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Neutron Stars and Gravitational Wave Observations

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

Deadline for manuscript submissions: closed (31 December 2022) | Viewed by 15524

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Special Issue Editors

Prof. Dr. Ignazio Bombaci

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Guest Editor

1. Department of Physics “Enrico Fermi”, University of Pisa, 56127 Pisa, Italy
2. INFN, Sezione di Pisa, 56127 Pisa, Italy
Interests: nuclear many-body theory; nuclear matter; equation of state of hadronic matter under extreme conditions; quark deconfinement phase transition in dense matter; neutron star physics and related astrophysical phenomena

Prof. Dr. Rosa Poggiani

E-Mail Website
Guest Editor

Department of Physics, University of Pisa, 56126 Pisa, Italy
Interests: gravitational wave physics; high energy astrophysics; multimessenger astronomy; time domain astronomy; novae and cataclysmic variables; blazars; experimental physics of fundamental interactions; antimatter physics; cryogenics; ultrahigh vacuum techniques

Special Issue Information

Dear Colleagues,

Neutron stars are remarkable natural laboratories that allow us to investigate the fundamental constituents of matter and their interactions under extreme conditions that cannot be reproduced in terrestrial laboratories.

The global properties of neutron stars (mass, radius, maximum mass, maximum spin frequency, etc.) primarily depend on the equation of state (EoS) of strong interacting matter. The EoS of dense matter is also a basic ingredient for modeling various astrophysical phenomena related to neutron stars, such as core-collapse supernovae and binary neutron star mergers.

The detection of gravitational waves from the first binary neutron star mergers, GW170817, followed by another neutron star merger, GW190425, and by GW190814 (whose secondary could either be the lightest black hole or the heaviest neutron star in a double compact binary) has opened a new window to investigate the interior of neutron stars and to explore matter under extreme conditions. Gravitational wave astronomy thus offers a unique opportunity to test different dense matter EoS models.

The aim of this Special Issue is the presentation of the impact of gravitational wave observations on the understanding of neutron stars and dense matter physics and to provide a comprehensive update of the status of the art in the field. The collection aims to include contributions over a wide range of topics. From the point of view of gravitational observations, it will include topics related to the gravitational emission of binary neutron stars mergers; isolated and accreting neutron stars; and neutron star/black hole mergers. From the point of view of dense matter physics, it will include topics related to the equation of state of hot and dense nuclear matter; neutrino trapping and emission in neutron stars; and the role of “exotic” degrees of freedom (such as hyperons, meson condensates or deconfined quarks) on neutron star structure and on binary neutron star mergers.

Prof. Dr. Ignazio Bombaci
Prof. Dr. Rosa Poggiani
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.

Keywords

Gravitational waves
Neutron stars
Gravitational wave sources
Neutron star mergers
Equation of state of dense matter
Neutrino dynamics in dense matter
Strangeness in neutron star matter
Supernovae and newborn neutron stars

Published Papers (6 papers)

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Research

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12 pages, 319 KiB

Open AccessArticle

Continuous Gravitational Wave Emissions from Neutron Stars with Pinned Superfluids in the Core

by Brynmor Haskell, Marco Antonelli and Pierre Pizzochero

Universe 2022, 8(12), 619; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8120619 - 24 Nov 2022

Cited by 7 | Viewed by 1171

Abstract

We investigate the effect of a pinned superfluid component on the gravitational wave emissions of a rotating neutron star. The pinning of superfluid vortices to the flux-tubes in the outer core (where the protons are likely to form a type-II superconductor) is a possible mechanism to sustain long-lived and non-axisymmetric neutron currents in the interior, which break the axial symmetry of the unperturbed hydrostatic configuration. We consider pinning-induced perturbations to a stationary corotating configuration and determine the upper limits on the strength of gravitational wave emissions due to the pinning of vortices with a strong toroidal magnetic field of the kind predicted by recent magneto-hydrodynamic simulations of neutron star interiors. We estimate the contributions to gravitational wave emissions from both the mass and current multipole generated by the pinned vorticity in the outer core and find that the mass quadrupole can be large enough for gravitational waves to provide the dominant spindown torque in millisecond pulsars. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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25 pages, 14717 KiB

Open AccessArticle

Incorporating a Radiative Hydrodynamics Scheme in the Numerical-Relativity Code BAM

by Henrique Gieg, Federico Schianchi, Tim Dietrich and Maximiliano Ujevic

Universe 2022, 8(7), 370; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8070370 - 05 Jul 2022

Cited by 5 | Viewed by 1275

Abstract

To study binary neutron star systems and to interpret observational data such as gravitational-wave and kilonova signals, one needs an accurate description of the processes that take place during the final stages of the coalescence, for example, through numerical-relativity simulations. In this work, we present an updated version of the numerical-relativity code BAM in order to incorporate nuclear-theory-based equations of state and a simple description of neutrino interactions through a neutrino leakage scheme. Different test simulations, for stars undergoing a neutrino-induced gravitational collapse and for binary neutron stars systems, validate our new implementation. For the binary neutron stars systems, we show that we can evolve stably and accurately distinct microphysical models employing the different equations of state: SFHo, DD2, and the hyperonic BHB

Λ ϕ

. Overall, our test simulations have good agreement with those reported in the literature. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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17 pages, 4853 KiB

Open AccessArticle

Probing Dense Nuclear Matter in the Laboratory: Experiments at FAIR and NICA

by Peter Senger

Universe 2021, 7(6), 171; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7060171 - 30 May 2021

Cited by 10 | Viewed by 1751

Abstract

The poorly known properties of high-density strongly-interacting matter govern the structure of neutron stars and the dynamics of neutron star mergers. New insight has been and will be gained by astronomical observations, such as the measurement of mass and radius of neutron stars, and the detection of gravitational waves emitted from neutron star mergers. Alternatively, information on the Nuclear Matter Equation-of-State (EOS) and on a possible phase transition from hadronic to quark matter at high baryon densities can be obtained from laboratory experiments investigating heavy-ion collisions. Detector systems dedicated to such experiments are under construction at the “Facility for Antiproton and Ion Research” (FAIR) in Darmstadt, Germany, and at the “Nuclotron-based Ion Collider fAcility” (NICA) in Dubna, Russia. In heavy-ion collisions at these accelerator centers, one expects the creation of baryon densities of up to 10 times saturation density, where quark degrees-of-freedom should emerge. This article reviews the most promising observables in heavy-ion collisions, which are used to probe the high-density EOS and possible phase transition from hadronic to quark matter. Finally, the facilities and the experimental setups will be briefly described. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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Review

Jump to: Research

16 pages, 945 KiB

Open AccessReview

Phase Conversions in Neutron Stars: Implications for Stellar Stability and Gravitational Wave Astrophysics

by Germán Lugones and Ana Gabriela Grunfeld

Universe 2021, 7(12), 493; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7120493 - 13 Dec 2021

Cited by 12 | Viewed by 1949

Abstract

We review the properties of hybrid stars with a quark matter core and a hadronic mantle, focusing on the role of key micro-physical properties such as the quark/hadron surface and curvature tensions and the conversion speed at the interface between both phases. We summarize the results of works that have determined the surface and curvature tensions from microscopic calculations. If these quantities are large enough, mixed phases are energetically suppressed and the quark core would be separated from the hadronic mantle by a sharp interface. If the conversion speed at the interface is slow, a new class of dynamically stable hybrid objects is possible. Densities tens of times larger than the nuclear saturation density can be attained at the center of these objects. We discuss possible formation mechanisms for the new class of hybrid stars and smoking guns for their observational identification. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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33 pages, 638 KiB

Open AccessReview

Hyperons in Neutron Stars

by Domenico Logoteta

Universe 2021, 7(11), 408; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110408 - 28 Oct 2021

Cited by 18 | Viewed by 2037

Abstract

I review the issues related to the appearance of hyperons in neutron star matter, focusing in particular on the problem of the maximum mass supported by hyperonic equations of state. I discuss the general mechanism that leads to the formation of hyperons in the core of neutron stars and I review the main techniques and many-body methods used to construct an appropriate equation of state to describe the strongly interacting system of hadrons hosted in the core of neutron stars. I outline the consequences on the structure and internal composition of neutron stars and also discuss the possible signatures of the presence of hyperons in astrophysical dynamical systems like supernova explosions and binary neutron star mergers. Finally, I briefly report about the possible important role played by hyperons in the transport properties of neutron star matter and on the consequences of neutron star cooling and gravitational wave instabilities induced by the presence of hyperons. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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39 pages, 11834 KiB

Open AccessEditor’s ChoiceReview

Progress in Constraining Nuclear Symmetry Energy Using Neutron Star Observables Since GW170817

by Bao-An Li, Bao-Jun Cai, Wen-Jie Xie and Nai-Bo Zhang

Universe 2021, 7(6), 182; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7060182 - 04 Jun 2021

Cited by 114 | Viewed by 5832

Abstract

The density dependence of nuclear symmetry energy is among the most uncertain parts of the Equation of State (EOS) of dense neutron-rich nuclear matter. It is currently poorly known especially at suprasaturation densities partially because of our poor knowledge about isovector nuclear interactions at short distances. Because of its broad impacts on many interesting issues, pinning down the density dependence of nuclear symmetry energy has been a longstanding and shared goal of both astrophysics and nuclear physics. New observational data of neutron stars including their masses, radii, and tidal deformations since GW170817 have helped improve our knowledge about nuclear symmetry energy, especially at high densities. Based on various model analyses of these new data by many people in the nuclear astrophysics community, while our brief review might be incomplete and biased unintentionally, we learned in particular the following: (1) The slope parameter L of nuclear symmetry energy at saturation density

ρ_{0}

of nuclear matter from 24 new analyses of neutron star observables was about

L \approx 57.7 \pm 19

MeV at a 68% confidence level, consistent with its fiducial value from surveys of over 50 earlier analyses of both terrestrial and astrophysical data within error bars. (2) The curvature

K_{sym}

of nuclear symmetry energy at

ρ_{0}

from 16 new analyses of neutron star observables was about

K_{sym} \approx - 107 \pm 88

MeV at a 68% confidence level, in very good agreement with the systematics of earlier analyses. (3) The magnitude of nuclear symmetry energy at

2 ρ_{0}

, i.e.,

E_{sym} (2 ρ_{0}) \approx 51 \pm 13

MeV at a 68% confidence level, was extracted from nine new analyses of neutron star observables, consistent with the results from earlier analyses of heavy-ion reactions and the latest predictions of the state-of-the-art nuclear many-body theories. (4) While the available data from canonical neutron stars did not provide tight constraints on nuclear symmetry energy at densities above about

2 ρ_{0}

, the lower radius boundary

R_{2.01} = 12.2

km from NICER’s very recent observation of PSR J0740+6620 of mass

2.08 \pm 0.07

M_{⊙}

and radius

R = 12.2 - 16.3

km at a 68% confidence level set a tight lower limit for nuclear symmetry energy at densities above

2 ρ_{0}

. (5) Bayesian inferences of nuclear symmetry energy using models encapsulating a first-order hadron–quark phase transition from observables of canonical neutron stars indicated that the phase transition shifted appreciably both L and

K_{sym}

to higher values, but with larger uncertainties compared to analyses assuming no such phase transition. (6) The high-density behavior of nuclear symmetry energy significantly affected the minimum frequency necessary to rotationally support GW190814’s secondary component of mass (2.50–2.67)

M_{⊙}

as the fastest and most massive pulsar discovered so far. Overall, thanks to the hard work of many people in the astrophysics and nuclear physics community, new data of neutron star observations since the discovery of GW170817 have significantly enriched our knowledge about the symmetry energy of dense neutron-rich nuclear matter. Full article

(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)

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