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Universe
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Neutron Star Astrophysics
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Neutron Star Astrophysics

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (31 October 2020) | Viewed by 19019

Special Issue Editor

Prof. Dr. Jose A. Pons

E-Mail Website
Guest Editor
University of Alicante, Alicante, Spain
Interests: Neutron stars; relativistic hydrodynamics and magnetohydrodynamics (MHD); nuclear equation of state; gravitational waves

Special Issue Information

Dear Colleagues,

Neutron stars are ideal laboratories to study matter under extreme physical conditions of density, temperature, gravity, or magnetic field, being the only environment in the Universe where such extremes are realized simultaneously. In the last few decades, the improved quality and large amount of observational data collected have stimulated the interest of different communities (nuclear physics, particle physics, relativity physics, condensed matter physics) looking at neutron stars as laboratories to probe their new theories at the frontiers of physics.

The direct detection of gravitational waves (GWs) from a binary neutron star merger, in coincidence with a variety of observations at different wavelengths of the electromagnetic spectrum, has officially opened the new era of multimessenger astronomy. Research in the near and midterm future is equally promising, with more neutron star binary merger events to come, including, perhaps, the long-expected galactic Supernova, which will also be observed with neutrino detectors. Given the current exciting developments, we think that it is time to publish a Special Issue on the topic of neutron star astrophysics.

The purpose of this Special Issue is to collect contributions on current theoretical research on the physical and astrophysical processes related to any of the neutron star manifestations (magnetars, radio-pulsars, X-ray pulsars, proto-neutron stars, exotic compact stars), as well as new (or soon-to-come) observational results (NICER, SKA, and other future missions).  We aim to provide the reader with an updated overview of the recent advances in the field from the theoretical and observational points of view.  We wish to invite both original and review papers to this Special Issue along any of the lines related to this fascinating area of research.

Prof. Dr. Jose A. Pons
Guest Editor

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

  • Neutron stars
  • pulsars
  • magnetars
  • gravitational waves
  • equation of state of dense matter
  • transport properties in dense matter
  • binary systems with neutron stars

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Published Papers (5 papers)

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Research

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16 pages, 1038 KiB  
Article
Constraint on Hybrid Stars with Gravitational Wave Events
by Kilar Zhang and Feng-Li Lin
Universe 2020, 6(12), 231; https://doi.org/10.3390/universe6120231 - 4 Dec 2020
Cited by 10 | Viewed by 1961
Abstract
Motivated by the recent discoveries of compact objects from LIGO/Virgo observations, we study the possibility of identifying some of these objects as compact stars made of dark matter called dark stars, or the mix of dark and nuclear matters called hybrid stars. In [...] Read more.
Motivated by the recent discoveries of compact objects from LIGO/Virgo observations, we study the possibility of identifying some of these objects as compact stars made of dark matter called dark stars, or the mix of dark and nuclear matters called hybrid stars. In particular, in GW190814, a new compact object with 2.6 M⊙ is reported. This could be the lightest black hole, the heaviest neutron star, and a dark or hybrid star. In this work, we extend the discussion on the interpretations of the recent LIGO/Virgo events as hybrid stars made of various self-interacting dark matter (SIDM) in the isotropic limit. We pay particular attention to the saddle instability of the hybrid stars which will constrain the possible SIDM models. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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15 pages, 540 KiB  
Article
Neutron Stars and Dark Matter
by Antonino Del Popolo, Morgan Le Delliou and Maksym Deliyergiyev
Universe 2020, 6(12), 222; https://doi.org/10.3390/universe6120222 - 26 Nov 2020
Cited by 14 | Viewed by 3396
Abstract
Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic [...] Read more.
Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic center, the density increase in dark matter results in increased accretion. At distances of some fraction of a parsec, the neutron star acquire enough dark matter to have its structure changed. We show that the neutron star mass decreases going towards the galactic centre, and that dark matter accumulation beyond a critical value collapses the neutron star into a black hole. Calculations cover cases varying the dark matter particle mass, self-interaction strength, and ratio between the pressure of dark matter and ordinary matter. This allow us to constrain the interaction cross section, σdm, between nucleons and dark matter particles, as well as the dark matter self-interaction cross section. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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12 pages, 576 KiB  
Article
Neutron Star Properties: Quantifying the Effect of the Crust–Core Matching Procedure
by Márcio Ferreira and Constança Providência
Universe 2020, 6(11), 220; https://doi.org/10.3390/universe6110220 - 23 Nov 2020
Cited by 9 | Viewed by 2111
Abstract
The impact of the equation of state (EoS) crust-core matching procedure on neutron star (NS) properties is analyzed within a meta-modeling approach. Using a Taylor expansion to parametrize the core equation of state (EoS) and the SLy4 crust EoS, we create two distinct [...] Read more.
The impact of the equation of state (EoS) crust-core matching procedure on neutron star (NS) properties is analyzed within a meta-modeling approach. Using a Taylor expansion to parametrize the core equation of state (EoS) and the SLy4 crust EoS, we create two distinct EoS datasets employing two matching procedures. Each EoS describes cold NS matter in a β equilibrium that is thermodynamically stable and causal. It is shown that the crust-core matching procedure affects not only the crust-core transition but also the nuclear matter parameter space of the core EoS, and thus the most probable nuclear matter properties. An uncertainty of as much as 5% (8%) on the determination of low mass NS radii (tidal deformability) is attributed to the complete matching procedure, including the effect on core EoS. By restricting the analysis, imposing that the same set of core EoS is retained in both matching procedures, the uncertainty on the NS radius drops to 3.5% and below 1.5% for 1.9M⊙. Moreover, under these conditions, the crust-core matching procedure has a strong impact on the Love number k2, of almost 20% for 1.0M⊙ stars and 7% for 1.9M⊙ stars, but it shows a very small impact on the tidal deformability Λ, below 1%. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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22 pages, 550 KiB  
Article
Estimation of Electrical Conductivity and Magnetization Parameter of Neutron Star Crusts and Applied to the High-Braking-Index Pulsar PSR J1640-4631
by Hui Wang, Zhi-Fu Gao, Huan-Yu Jia, Na Wang and Xiang-Dong Li
Universe 2020, 6(5), 63; https://doi.org/10.3390/universe6050063 - 1 May 2020
Cited by 24 | Viewed by 3309
Abstract
Young pulsars are thought to be highly magnetized neutron stars (NSs). The crustal magnetic field of a NS usually decays at different timescales in the forms of Hall drift and Ohmic dissipation. The magnetization parameter ω B τ is defined as the ratio [...] Read more.
Young pulsars are thought to be highly magnetized neutron stars (NSs). The crustal magnetic field of a NS usually decays at different timescales in the forms of Hall drift and Ohmic dissipation. The magnetization parameter ω B τ is defined as the ratio of the Ohmic timescale τ O h m to the Hall drift timescale τ H a l l . During the first several million years, the inner temperature of the newly born neutron star cools from T = 10 9 K to T = 1.0 × 10 8 K, and the crustal conductivity increases by three orders of magnitude. In this work, we adopt a unified equations of state for cold non-accreting neutron stars with the Hartree–Fock–Bogoliubov method, developed by Pearson et al. (2018), and choose two fiducial dipole magnetic fields of B = 1.0 × 10 13 G and B = 1.0 × 10 14 G, four different temperatures, T, and two different impurity concentration parameters, Q, and then calculate the conductivity of the inner crust of NSs and give a general expression of magnetization parameter for young pulsars: ω B τ ≃ ( 1 − 50 ) B 0 / ( 10 13 G) by using numerical simulations. It was found when B ≤ 10 15 G, due to the quantum effects, the conductivity increases slightly with the increase in the magnetic field, the enhanced magnetic field has a small effect on the matter in the low-density regions of the crust, and almost has no influence the matter in the high-density regions. Then, we apply the general expression of the magnetization parameter to the high braking-index pulsar PSR J1640-4631. By combining the observed arrival time parameters of PSR J1640-4631 with the magnetic induction equation, we estimated the initial rotation period P 0 , the initial dipole magnetic field B 0 , the Ohm dissipation timescale τ O h m and Hall drift timescale τ H a l l . We model the magnetic field evolution and the braking-index evolution of the pulsar and compare the results with its observations. It is expected that the results of this paper can be applied to more young pulsars. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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Review

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36 pages, 1380 KiB  
Review
Continuous Gravitational Waves from Neutron Stars: Current Status and Prospects
by Magdalena Sieniawska and Michał Bejger
Universe 2019, 5(11), 217; https://doi.org/10.3390/universe5110217 - 31 Oct 2019
Cited by 85 | Viewed by 6719
Abstract
Gravitational waves astronomy allows us to study objects and events invisible in electromagnetic waves. It is crucial to validate the theories and models of the most mysterious and extreme matter in the Universe: the neutron stars. In addition to inspirals and mergers of [...] Read more.
Gravitational waves astronomy allows us to study objects and events invisible in electromagnetic waves. It is crucial to validate the theories and models of the most mysterious and extreme matter in the Universe: the neutron stars. In addition to inspirals and mergers of neutrons stars, there are currently a few proposed mechanisms that can trigger radiation of long-lasting gravitational radiation from neutron stars, such as e.g., elastically and/or magnetically driven deformations: mountains on the stellar surface supported by the elastic strain or magnetic field, free precession, or unstable oscillation modes (e.g., the r-modes). The astrophysical motivation for continuous gravitational waves searches, current LIGO and Virgo strategies of data analysis and prospects are reviewed in this work. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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