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Open AccessArticle

Bayesian Exploration of Phenomenological EoS of Neutron/Hybrid Stars with Recent Observations

by

,

,

and

Particles 2023, 6(1), 198-216; https://doi.org/10.3390/particles6010011 - 02 Feb 2023

Cited by 1 | Viewed by 1196

Abstract

The description of the stellar interior of compact stars remains as a big challenge for the nuclear astrophysics community. The consolidated knowledge is restricted to density regions around the saturation of hadronic matter [...] Read more.

The description of the stellar interior of compact stars remains as a big challenge for the nuclear astrophysics community. The consolidated knowledge is restricted to density regions around the saturation of hadronic matter

ρ_{0} = 2.8 \times 10^{14} g {cm}^{- 3}

, regimes where our nuclear models are successfully applied. As one moves towards higher densities and extreme conditions up to the quark/gluons deconfinement, little can be said about the microphysics of the equation of state (EoS). Here, we employ a Markov Chain Monte Carlo (MCMC) strategy to access the variability at high density regions of polytropic piecewise models for neutron star (NS) EoS or possible hybrid stars, i.e., a NS with a small quark-matter core. With a fixed description of the hadronic matter for low density, below the nuclear saturation density, we explore a variety of models for the high density regimes leading to stellar masses near to

2.5 M_{⊙}

, in accordance with the observations of massive pulsars. The models are constrained, including the observation of the merger of neutrons stars from VIRGO-LIGO and with the pulsar observed by NICER. In addition, we also discuss the possibility of the use of a Bayesian power regression model with heteroscedastic error. The set of EoS from the Laser Interferometer Gravitational-Wave Observatory (LIGO) was used as input and treated as the data set for the testing case. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessEditor’s ChoiceArticle

Constraints on Nuclear Symmetry Energy Parameters

by

James M. Lattimer

Particles 2023, 6(1), 30-56; https://doi.org/10.3390/particles6010003 - 04 Jan 2023

Cited by 13 | Viewed by 1714

Abstract

A review is made of constraints on the nuclear symmetry energy parameters arising from nuclear binding energy measurements, theoretical chiral effective field predictions of neutron matter properties, the unitary gas conjecture, and measurements of neutron skin thicknesses and dipole polarizabilities. While most studies [...] Read more.

A review is made of constraints on the nuclear symmetry energy parameters arising from nuclear binding energy measurements, theoretical chiral effective field predictions of neutron matter properties, the unitary gas conjecture, and measurements of neutron skin thicknesses and dipole polarizabilities. While most studies have been confined to the parameters

S_{V}

and L, the important roles played by, and constraints on

K_{sym}

, or, equivalently, the neutron matter incompressibility

K_{N}

, are discussed. Strong correlations among

S_{V}, L

, and

K_{N}

are found from both nuclear binding energies and neutron matter theory. However, these correlations somewhat differ in the two cases, and those from neutron matter theory have smaller uncertainties. To 68% confidence, it is found from neutron matter theory that

S_{V} = 32.0 \pm 1.1

MeV,

L = 51.9 \pm 7.9

MeV and

K_{N} = 152.2 \pm 38.1

MeV. Theoretical predictions for neutron skin thickness and dipole polarizability measurements of the neutron-rich nuclei

^{48}

Ca,

^{120}

Sn, and

^{208}

Pb are compared to recent experimental measurements, most notably the CREX and PREX neutron skin experiments from Jefferson Laboratory. By themselves, PREX I+II measurements of

^{208}

Pb and CREX measurement of

^{48}

Ca suggest

L = 121 \pm 47

MeV and

L = - 5 \pm 40

MeV, respectively, to 68% confidence. However, we show that nuclear interactions optimally satisfying both measurements imply

L = 53 \pm 13

MeV, nearly the range suggested by either nuclear mass measurements or neutron matter theory, and is also consistent with nuclear dipole polarizability measurements. This small parameter range implies

R_{1.4} = 11.6 \pm 1.0

km and

Λ_{1.4} = 228_{- 90}^{+ 148}

, which are consistent with NICER X-ray and LIGO/Virgo gravitational wave observations of neutron stars. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessArticle

Recovering the Conformal Limit of Color Superconducting Quark Matter within a Confining Density Functional Approach

by

Oleksii Ivanytskyi

and

David B. Blaschke

Particles 2022, 5(4), 514-534; https://doi.org/10.3390/particles5040038 - 28 Nov 2022

Cited by 13 | Viewed by 1172

Abstract

We generalize a recently proposed confining relativistic density-functional approach to the case of density-dependent vector and diquark couplings. The particular behavior of these couplings is motivated by the non-perturbative gluon exchange in dense quark matter and provides the conformal limit at asymptotically high [...] Read more.

We generalize a recently proposed confining relativistic density-functional approach to the case of density-dependent vector and diquark couplings. The particular behavior of these couplings is motivated by the non-perturbative gluon exchange in dense quark matter and provides the conformal limit at asymptotically high densities. We demonstrate that this feature of the quark matter EoS is consistent with a significant stiffness in the density range typical for the interiors of neutron stars. In order to model these astrophysical objects, we construct a family of hybrid quark-hadron EoSs of cold stellar matter. We also confront our approach with the observational constraints on the mass–radius relation of neutron stars and their tidal deformabilities and argue in favor of a quark matter onset at masses below

1.0 M_{⊙}

. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessArticle

Bulk Viscosity of Relativistic npeμ Matter in Neutron-Star Mergers

by

Mark Alford

,

Arus Harutyunyan

and

Armen Sedrakian

Particles 2022, 5(3), 361-376; https://doi.org/10.3390/particles5030029 - 06 Sep 2022

Cited by 9 | Viewed by 1081

Abstract

We discuss the bulk viscosity of hot and dense

n p e μ

matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with

T \leq T_{tr}

(

T_{tr} ≃ 5 \div 10

MeV [...] Read more.

We discuss the bulk viscosity of hot and dense

n p e μ

matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with

T \leq T_{tr}

(

T_{tr} ≃ 5 \div 10

MeV is the neutrino-trapping temperature); and (b) the neutrino-trapped regime with

T \geq T_{tr}

. Nuclear matter is modeled in relativistic density functional approach with density-dependent parametrization DDME2. The maximum of the bulk viscosity is achieved at temperatures

T ≃ 5 \div 6

MeV in the neutrino-transparent regime, then it drops rapidly at higher temperatures where neutrino-trapping occurs. As an astrophysical application, we estimate the damping timescales of density oscillations by the bulk viscosity in neutron star mergers and find that, e.g., at the oscillation frequency

f = 10

kHz, the damping will be very efficient at temperatures

4 \leq T \leq 7

MeV where the bulk viscosity might affect the evolution of the post-merger object. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessArticle

Quick Guides for Use of the CompOSE Data Base

by

Veronica Dexheimer

,

Marco Mancini

,

Micaela Oertel

,

Constança Providência

,

Laura Tolos

and

Stefan Typel

Particles 2022, 5(3), 346-360; https://doi.org/10.3390/particles5030028 - 05 Sep 2022

Cited by 2 | Viewed by 1206

Abstract

We present a combination of two quick guides aimed at summarizing relevant information about the CompOSE nuclear equation of state repository. The first is aimed at nuclear physicists and describes how to provide standard equation of state tables. The second quick guide is [...] Read more.

We present a combination of two quick guides aimed at summarizing relevant information about the CompOSE nuclear equation of state repository. The first is aimed at nuclear physicists and describes how to provide standard equation of state tables. The second quick guide is meant for users and describes the basic procedures to obtain customized tables with equation of state data. Several examples are included to help providers and users to understand and benefit from the CompOSE database. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessArticle

Stability of Spherical Nuclei in the Inner Crust of Neutron Stars

by

Nikita A. Zemlyakov

and

Andrey I. Chugunov

Particles 2022, 5(3), 225-234; https://doi.org/10.3390/particles5030020 - 01 Jul 2022

Cited by 3 | Viewed by 1462

Abstract

Neutron stars are the densest objects in the Universe. In this paper, we consider the so-called inner crust—the layer where neutron-excess nuclei are immersed in the degenerate gas of electrons and a sea of quasi-free neutrons. It was generally believed that spherical nuclei [...] Read more.

Neutron stars are the densest objects in the Universe. In this paper, we consider the so-called inner crust—the layer where neutron-excess nuclei are immersed in the degenerate gas of electrons and a sea of quasi-free neutrons. It was generally believed that spherical nuclei become unstable with respect to quadrupole deformations at high densities, and here, we consider this instability. Within the perturbative approach, we show that spherical nuclei with equilibrium number density are, in fact, stable with respect to infinitesimal quadrupole deformation. This is due to the background of degenerate electrons and associated electrostatic potential, which maintain stability of spherical nuclei. However, if the number of atomic nuclei per unit volume is much less than the equilibrium value, instability can arise. To avoid confusion, we stress that our results are limited to infinitesimal deformations and do not guarantee strict thermodynamic stability of spherical nuclei. In particular, they do not exclude that substantially non-spherical nuclei (so-called pasta phase) represent a thermodynamic equilibrium state of the densest layers of the neutron star crust. Rather, our results point out that spherical nuclei can be metastable even if they are not energetically favourable, and the timescale of transformation of spherical nuclei to the pasta phases should be estimated subsequently. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessArticle

Stripping Model for Short Gamma-Ray Bursts in Neutron Star Mergers

by

,

,

and

Particles 2022, 5(2), 198-209; https://doi.org/10.3390/particles5020018 - 16 Jun 2022

Cited by 5 | Viewed by 1517

Abstract

We overview the current status of the stripping model for short gamma-ray bursts. After the historical joint detection of the gravitational wave event GW170817 and the accompanying gamma-ray burst GRB170817A, the relation between short gamma-ray bursts and neutron star mergers has been reliably [...] Read more.

We overview the current status of the stripping model for short gamma-ray bursts. After the historical joint detection of the gravitational wave event GW170817 and the accompanying gamma-ray burst GRB170817A, the relation between short gamma-ray bursts and neutron star mergers has been reliably confirmed. Many properties of GRB170817A, which turned out to be peculiar in comparison with other short gamma-ray bursts, are naturally explained in the stripping model, suggested by one of us in 1984. We point out the role of late Dmitriy Nadyozhin (1937–2020) in predicting the GRB and kilonova properties in 1990. We also review the problems to be solved in the context of this model. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Review

Jump to: Research

Open AccessReview

Future of Neutron Star Studies with Fast Radio Bursts

by

Sergei B. Popov

and

Maxim S. Pshirkov

Particles 2023, 6(1), 451-469; https://doi.org/10.3390/particles6010025 - 21 Mar 2023

Cited by 1 | Viewed by 1318

Abstract

Fast radio bursts (FRBs) were discovered only in 2007. However, the number of known events and sources of repeating bursts grows very rapidly. In the near future, the number of events will be ≳10⁴ and the number of repeaters ≳100. Presently, there [...] Read more.

Fast radio bursts (FRBs) were discovered only in 2007. However, the number of known events and sources of repeating bursts grows very rapidly. In the near future, the number of events will be ≳10⁴ and the number of repeaters ≳100. Presently, there is a consensus that most of the sources of FRBs might be neutron stars (NSs) with large magnetic fields. These objects might have different origin as suggested by studies of their host galaxies which represent a very diverse sample: from regions of very active star formation to old globular clusters. Thus, in the following decade we expect to have a very large sample of events directly related to extragalactic magnetars of different origin. This might open new possibilities to probe various aspects of NS physics. In the review we briefly discuss the main directions of such future studies and summarize our present knowledge about FRBs and their sources. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

Open AccessReview

Formation, Possible Detection and Consequences of Highly Magnetized Compact Stars

by

Banibrata Mukhopadhyay

and

Mukul Bhattacharya

Particles 2022, 5(4), 493-513; https://doi.org/10.3390/particles5040037 - 17 Nov 2022

Cited by 3 | Viewed by 1106

Abstract

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as [...] Read more.

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as high as ∼2.5–2.67

M_{⊙}

within the so-called mass gap, indicating the existence of highly massive neutron stars. One of the primary goals to invoke massive compact objects was to explain the recent detections of over a dozen Type Ia supernovae, whose peculiarity lies with their unusual light curve, in particular the high luminosity and low ejecta velocity. In a series of recent papers, our group has proposed that highly magnetised white dwarfs with super-Chandrasekhar masses can be promising candidates for the progenitors of these peculiar supernovae. The mass-radius relations of these magnetised stars are significantly different from those of their non-magnetised counterparts, which leads to a revised super-Chandrasekhar mass-limit. These compact stars have wider ranging implications, including those for soft gamma-ray repeaters, anomalous X-ray pulsars, white dwarf pulsars and gravitational radiation. Here we review the development of the subject over the last decade or so, describing the overall state of the art of the subject as it stands now. We mainly touch upon the possible formation channels of these intriguing stars as well as the effectiveness of direct detection methods. These magnetised stars can have many interesting consequences, including reconsideration of them as possible standard candles. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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Open AccessFeature PaperReview

Energy-Momentum Complex in Higher Order Curvature-Based Local Gravity

by

Salvatore Capozziello

,

Maurizio Capriolo

and

Gaetano Lambiase

Particles 2022, 5(3), 298-330; https://doi.org/10.3390/particles5030026 - 10 Aug 2022

Cited by 2 | Viewed by 1318

Abstract

An unambiguous definition of gravitational energy remains one of the unresolved issues of physics today. This problem is related to the non-localization of gravitational energy density. In General Relativity, there have been many proposals for defining the gravitational energy density, notably those proposed [...] Read more.

An unambiguous definition of gravitational energy remains one of the unresolved issues of physics today. This problem is related to the non-localization of gravitational energy density. In General Relativity, there have been many proposals for defining the gravitational energy density, notably those proposed by Einstein, Tolman, Landau and Lifshitz, Papapetrou, Møller, and Weinberg. In this review, we firstly explored the energy–momentum complex in an

n^{t h}

order gravitational Lagrangian

L = L (g_{μ ν}, g_{μ ν, i_{1}}, g_{μ ν, i_{1} i_{2}}, g_{μ ν, i_{1} i_{2} i_{3}}, \dots, g_{μ ν, i_{1} i_{2} i_{3} \dots i_{n}})

and then in a gravitational Lagrangian as

L_{g} = (\bar{R} + a_{0} R^{2} + \sum_{k = 1}^{p} a_{k} R □^{k} R) \sqrt{- g}

. Its gravitational part was obtained by invariance of gravitational action under infinitesimal rigid translations using Noether’s theorem. We also showed that this tensor, in general, is not a covariant object but only an affine object, that is, a pseudo-tensor. Therefore, the pseudo-tensor

τ_{α}^{η}

becomes the one introduced by Einstein if we limit ourselves to General Relativity and its extended corrections have been explicitly indicated. The same method was used to derive the energy–momentum complex in

f (R)

gravity both in Palatini and metric approaches. Moreover, in the weak field approximation the pseudo-tensor

τ_{α}^{η}

to lowest order in the metric perturbation h was calculated. As a practical application, the power per unit solid angle

Ω

emitted by a localized source carried by a gravitational wave in a direction

\hat{x}

for a fixed wave number

k

under a suitable gauge was obtained, through the average value of the pseudo-tensor over a suitable spacetime domain and the local conservation of the pseudo-tensor. As a cosmological application, in a flat Friedmann–Lemaître–Robertson–Walker spacetime, the gravitational and matter energy density in

f (R)

gravity both in Palatini and metric formalism was proposed. The gravitational energy–momentum pseudo-tensor could be a useful tool to investigate further modes of gravitational radiation beyond two standard modes required by General Relativity and to deal with non-local theories of gravity involving

□^{- k}

terms. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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