Universe: Feature Papers − Compact Objects

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Compact Objects".

Deadline for manuscript submissions: closed (30 September 2021) | Viewed by 22892

Special Issue Editors

1. Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetsky pr. 13, Moscow 119234, Russia
2. Department of Physics, National Research University ‘Higher School of Economics’, Myasnitskaya str. 20, Moscow 101000, Russia
Interests: neutron stars; black holes; binary systems; pulsars
Special Issues, Collections and Topics in MDPI journals
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
Interests: black holes; accretion; radiation transport; gravitational theory; computational physics

Special Issue Information

Dear Colleagues,

Compact objects such as white dwarfs, neutron stars, and black holes are natural test-beds which enable the study of physical processes under conditions inaccessible to terrestrial laboratories. Different branches of fundamental physics, from quantum electrodynamics to strong field gravity, and from chromodynamics to particle physics, stand to benefit greatly from astrophysical studies of compact objects.

In recent years, primarily due to multimessenger (and often multiwavelength) approaches, our understanding of compact object physics has advanced significantly. Observational facilities like LIGO/VIRGO, the Event Horizon Telescope, as well as Gaia, Chandra and XMM-Newton, large ground-based optical telescopes, and many others, provide us with a plethora of new data which warrant both deep and careful analysis, and theoretical interpretation. This volume aims to present new results in this vast field of theoretical and observational astrophysics.

Many aspects of the astrophysics of compact objects are linked to each other. Thanks to studies of Sgr A* and results from the Event Horizon Telescope on M87, strong field gravity is linked to accretion physics. Studies of neutron star cooling demonstrate that magnetic field evolution is linked to properties of high density matter. In the case of coalescence of compact objects, nuclear physics appears to be connected to gravitational wave observations. We hope that reviews, comprehensive papers, and results of original studies in this Feature Paper Volume will exhibit the richness of the multifaceted and interrelated properties of black holes, neutron stars, and white dwarfs.

We anticipate the manuscripts to not just cover our present day understanding, but also to devote some attention to addressing current observational and theoretical challenges, major ongoing developments, and some discussion of future prospects and the scientific explorations and insights these will enable.

We invite colleagues to submit contributing topics on the broad subject of “Compact Objects”, addressing both observational and theoretical research concerning:

(1) Accretion flows and jets;

(2) Coalescence of compact objects;

(3) Properties of matter inside neutron stars and white dwarfs;

(4) Magnetic fields of compact objects and magnetospheric processes;

(5) Supermassive black holes and their associated physical phenomena;

(6) Multi-messenger studies of compact objects and related physical phenomena;

(7) Astroparticle physics with compact objects;

(8) Probes of fundamental theories and new physics with compact objects;

(9) Compact binary evolution and dynamical gravitational fields.

Prof. Dr. Sergei Popov
Dr. Ziri Younsi
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. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. 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.

Published Papers (13 papers)

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Editorial

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3 pages, 179 KiB  
Editorial
Editorial to the Special Issue “Feature Papers—Compact Objects”
Universe 2022, 8(12), 662; https://doi.org/10.3390/universe8120662 - 16 Dec 2022
Viewed by 846
Abstract
Astrophysical studies of compact objects unite very different fields of research: from observations of known sources to sophisticated theoretical models involving new physics [...] Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)

Research

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18 pages, 880 KiB  
Article
Rotational Energy Extraction from the Kerr Black Hole’s Mimickers
Universe 2022, 8(11), 571; https://doi.org/10.3390/universe8110571 - 30 Oct 2022
Cited by 10 | Viewed by 1099
Abstract
In this paper, the Penrose process is used to extract rotational energy from regular black holes. Initially, we consider the rotating Simpson–Visser regular spacetime, which describes the class of geometries of Kerr black hole mimickers. The Penrose process is then studied through conformally [...] Read more.
In this paper, the Penrose process is used to extract rotational energy from regular black holes. Initially, we consider the rotating Simpson–Visser regular spacetime, which describes the class of geometries of Kerr black hole mimickers. The Penrose process is then studied through conformally transformed rotating singular and regular black hole solutions. Both the Simpson–Visser and conformally transformed geometries depend on mass, spin, and an additional regularisation parameter l. In both cases, we investigate how the spin and regularisation parameter l affect the configuration of an ergoregion and event horizons. Surprisingly, we find that the energy extraction efficiency from the event horizon surface is not dependent on the regularisation parameter l in the Simpson–Visser regular spacetimes, and hence, it does not vary from that of the Kerr black hole. Meanwhile, in conformally transformed singular and regular black holes, we obtain that the efficiency rate of extracted energies is extremely high compared to that of the Kerr black hole. This distinct signature of conformally transformed singular and regular black holes is useful to distinguish them from Kerr black holes in observation. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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16 pages, 586 KiB  
Article
Observability of HOFNARs at SRG/eROSITA
Universe 2022, 8(7), 354; https://doi.org/10.3390/universe8070354 - 27 Jun 2022
Cited by 3 | Viewed by 1210
Abstract
Neutron stars can appear as sources of different nature. In this paper we address the observability of a hypothetical class of neutron stars—HOt and Fast Non-Accreting Rotators, HOFNARs. These objects are heated due to the r-mode instability. With surface temperatures ∼106 K [...] Read more.
Neutron stars can appear as sources of different nature. In this paper we address the observability of a hypothetical class of neutron stars—HOt and Fast Non-Accreting Rotators, HOFNARs. These objects are heated due to the r-mode instability. With surface temperatures ∼106 K they are expected to be thermal soft X-ray emitters. We perform a population synthesis modeling of HOFNARs to predict the number of potentially detectable sources in the eROSITA all-sky survey. For surface temperatures ∼106 K we obtain ∼500 sources above the detection limit 0.01 cts s−1 and ∼100 easier identifiable sources with >0.1 cts s−1. Temperatures ≳1.2 × 106 K start to be in contradiction with non-detection of HOFNARs by ROSAT. Only for T ≲ 5 × 105 K numbers predicted for eROSITA turn out to be so low that identification does not look possible. We conclude that eROSITA has good chances to discover HOFNARs, if they exist. Non-detection will put very stringent limits on the properties of this type of neutron stars. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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23 pages, 880 KiB  
Article
Shadow and Weak Deflection Angle of a Black Hole in Nonlocal Gravity
Universe 2022, 8(7), 341; https://doi.org/10.3390/universe8070341 - 21 Jun 2022
Cited by 4 | Viewed by 1141
Abstract
Black hole shadow and gravitational lensing play important roles in testing gravitational theories in the strong field regime. As the first-order modifications from quantum gravity, the nonlocality can be manifested by black hole shadow and gravitational lensing. For example, the nonlocal parameter introduced [...] Read more.
Black hole shadow and gravitational lensing play important roles in testing gravitational theories in the strong field regime. As the first-order modifications from quantum gravity, the nonlocality can be manifested by black hole shadow and gravitational lensing. For example, the nonlocal parameter introduced by nonlocality will affect the shape and size of the black hole shadow, and also affect the deflection angle of light rays. In this paper, we mainly investigate the effects of the nonlocality on the black hole shadow and the gravitational lensing for two types of rotating black holes in nonlocal gravity. It is found that the size of the black hole shadow decreases with the nonlocal parameter since the nonlocality weakens the gravitational constant, and the shape of the shadow becomes more deformed with the increase in the nonlocal parameter. However, if the rotation parameter is small, the shape of the shadow is almost a circle even though the nonlocal parameter approaches its maximum. The energy emission rate in both models is also studied. The results show that there is a peak for each curve and the peak decreases and shifts to the low frequency with the increase in the nonlocal parameter. In addition, we also explore the shadow of both types of black holes surrounded by a nonmagnetized pressureless plasma which satisfies the separability condition. It is found that the plasma has a frequency-dependent dispersive effect on the size and shape of the black hole shadow. For the gravitational lensing, we find that the nonlocal parameter of model A makes a positive contribution to the deflection angle, which can be compared with the contribution of the rotation parameter, while the nonlocal parameter of model B makes a negative contribution which can be ignored. These results may be helpful for probing nonlocal gravity in future observations. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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16 pages, 3125 KiB  
Article
Multi-Messenger Astrophysics of a Millisecond Pulsar Orbiting around a Massive Black Hole
Universe 2022, 8(2), 78; https://doi.org/10.3390/universe8020078 - 27 Jan 2022
Cited by 3 | Viewed by 2081
Abstract
Extreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar are gravitational-wave sources that emit also electromagnetic radiation. The millisecond pulsars in these binaries have complex orbital and spin dynamics, which are observable because of spin–orbit and spin–spin coupling (through spin–curvature interaction). The strengths of [...] Read more.
Extreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar are gravitational-wave sources that emit also electromagnetic radiation. The millisecond pulsars in these binaries have complex orbital and spin dynamics, which are observable because of spin–orbit and spin–spin coupling (through spin–curvature interaction). The strengths of the couplings generally depends on the mass ratio between the pulsar and the black hole. The narrow mass range of neutron stars gives an advantage in parameter extraction as it greatly reduces the search space, in particular, in the determination of the black-hole mass, in gravitational wave experiments and radio pulsar timing observations. Extreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar will help to resolve the astrophysical problems, concerning the applicability of the M-σ relation for galactic spheroids extending to the very low-mass galaxies and whether or not low-mass dwarf galaxies and globular clusters would harbour a nuclear intermediate-mass black hole. The high-precision that can be achieved in gravitational wave experiments and radio pulsar timing observations will provide an opportunity to directly detect gravitational clock effects that are arisen from spin couplings. Radio monitoring of the orbital and spin evolution of the millisecond pulsar in an extreme-mass-ratio binary can be used as a bootstrap method for correcting the drifts in the phases in the gravitational waves from the extreme-mass-ratio and intermediate-mass-ratio binaries caused by self-force. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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16 pages, 362 KiB  
Article
Spin-1/2 Particles under the Influence of a Uniform Magnetic Field in the Interior Schwarzschild Solution
Universe 2021, 7(12), 467; https://doi.org/10.3390/universe7120467 - 30 Nov 2021
Cited by 2 | Viewed by 1634
Abstract
The relativistic wave equation for spin-1/2 particles in the interior Schwarzschild solution in the presence of a uniform magnetic field is obtained. The fully relativistic regime is considered, and the energy levels occupied by the particles are derived as functions of the magnetic [...] Read more.
The relativistic wave equation for spin-1/2 particles in the interior Schwarzschild solution in the presence of a uniform magnetic field is obtained. The fully relativistic regime is considered, and the energy levels occupied by the particles are derived as functions of the magnetic field, the radius of the massive sphere and the total mass of the latter. As no assumption is made on the relative strengths of the particles’ interaction with the gravitational and magnetic fields, the relevance of our results to the physics of the interior of neutron stars, where both the gravitational and the magnetic fields are very intense, is discussed. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
18 pages, 819 KiB  
Article
Analytical Solution and Quasi-Periodic Behavior of a Charged Dilaton Black Hole
Universe 2021, 7(10), 377; https://doi.org/10.3390/universe7100377 - 09 Oct 2021
Cited by 3 | Viewed by 1170
Abstract
With the vast breakthrough brought by the Event Horizon Telescope, the theoretical analysis of various black holes has become more critical than ever. In this paper, the second-order asymptotic analytical solution of the charged dilaton black hole flow in the spinodal region is [...] Read more.
With the vast breakthrough brought by the Event Horizon Telescope, the theoretical analysis of various black holes has become more critical than ever. In this paper, the second-order asymptotic analytical solution of the charged dilaton black hole flow in the spinodal region is constructed from the perspective of dynamics by using the two-timing scale method. Through a numerical comparison with the original charged dilaton black hole system, it is found that the constructed analytical solution is highly consistent with the numerical solution. In addition, several quasi-periodic motions of the charged dilaton black hole flow are numerically obtained under different groups of irrational frequency ratios, and the phase portraits of the black hole flow with sufficiently small thermal parameter perturbation display good stability. Finally, the final evolution state of black hole flow over time is studied according to the obtained analytical solution. The results show that the smaller the integral constant of the system, the greater the periodicity of the black hole flow. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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20 pages, 2668 KiB  
Article
The 35-Day Cycle of Hercules X-1 in Multiple Energy Bands from MAXI and Swift/BAT Monitoring
Universe 2021, 7(6), 160; https://doi.org/10.3390/universe7060160 - 23 May 2021
Cited by 4 | Viewed by 1301
Abstract
Hercules X-1 (Her X-1) has been monitored by MAXI and by Swift/BAT for over a decade. Those observations are analyzed to measure the shape and energy dependence of the long-term average of the 35-day cycle of Her X-1. The cross-correlation (CC) method is [...] Read more.
Hercules X-1 (Her X-1) has been monitored by MAXI and by Swift/BAT for over a decade. Those observations are analyzed to measure the shape and energy dependence of the long-term average of the 35-day cycle of Her X-1. The cross-correlation (CC) method is used to determine peak times and cycle lengths. Swift/BAT data produces better 35-day times because of the gaps in the MAXI data. Using Swift/BAT-derived times, average 35-day cycle light-curves are created for multiple energy bands: MAXI’s 2–20 keV, 2–4 keV, 4–10 keV and 10–20 keV bands and Swift/BAT’s 15–50 keV band. The durations of the different states of the 35-day cycle are measured better than previously. We find clear changes in X-ray softness ratio with 35-day phase, and detect persistent features in the 35-day cycle. These include column density changes during turn-on of Main High and of Short High states, and persistent absorption dips during the bright part of Main High and of Short High states. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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23 pages, 598 KiB  
Article
Study on Anisotropic Strange Stars in f ( T , T ) Gravity
Universe 2020, 6(10), 167; https://doi.org/10.3390/universe6100167 - 03 Oct 2020
Cited by 27 | Viewed by 2387
Abstract
In this work, we study the existence of strange stars in the background of f(T,T) gravity in the Einstein spacetime geometry, where T is the torsion tensor and T is the trace of the energy-momentum tensor. The equations [...] Read more.
In this work, we study the existence of strange stars in the background of f(T,T) gravity in the Einstein spacetime geometry, where T is the torsion tensor and T is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified Tolman–Oppenheimer–Volkoff (TOV) equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in f(T,T) gravity as well as their observational signatures. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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Review

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21 pages, 2109 KiB  
Review
GRMHD Simulations and Modeling for Jet Formation and Acceleration Region in AGNs
Universe 2022, 8(2), 85; https://doi.org/10.3390/universe8020085 - 28 Jan 2022
Cited by 13 | Viewed by 2545
Abstract
Relativistic jets are collimated plasma outflows with relativistic speeds. Astrophysical objects involving relativistic jets are a system comprising a compact object such as a black hole, surrounded by rotating accretion flows, with the relativistic jets produced near the central compact object. The most [...] Read more.
Relativistic jets are collimated plasma outflows with relativistic speeds. Astrophysical objects involving relativistic jets are a system comprising a compact object such as a black hole, surrounded by rotating accretion flows, with the relativistic jets produced near the central compact object. The most accepted models explaining the origin of relativistic jets involve magnetohydrodynamic (MHD) processes. Over the past few decades, many general relativistic MHD (GRMHD) codes have been developed and applied to model relativistic jet formation in various conditions. This short review provides an overview of the recent progress of GRMHD simulations in generating relativistic jets and their modeling for observations. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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12 pages, 11715 KiB  
Review
The Effect of Charge, Isospin, and Strangeness in the QCD Phase Diagram Critical End Point
Universe 2021, 7(11), 454; https://doi.org/10.3390/universe7110454 - 20 Nov 2021
Cited by 4 | Viewed by 1287
Abstract
In this work, we discuss the deconfinement phase transition to quark matter in hot/dense matter. We examine the effect that different charge fractions, isospin fractions, net strangeness, and chemical equilibrium with respect to leptons have on the position of the coexistence line between [...] Read more.
In this work, we discuss the deconfinement phase transition to quark matter in hot/dense matter. We examine the effect that different charge fractions, isospin fractions, net strangeness, and chemical equilibrium with respect to leptons have on the position of the coexistence line between different phases. In particular, we investigate how different sets of conditions that describe matter in neutron stars and their mergers, or matter created in heavy-ion collisions affect the position of the critical end point, namely where the first-order phase transition becomes a crossover. We also present an introduction to the topic of critical points, including a review of recent advances concerning QCD critical points. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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26 pages, 432 KiB  
Review
Astroparticle Physics with Compact Objects
Universe 2021, 7(11), 401; https://doi.org/10.3390/universe7110401 - 25 Oct 2021
Cited by 14 | Viewed by 1585
Abstract
Probing the existence of hypothetical particles beyond the Standard model often deals with extreme parameters: large energies, tiny cross-sections, large time scales, etc. Sometimes, laboratory experiments can test required regions of parameter space, but more often natural limitations lead to poorly restrictive upper [...] Read more.
Probing the existence of hypothetical particles beyond the Standard model often deals with extreme parameters: large energies, tiny cross-sections, large time scales, etc. Sometimes, laboratory experiments can test required regions of parameter space, but more often natural limitations lead to poorly restrictive upper limits. In such cases, astrophysical studies can help to expand the range of values significantly. Among astronomical sources, used in interests of fundamental physics, compact objects—neutron stars and white dwarfs—play a leading role. We review several aspects of astroparticle physics studies related to observations and properties of these celestial bodies. Dark matter particles can be collected inside compact objects resulting in additional heating or collapse. We summarize regimes and rates of particle capturing as well as possible astrophysical consequences. Then, we focus on a particular type of hypothetical particles—axions. Their existence can be uncovered due to observations of emission originated due to the Primakoff process in magnetospheres of neutron stars or white dwarfs. Alternatively, they can contribute to the cooling of these compact objects. We present results in these areas, including upper limits based on recent observations. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
34 pages, 1581 KiB  
Review
Evolution of Neutron Star Magnetic Fields
Universe 2021, 7(9), 351; https://doi.org/10.3390/universe7090351 - 20 Sep 2021
Cited by 50 | Viewed by 3149
Abstract
Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects, its magnetic field determines the appearance of [...] Read more.
Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects, its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for the interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review, we first briefly describe theoretical models of the formation and evolution of the magnetic field of neutron stars, paying special attention to field decay processes. Then, we present important observational results related to the field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, and sources in binary systems. After that, we discuss which observations can shed light on the obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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