpii can possibly induce potentials to accelerate charged particles [...] Read more.
In this paper, the possibility of periodic pulsar-like gamma-ray emission from the white dwarfs in AE Aquarii and AR Scorpii is investigated. We show that the white dwarf magnetospheres in AE Aquarii and AR Scorpii can possibly induce potentials to accelerate charged particles to energies in excess of one tera electronvolt (TeV) with associated gamma-ray emission through processes such as curvature radiation, inverse Compton, and hadronic processes such as neutral pion decay. We report here pulsed gamma-ray signatures at or close to the spin period of white dwarfs in both AE Aquarii and AR Scorpii in the Fermi-LAT dataset. This may indicate that both these white dwarfs possibly contain a particle accelerator that can produce relativistic electrons and ions and associated high energy radiation. The possibility of pair production is also investigated, which could provide a source for relativistic e± pairs in the magnetosphere. This could possibly be a driver for other forms of lepton-induced multi-wavelength pulsar-like emission from these two systems as well, for example, to explain the recently detected pulsed radio emission from AE Aquarii and R Scorpii in MeerKAT observations at the spin period of the white dwarf. The possibility of future detection of AE Aquarii and AR Scorpii with the Cherenkov Telescope Array (CTA) is also discussed. The future Vera Rubin Observatory will make a revolutionary contribution to time-domain astrophysics, which may lead to the discovery of thousands of new transient sources, possibly also many more close binaries with highly spun-up magnetized white dwarfs such as AE Aquarii and AR Scorpii for future investigation. 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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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 1749
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
(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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23 pages, 631 KiB  
Article
Gravitational-Wave Instabilities in Rotating Compact Stars
by Eric L. Bratton II, Zikun Lin, Fridolin Weber, Milva G. Orsaria, Ignacio F. Ranea-Sandoval and Nathaniel Saavedra
Galaxies 2022, 10(5), 94; https://doi.org/10.3390/galaxies10050094 - 30 Aug 2022
Cited by 3 | Viewed by 1868
Abstract
It is generally accepted that the limit on the stable rotation of neutron stars is set by gravitational-radiation reaction (GRR) driven instabilities, which cause the stars to emit gravitational waves that carry angular momentum away from them. The instability modes are moderated by [...] Read more.
It is generally accepted that the limit on the stable rotation of neutron stars is set by gravitational-radiation reaction (GRR) driven instabilities, which cause the stars to emit gravitational waves that carry angular momentum away from them. The instability modes are moderated by the shear viscosity and the bulk viscosity of neutron star matter. Among the GRR instabilities, the f-mode instability plays a historically predominant role. In this work, we determine the instability periods of this mode for three different relativistic models for the nuclear equation of state (EoS) named DD2, ACB4, and GM1L. The ACB4 model for the EoS accounts for a strong first-order phase transition that predicts a new branch of compact objects known as mass-twin stars. DD2 and GM1L are relativistic mean field (RMF) models that describe the meson-baryon coupling constants to be dependent on the local baryon number density. Our results show that the f-mode instability associated with m=2 sets the limit of stable rotation for cold neutron stars (T≲1010 K) with masses between 1M⊙ and 2M⊙. This mode is excited at rotation periods between 1 and 1.4 ms (∼20% to ∼40% higher than the Kepler periods of these stars). For cold hypothetical mass-twin compact stars with masses between 1.96M⊙ and 2.10M⊙, the m=2 instability sets in at rotational stellar periods between 0.8 and 1 millisecond (i.e., ∼25% to ∼30% above the Kepler period). 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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14 pages, 482 KiB  
Article
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 1652
Abstract
The source, origin, and acceleration mechanisms of ultra-high-energy cosmic rays (UHECR) (E>1020 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>1020 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  
Article
Sensitivity of Cross-Correlation Studies by Using Xmax 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 1363
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 Xmax 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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7 pages, 578 KiB  
Article
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 1588
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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Review

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28 pages, 2221 KiB  
Review
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 2665
Abstract
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 1015 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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Other

9 pages, 2205 KiB  
Essay
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 1802
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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