The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (8 January 2024) | Viewed by 7529

Special Issue Editors


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Guest Editor
INAF, Institute for Space Astrophysics, 20133 Milan, Italy
Interests: stellar astrophysics; compact objects; multi-wavelength observations
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Guest Editor
Department of Physics, University of Pisa, 56127 Pisa, Italy
Interests: gravitational waves; multimessenger astrophysics; pulsars; computing and machine learning

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Guest Editor
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

Special Issue Information

Dear Colleagues,

Neutron stars (NSs), stellar corpses born from the core collapse of massive stars (~10 Msun) which underwent supernova (SN) explosion, are probably the stars with the most unique properties in the Universe, letting black holes (BHs) aside, where physics laws can be tested at their extreme. Overall, NSs are endowed with the highest magnetic fields (up to 1013-14 G), with a mass of about 1.4 Msun compressed in a radius of about 12 km they have supra-nuclear densities of 1014 g cm-3, they have the highest surface temperatures, up to a few million degrees, and they have the highest spatial velocity, ~400 km s-1 on average, resulting from the natal kick imparted by the SN explosion. Therefore, NSs are ideal cosmic laboratories to perform a number of studies in fundamental physics unattainable on Earth, e.g., the radiation emission processes under extreme magnetic fields, the behavior of matter at supra-nuclear densities and its equation of state (EoS), the cooling of this hyper-dense matter and its composition, general relativity (GR) effects in extreme gravitational fields, gravitational wave (GW) emission in hyper-dense, fast-rotating (periods as low as few ms), objects, and the confinement of relativistic charged particles injected in the interstellar medium (ISM) from a source moving at supersonic velocities.

In the last 20 years, important progress has been made in the study of NSs. An energy range was discovered using radio over 50 years ago; most NSs today are still detected as radio pulsars (~3000). Observations made using space and ground-based telescopes have consolidated the view of NSs as multi-wavelength emitters, from the sub mm to very-high-energy gamma rays, with, e.g., about 300 of them identified as gamma-ray pulsars by the Fermi Gamma-ray Space Telescope. This harvest of data now paves the way to unprecedented studies of the emission from the NS magnetosphere, possibly yielding to a unified picture, and of the thermal emission from the NS surface, exploiting the synergy between X-ray and ultraviolet (UV) observations. At the same time, radio/optical observations of pulsars in binary systems led to very accurate measurements of the NS masses, breaking the paradigm of the assumed value of 1.4 Msun and indicating that NSs as massive as ~2 Msun indeed exist, suggesting that pulsars in binary systems which might have undergone an accretion phase and spin-up from the companion star, hence dubbed ms-pulsars, span a different mass range wrt. Isolated NSs. Wherever this is due to accretion alone or to a different evolutionary path of the pulsar progenitor, it has outstanding implications on the determination of the NS EoS.

The aim of this Special Issue is to set the state of the art of neutron star astrophysics, with particular emphasis on the progress accomplished in the last 20 years, and to discuss future challenges in this field. Review articles and research articles are equally welcome.

Selected topics include (but not only):

  • Emission from the NS magnetosphere;
  • NS cooling;
  • NS polarimetry and QED;
  • The NS variety and the NS census (isolated, binary, magnetars, etc.);
  • NS masses and radii;
  • NS EoS;
  • NS and general relativity;
  • Future challenges and goals;
  • Observations with future facilities.

Prof. Dr. Roberto Mignani
Dr. Massimiliano Razzano
Prof. Dr. Sergei B. Popov
Guest Editors

Manuscript Submission Information

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Keywords

  • emission from the NS magnetosphere
  • NS cooling
  • NS polarimetry and QED
  • the NS variety and the NS census (isolated, binary, magnetars, etc.)
  • NS masses and radii
  • NS EoS
  • NS and general relativity
  • future challenges and goals
  • observations with future facilities.

Published Papers (5 papers)

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Research

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15 pages, 782 KiB  
Article
Radial Oscillations in Neutron Stars from Unified Hadronic and Quarkyonic Equation of States
by Souhardya Sen, Shubham Kumar, Athul Kunjipurayil, Pinku Routaray, Sayantan Ghosh, Probit J. Kalita, Tianqi Zhao and Bharat Kumar
Galaxies 2023, 11(2), 60; https://doi.org/10.3390/galaxies11020060 - 19 Apr 2023
Cited by 10 | Viewed by 1754
Abstract
We study radial oscillations in non-rotating neutron stars by considering the unified equation of states (EoSs), which support the 2 M star criterion. We solve the Sturm–Liouville problem to compute the 20 lowest radial oscillation modes and their eigenfunctions for a neutron [...] Read more.
We study radial oscillations in non-rotating neutron stars by considering the unified equation of states (EoSs), which support the 2 M star criterion. We solve the Sturm–Liouville problem to compute the 20 lowest radial oscillation modes and their eigenfunctions for a neutron star modeled with eight selected unified EoSs from distinct Skyrme–Hartree–Fock, relativistic mean field and quarkyonic models. We compare the behavior of the computed eigenfrequency for an NS modeled with hadronic to one with quarkyonic EoSs while varying the central densities. The lowest-order f-mode frequency varies substantially between the two classes of the EoS at 1.4 M but vanishes at their respective maximum masses, consistent with the stability criterion M/ρc>0. Moreover, we also compute large frequency separation and discover that higher-order mode frequencies are significantly reduced by incorporating a crust in the EoS. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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Review

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43 pages, 2383 KiB  
Review
Spin Evolution of Neutron Stars
by Pavel Abolmasov, Anton Biryukov and Sergei B. Popov
Galaxies 2024, 12(1), 7; https://doi.org/10.3390/galaxies12010007 - 10 Feb 2024
Cited by 1 | Viewed by 1366
Abstract
In this paper we review the basics of magneto-rotational properties of neutron stars focusing on spin-up/spin-down behavior at different evolutionary stages. The main goal is to provide equations for the spin frequency changes in various regimes (radio pulsar, propeller, accretor, etc.). Since presently [...] Read more.
In this paper we review the basics of magneto-rotational properties of neutron stars focusing on spin-up/spin-down behavior at different evolutionary stages. The main goal is to provide equations for the spin frequency changes in various regimes (radio pulsar, propeller, accretor, etc.). Since presently the spin behavior of neutron stars at all stages remains a subject of many uncertainties, we review different suggestions made over the years in the literature. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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27 pages, 2182 KiB  
Review
X-ray Polarization from Magnetar Sources
by Roberto Taverna  and Roberto Turolla 
Galaxies 2024, 12(1), 6; https://doi.org/10.3390/galaxies12010006 - 10 Feb 2024
Cited by 1 | Viewed by 1198
Abstract
The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2–8keV band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more [...] Read more.
The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2–8keV band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more than 60 sources, including four magnetars, neutron stars with magnetic fields in the petaGauss range. In this paper we summarize the IXPE main findings and discuss their implications for the physics of ultra-magnetized neutron stars. Polarimetric observations confirmed theoretical predictions, according to which X-ray radiation from magnetar sources is highly polarized, up to ≈80%, the highest value detected so far. This provides an independent confirmation that magnetars are indeed endowed with a super-strong magnetic field and that the twisted magnetosphere scenario is the most likely explanation for their soft X-ray emission. Polarization measurements allowed us to probe the physical conditions of the star’s outermost layers, showing that the cooler surface regions are in a condensed state, with no atmosphere on top. Although no smoking-gun of vacuum QED effects was found, the phase-dependent behavior of the polarization angle strongly hints that vacuum birefringence is indeed at work in magnetar magnetospheres. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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11 pages, 326 KiB  
Review
Spin Equilibrium of Rapidly Spinning Neutron Stars via Transient Accretion
by Sudip Bhattacharyya
Galaxies 2023, 11(5), 103; https://doi.org/10.3390/galaxies11050103 - 1 Oct 2023
Viewed by 1150
Abstract
The concept of spin equilibrium due to an interaction between the stellar magnetosphere and a thin, Keplerian accretion disk, and a well-known formula of the corresponding equilibrium spin frequency, provide a key understanding of spin evolution and the distribution of rapidly spinning neutron [...] Read more.
The concept of spin equilibrium due to an interaction between the stellar magnetosphere and a thin, Keplerian accretion disk, and a well-known formula of the corresponding equilibrium spin frequency, provide a key understanding of spin evolution and the distribution of rapidly spinning neutron stars, viz., millisecond pulsars. However, this concept and formula are for stable accretion, but the mass transfer to most accreting millisecond pulsars is transient and the accretion rate evolves by orders of magnitude during an outburst. In this short and focussed review, we briefly discuss a relatively new concept of the spin equilibrium condition and a new formula for the equilibrium spin frequency for transiently accreting millisecond pulsars. We also review a new method to estimate this equilibrium spin frequency for observed transiently accreting millisecond pulsars, even when a pulsar has not yet attained the spin equilibrium. These will be crucial to probe the spin evolution and distribution of millisecond pulsars, and should also be applicable to all magnetic stars transiently accreting via a thin, Keplerian accretion disk. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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18 pages, 5609 KiB  
Review
Gamma Ray Pulsars and Opportunities for the MACE Telescope
by Atul Pathania, Krishna Kumar Singh and Kuldeep Kumar Yadav
Galaxies 2023, 11(4), 91; https://doi.org/10.3390/galaxies11040091 - 17 Aug 2023
Viewed by 1134
Abstract
Rapidly rotating neutron stars with very strong surface magnetic fields are observed to emit pulsed emission in the whole range of electromagnetic spectrum from radio to high-energy gamma rays. These so-called pulsars are known for their exceptional rotational stability. The radio emission from [...] Read more.
Rapidly rotating neutron stars with very strong surface magnetic fields are observed to emit pulsed emission in the whole range of electromagnetic spectrum from radio to high-energy gamma rays. These so-called pulsars are known for their exceptional rotational stability. The radio emission from pulsars is generally believed to be powered by the rotational energy of neutron stars. More than 3000 pulsars have been currently known from radio observations; however, only about 10% are observed in the high-energy gamma ray band. The Fermi-LAT observations in the energy range above 100 MeV have discovered more than 300 pulsars. However, the origin of high-energy non-thermal radiation from pulsars is not completely understood and remains an active area of research. In this contribution, we report a summary of observational features of the gamma ray pulsars and briefly discuss observability for the MACE gamma ray telescope, which has just started its regular science operation at Hanle in India. Six gamma ray pulsars, other than the well-known Crab and Geminga, are identified as probable candidates for MACE observations. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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