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15 pages, 3604 KiB

Open AccessArticle

Off-Axis Color Characteristics of Binary Neutron Star Merger Events: Applications for Space Multi-Band Variable Object Monitor and James Webb Space Telescope

by Hongyu Gong, Daming Wei and Zhiping Jin

Universe 2024, 10(10), 403; https://doi.org/10.3390/universe10100403 - 19 Oct 2024

Viewed by 574

Abstract

With advancements in gravitational wave detection technology, an increasing number of binary neutron star (BNS) merger events are expected to be detected. Due to the narrow opening angle of jet cores, many BNS merger events occur off-axis, resulting in numerous gamma-ray bursts (GRBs) [...] Read more.

With advancements in gravitational wave detection technology, an increasing number of binary neutron star (BNS) merger events are expected to be detected. Due to the narrow opening angle of jet cores, many BNS merger events occur off-axis, resulting in numerous gamma-ray bursts (GRBs) going undetected. Models suggest that kilonovae, which can be observed off-axis, offer more opportunities to be detected in the optical/near-infrared band as electromagnetic counterparts of BNS merger events. In this study, we calculate kilonova emission using a three-dimensional semi-analytical code and model the GRB afterglow emission with the open-source Python package afterglowpy at various inclination angles. Our results show that it is possible to identify the kilonova signal from the observed color evolution of BNS merger events. We also deduce the optimal observing window for SVOM/VT and JWST/NIRCam, which depends on the viewing angle, jet opening angle, and circumburst density. These parameters can be cross-checked with the multi-band afterglow fitting. We suggest that kilonovae are more likely to be identified at larger inclination angles, which can also help determine whether the observed signals without accompanying GRBs originate from BNS mergers. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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16 pages, 2282 KiB

Open AccessArticle

Hybrid Isentropic Twin Stars

by Juan Pablo Carlomagno, Gustavo A. Contrera, Ana Gabriela Grunfeld and David Blaschke

Universe 2024, 10(9), 336; https://doi.org/10.3390/universe10090336 - 23 Aug 2024

Cited by 3 | Viewed by 526

Abstract

We present a study of hybrid neutron stars with color superconducting quark matter cores at a finite temperature that results in sequences of stars with constant entropy per baryon,

s / n_{B} = const

. For the quark matter equation of state, [...] Read more.

We present a study of hybrid neutron stars with color superconducting quark matter cores at a finite temperature that results in sequences of stars with constant entropy per baryon,

s / n_{B} = const

. For the quark matter equation of state, we employ a recently developed nonlocal chiral quark model, while nuclear matter is described with a relativistic density functional model of the DD2 class. The phase transition is obtained through a Maxwell construction under isothermal conditions. We find that traversing the mixed phase on a trajectory at low

s / n_{B} ≲ 2

in the phase diagram shows a heating effect, while at larger

s / n_{B}

the temperature drops. This behavior may be attributed to the presence of a color superconducting quark matter phase at low temperatures and the melting of the diquark condensate which restores the normal quark matter phase at higher temperatures. While the isentropic hybrid star branch at low

s / n_{B} ≲ 2

is connected to the neutron star branch, it becomes disconnected at higher entropy per baryon so that the “thermal twin” phenomenon is observed. We find that the transition from connected to disconnected hybrid star sequences may be estimated with the Seidov criterion for the difference in energy densities. The radii and masses at the onset of deconfinement exhibit a linear relationship and thus define a critical compactness of the isentropic star configuration for which the transition occurs and which, for large enough

s / n_{B} ≳ 2

values, is accompanied by instability. The results of this study may be of relevance for uncovering the conditions for the supernova explodability of massive blue supergiant stars using the quark deconfinement mechanism. The accretion-induced deconfinement transition with thermal twin formation may contribute to explaining the origin of eccentric orbits in some binary systems and the origin of isolated millisecond pulsars. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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10 pages, 267 KiB

Open AccessArticle

Estimate for the Neutrino Magnetic Moment from Pulsar Kick Velocities Induced at the Birth of Strange Quark Matter Neutron Stars

by Alejandro Ayala, Santiago Bernal-Langarica and Daryel Manreza-Paret

Universe 2024, 10(7), 301; https://doi.org/10.3390/universe10070301 - 20 Jul 2024

Viewed by 844

Abstract

We estimate the magnetic moment of electron neutrinos by computing the neutrino chirality flip rate that can occur in the core of a strange quark matter neutron star at birth. We show that this process allows neutrinos to anisotropically escape, thus inducing the [...] Read more.

We estimate the magnetic moment of electron neutrinos by computing the neutrino chirality flip rate that can occur in the core of a strange quark matter neutron star at birth. We show that this process allows neutrinos to anisotropically escape, thus inducing the star kick velocity. Although the flip from left- to right-handed neutrinos is assumed to happen in equilibrium, the no-go theorem does not apply because right-handed neutrinos do not interact with matter and the reverse process does not happen, producing the loss of detailed balance. For simplicity, we model the star core as consisting of strange quark matter. We find that even when the energy released in right-handed neutrinos is a small fraction of the total energy released in left-handed neutrinos, the process describes kick velocities for natal conditions, which are consistent with the observed ones and span the correct range of radii, temperatures and chemical potentials for typical magnetic field intensities. The neutrino magnetic moment is estimated to be

μ_{ν} \sim 3.6 \times 10^{- 18} μ_{B}

, where

μ_{B}

is the Bohr magneton. This value is more stringent than the bound found for massive neutrinos in a minimal extension of the standard model. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

30 pages, 2160 KiB

Open AccessArticle

Isospin QCD as a Laboratory for Dense QCD

by Toru Kojo, Daiki Suenaga and Ryuji Chiba

Universe 2024, 10(7), 293; https://doi.org/10.3390/universe10070293 - 12 Jul 2024

Cited by 4 | Viewed by 858

Abstract

QCD with the isospin chemical potential

μ_{I}

is a useful laboratory to delineate the microphysics in dense QCD. To study the quark–hadron continuity, we use a quark–meson model that interpolates hadronic and quark matter physics at microscopic level. The equation of state [...] Read more.

QCD with the isospin chemical potential

μ_{I}

is a useful laboratory to delineate the microphysics in dense QCD. To study the quark–hadron continuity, we use a quark–meson model that interpolates hadronic and quark matter physics at microscopic level. The equation of state is dominated by mesons at low density but taken over by quarks at high density. We extend our previous studies with two flavors to the three-flavor case to study the impact of the strangeness, which may be brought by kaons

(K_{+}, K_{0}) = (u \bar{s}, s \bar{d})

and the U_A(1) anomaly. In the normal phase, the excitation energies of kaons are reduced by

μ_{I}

in the same way as hyperons in nuclear matter at the finite baryon chemical potential. Once pions condense, kaon excitation energies increase as

μ_{I}

does. Moreover, strange quarks become more massive through the U_A(1) coupling to the condensed pions. Hence, at zero and low temperature, the strange hadrons and quarks are highly suppressed. The previous findings in two-flavor models, sound speed peak, negative trace anomaly, gaps insensitive to

μ_{I}

, persist in our three-flavor model and remain consistent with the lattice results to

μ_{I} \sim

1 GeV. We discuss the non-perturbative power corrections and quark saturation effects as important ingredients to understand the crossover equations of state measured on the lattice. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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17 pages, 3765 KiB

Open AccessArticle

Strange Quark Stars: The Role of Excluded Volume Effects

by G. Lugones and Ana G. Grunfeld

Universe 2024, 10(6), 233; https://doi.org/10.3390/universe10060233 - 24 May 2024

Cited by 2 | Viewed by 746

Abstract

We study cold strange quark stars employing an enhanced version of the quark-mass density-dependent model, which incorporates excluded volume effects to address non-perturbative QCD repulsive interactions. We provide a comparative analysis of our mass formula parametrization with previous models from the literature. We [...] Read more.

We study cold strange quark stars employing an enhanced version of the quark-mass density-dependent model, which incorporates excluded volume effects to address non-perturbative QCD repulsive interactions. We provide a comparative analysis of our mass formula parametrization with previous models from the literature. We identify the regions within the parameter space where three-flavor quark matter is more stable than the most tightly bound atomic nucleus (stability window). Specifically, we show that excluded volume effects do not change the Gibbs free energy per baryon at zero pressure, rendering the stability window unaffected. The curves of pressure versus energy density exhibit various shapes—convex upward, concave downward, or nearly linear—depending on the mass parametrization. This behavior results in different patterns of increase, decrease, or constancy in the speed of sound as a function of baryon number density. We analyze the mass–radius relationship of strange quark stars, revealing a significant increase in maximum gravitational mass and a shift in the curves toward larger radii as the excluded volume effect intensifies. Excluded volume effects render our models compatible with all modern astrophysical constraints, including the properties of the recently observed low-mass compact object HESSJ1731. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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20 pages, 591 KiB

Open AccessArticle

Nuclear Matter Equation of State in the Brueckner–Hartree–Fock Approach and Standard Skyrme Energy Density Functionals

by Isaac Vidaña, Jérôme Margueron and Hans-Josef Schulze

Universe 2024, 10(5), 226; https://doi.org/10.3390/universe10050226 - 17 May 2024

Cited by 1 | Viewed by 928

Abstract

The equation of state of asymmetric nuclear matter as well as the neutron and proton effective masses and their partial-wave and spin–isospin decomposition are analyzed within the Brueckner–Hartree–Fock approach. Theoretical uncertainties for all these quantities are estimated by using several phase-shift-equivalent nucleon–nucleon forces [...] Read more.

The equation of state of asymmetric nuclear matter as well as the neutron and proton effective masses and their partial-wave and spin–isospin decomposition are analyzed within the Brueckner–Hartree–Fock approach. Theoretical uncertainties for all these quantities are estimated by using several phase-shift-equivalent nucleon–nucleon forces together with two types of three-nucleon forces, phenomenological and microscopic. It is shown that the choice of the three-nucleon force plays an important role above saturation density, leading to different density dependencies of the energy per particle. These results are compared to the standard form of the Skyrme energy density functional, and we find that it is not possible to reproduce the BHF predictions in the

(S, T)

channels in symmetric and neutron matter above saturation density, already at the level of the two-body interaction, and even more including the three-body interaction. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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14 pages, 1170 KiB

Open AccessArticle

Compact Stars in the vBag Model and Its f-Mode Oscillations

by Heng-Yi Zhou, Wei Wei and Xia Zhou

Universe 2023, 9(6), 285; https://doi.org/10.3390/universe9060285 - 10 Jun 2023

Viewed by 1521

Abstract

Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced [...] Read more.

Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced bag model (vBag model) is employed to investigate strange and hybrid stars’ properties. The parameters of the vBag model are constrained using multi-messenger observations, revealing that strange stars are incompatible with current observations. In contrast, hybrid stars can exhibit a substantial mixed phase region and a thin hadronic shell. Furthermore, we present the frequencies and damping time of fundamental mode (f-mode) oscillations of hybrid stars and test their universal relations with compactness and tidal deformability. The findings indicate that the presence of mixed phase components leads to larger frequencies and shorter damping time of the f-mode oscillation of hybrid stars, and the softer equation of state (EoS) affects this behavior more significantly. The universal relations of hybrid stars in the vBag model can be described by fourth-order/seventh-order polynomials, which do not break the previous results. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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Review

Jump to: Research

17 pages, 998 KiB

Open AccessReview

Strange Dwarfs: A Review on the (in)Stability

by Francesco Di Clemente, Alessandro Drago and Giuseppe Pagliara

Universe 2024, 10(8), 322; https://doi.org/10.3390/universe10080322 - 9 Aug 2024

Cited by 1 | Viewed by 750

Abstract

White dwarfs are the remnants of stars not massive enough to become supernovae. This review explores the concept of strange dwarfs, a unique class of white dwarfs that contain cores of strange quark matter. Strange dwarfs have different sizes, masses, and evolutionary paths [...] Read more.

White dwarfs are the remnants of stars not massive enough to become supernovae. This review explores the concept of strange dwarfs, a unique class of white dwarfs that contain cores of strange quark matter. Strange dwarfs have different sizes, masses, and evolutionary paths with respect to white dwarfs. They might form through the accumulation of normal matter on strange quark stars or by the capture of strangelets. The stability of strange dwarfs has been debated, with initial studies suggesting stability, while later analyses indicated potential instability. This review revisits these discussions, focusing on the critical role of boundary conditions between nuclear and quark matter in determining stability. It also offers insights into their formation, structure, and possible detection in the universe. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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25 pages, 2208 KiB

Open AccessReview

Measuring the Lense–Thirring Orbital Precession and the Neutron Star Moment of Inertia with Pulsars

by Huanchen Hu and Paulo C. C. Freire

Universe 2024, 10(4), 160; https://doi.org/10.3390/universe10040160 - 28 Mar 2024

Cited by 3 | Viewed by 1487

Abstract

Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on [...] Read more.

Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on the equation of state (EoS) of dense matter, accurately determining the radius or moment of inertia (MoI) of an NS remains a major challenge. This article presents a detailed review on measuring the Lense–Thirring (LT) precession effect in the orbit of binary pulsars, which would give access to the MoI of NSs and offer further constraints on the EoS. We discuss the suitability of certain classes of binary pulsars for measuring the LT precession from the perspective of binary star evolution and highlight five pulsars that exhibit properties promising to realise these goals in the near future. Finally, discoveries of compact binaries with shorter orbital periods hold the potential to greatly enhance measurements of the MoI of NSs. The MoI measurements of binary pulsars are pivotal to advancing our understanding of matter at supranuclear densities, as well as improving the precision of gravity tests, such as the orbital decay due to gravitational wave emission, and of tests of alternative gravity theories. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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