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Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables

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

Deadline for manuscript submissions: closed (31 March 2026) | Viewed by 4864

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Prof. Dr. Efrain J. Ferrer

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Guest Editor

Department of Physics and Astronomy, University of Texas Rio Grande Valley, Edinburg, TX 78539, USA
Interests: nuclear physics; high energy physics; astrophysics
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Vivian de la Incera

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Department of Physics and Astronomy, University of Texas Rio Grande Valley, Edinburg, TX 78539, USA
Interests: quantum field theory; QCD under extreme conditions; QFT in magnetic fields

Dr. Cristina Manuel

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1. Instituto de Ciencias del Espacio (ICE, CSIC), C. Can Magrans s.n., 08193 Cerdanyola del Vallès, Catalonia, Spain
2. Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, Barcelona, Catalonia, Spain
Interests: QCD

Special Issue Information

Dear Colleagues,

This Special Issue is a collection of articles on the properties of highly dense matter in strong magnetic fields and the implications for the physics of compact objects such as neutron stars. Neutron stars are known to have strong surface magnetic fields, with magnetars having surface fields as large as 10¹⁵G. Interior fields can potentially be much larger, estimated to reach values a few times 10¹⁷ G, according to studies of magnetars based on GR magnetohydrodynamics. With densities a several times larger than than nuclear saturation density and the largest magnetic fields in nature, neutron stars are a unique natural laboratory for investigating these extreme conditions. Questions such as how the magnetic field can influence the inner matter phase will have implications for the EoS and mass/radius ratio of these compact objects; similarly, the field may affect the star cooling and other transport properties of relevance. These and other potential implications of the magnetic field will be discussed by specialists who have contributed fundamental research to this area of physics and astrophysics.

Prof. Dr. Efrain J. Ferrer
Prof. Dr. Vivian de la Incera
Dr. Cristina Manuel
Guest Editors

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Keywords

magnetized relativistic systems
magnetized dense systems
equation of state in magnetized dense systems
rotation in magnetized dense system
cooling in magnetized dense systems
transport properties of magnetized dense systems
topological effects in magnetized dense systems
hadron–quark phase transition in a magnetic field

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Published Papers (10 papers)

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Research

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24 pages, 1307 KB

Open AccessArticle

Finite-Size Effects on the Critical End Point of Magnetized Quark Matter in the Nonlocal PNJL Model

by G. Lugones, S. A. Ferraris and A. G. Grunfeld

Universe 2026, 12(5), 149; https://doi.org/10.3390/universe12050149 - 20 May 2026

Viewed by 86

Abstract

We investigate finite-size effects in the T–

μ

We investigate finite-size effects in the T–

μ

phase diagram of magnetized quark matter within the framework of a nonlocal extension of the Polyakov–Nambu–Jona-Lasinio (PNJL) model. Finite-size corrections are incorporated through the multiple reflection expansion (MRE) formalism, which describes a spherical quark droplet of radius R and modifies the density of states by including surface and curvature contributions. We consider two-flavor quark matter at finite temperature and chemical potential in the presence of a uniform magnetic field with strengths ranging from

e B = 0

to 1

{GeV}^{2}

, and droplet radii from

R = 3

fm to the bulk limit. The nonlocal PNJL (nlPNJL) model naturally reproduces both magnetic catalysis at low temperatures and inverse magnetic catalysis near the chiral transition, in agreement with lattice QCD results. We analyze the chiral condensate, the traced Polyakov loop, the normalized quark condensate, and the corresponding susceptibilities. We find that finite-size effects do not modify the overall structure of the phase diagram, and that the coincidence of the chiral restoration and deconfinement transitions persists for all magnetic field strengths and system sizes explored, within the present implementation in which finite-size corrections are restricted to the fermionic sector. However, the critical end point (CEP) is notably shifted as a function of both magnetic field strength and system size: It moves toward higher chemical potentials and lower temperatures as system size decreases, an effect that is significantly amplified by strong magnetic fields. Our results have potential implications for the physics of phase conversion in compact stars and for the interpretation of relativistic heavy-ion collision experiments. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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29 pages, 1844 KB

Open AccessArticle

GRMHD Simulations of Magnetized Accretion Disk/Jet: Variabilities of Black Holes and Spectral Energy Distributions in Magnetic States

by Rohan Raha, Banibrata Mukhopadhyay and Koushik Chatterjee

Universe 2026, 12(5), 142; https://doi.org/10.3390/universe12050142 - 12 May 2026

Viewed by 192

Abstract

We perform three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of a near-maximally spinning black hole (spin parameter

a = 0.998

We perform three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of a near-maximally spinning black hole (spin parameter

a = 0.998

) with varying initial magnetic field geometries, systematically exploring the parameter space connecting magnetically arrested disk (MAD), intermediate (INT), and standard and normal evolution (SANE) accretion states. The magnetic flux threading the black hole horizon emerges as the fundamental state variable controlling jet efficiency, flow magnetization, and radiative output across all three states. We introduce complementary diagnostics—broadband spectral energy distributions spanning radio through hard X-ray frequencies and time-resolved X-ray light curves—that together connect simulation dynamics directly to multiwavelength observables. The radiative output follows a clear MAD > INT > SANE hierarchy in time-averaged luminosity, mean X-ray emission, as well as variability. Furthermore, MAD exhibits the highest fractional variability through quasi-periodic magnetic flux eruption events, and INT and SANE show moderate variability driven by episodic reconnection and stochastic MRI turbulence, respectively. Scaling to GRS 1915+105, Cyg X-1, and HLX-1, we demonstrate that all twelve temporal classes of GRS 1915+105 map naturally onto our three magnetic states, Cyg X-1’s persistent hard state is reproduced by a sustained INT configuration, and HLX-1’s extreme luminosities arise through efficient Blandford–Znajek extraction in MAD states scaled to higher black hole mass. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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18 pages, 316 KB

Open AccessArticle

Mechanical Equilibrium in the Magnetized Quark–Hadron Mixed Phase: A Covariant Generalization of the Gibbs Condition

by Aric Hackebill

Universe 2026, 12(5), 133; https://doi.org/10.3390/universe12050133 - 4 May 2026

Viewed by 190

Abstract

We formulate a covariant mechanical equilibrium condition for the quark–hadron mixed phase boundary in the presence of a magnetic-field-induced pressure anisotropy. Using the relativistic thin-shell formalism to describe the quark–hadron boundary, we interpret conservation of stress-energy across the interface as a set of generalized Young–Laplace conditions which characterize the geometry of the interface. In a comoving stationary frame, this provides a covariant description of mechanical equilibrium at the interface, which serves as a replacement for the scalar pressure-balance condition used in the isotropic Gibbs construction. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

22 pages, 2959 KB

Open AccessArticle

Magnetic Field Effects on the Structure of Neutron Stars

by Harsh Chandrakar, Ishfaq Ahmad Rather, Prashant Thakur, Tarun Kumar Jha, Rodrigo Negreiros, Carline Biesdorf, Mariana Dutra and Odilon Lourenço

Universe 2026, 12(5), 128; https://doi.org/10.3390/universe12050128 - 28 Apr 2026

Viewed by 432

Abstract

We investigate the impact of ultrastrong magnetic fields on the structure of neutron stars within a density-dependent relativistic mean-field framework (DDME2). In the first case, we incorporate a magnetic field framework through Landau quantization of charged particles, yielding anisotropic pressure contributions and showing that field-induced stiffening increases stellar radii, maximum masses, and tidal deformabilities. To capture anisotropic stresses and geometric distortions, we employ axisymmetric equilibrium configurations computed with the XNS 4.0 code under the extended conformally flat condition. For magnetic field strengths up to

4.5 \times 10^{17}

G, we analyze purely poloidal and toroidal geometries across a representative mass range (1.2–2.0

M_{⊙}

). Axisymmetric models reveal that purely toroidal fields induce prolate deformations reaching

| \bar{e} | \approx 0.67

for a 1.2

M_{⊙}

star, while purely poloidal fields drive oblate deformations with

\bar{e} \approx 0.24

, both diminishing with increasing stellar mass as greater gravitational binding resists magnetic reshaping. These macroscopic effects, combined with microphysical stiffening, have direct implications for gravitational-wave emission and systematic biases in radius measurements. Our study provides a systematic mapping between magnetic field strength, topology, and dense-matter stiffness, offering constraints relevant to multimessenger observations of magnetized neutron stars. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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17 pages, 1141 KB

Open AccessArticle

Effects of the Symmetry Energy Slope on Magnetized Neutron Stars

by Luiz L. Lopes, César O. V. Flores and Débora Peres Menezes

Universe 2026, 12(4), 117; https://doi.org/10.3390/universe12040117 - 15 Apr 2026

Viewed by 361

Abstract

In this work, we study the effect of the symmetry slope on the observables of weakly and strongly magnetized neutron stars within the chaotic magnetic field approximation. We investigate the impact of the symmetry energy slope in the equation of state, as well as on the observables of neutron stars, by calculating their masses, radii, redshifts, tidal deformabilities, and fundamental-mode gravitational-wave frequencies. We show that the effect of the magnetic field is strong on low mass stars, producing a softer equation of state and correspondingly lower values of radii. Furthermore, the magnetic field also causes a significant drop in the dimensionless tidal parameter even when the effects on the radii are small. At the end of the paper, we discuss the effects of the magnetic field on neutron stars’ universal relations. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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12 pages, 366 KB

Open AccessEditor’s ChoiceArticle

Stability Analysis of Magnetized Quark Matter in Tsallis Statistics

by Jia Zhang and Xin-Jian Wen

Universe 2025, 11(9), 312; https://doi.org/10.3390/universe11090312 - 12 Sep 2025

Cited by 1 | Viewed by 1083

Abstract

In this work, we employ the nonextensive Nambu–Jona-Lasinio model to analyze the thermodynamic properties of magnetized quark matter. The nonequilibrium state is described in Tsallis distribution by a dimensionless parameter q. We find that within a reasonable temperature range, the system undergoes a crossover transition at the critical chemical potential, which is decreased by the increase of both the temperature and q value. In contrast to the enhanced stability by magnetic field in Boltzmann statistics, it is found that the stability of chiral restored matter in Tsallis statistics would be reduced by an increase of the magnetic field. Conversely, the increase of the q would enhance the stability of quark matter. Finally, we display the different magnetic effects on the stability in the chiral broken and restored regions. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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Review

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22 pages, 8682 KB

Open AccessReview

Anisotropic Compact Stars: Theory and Simulation from Microphysical Models to Macroscopic Structure and Observables

by Zenia Zuraiq, Mayusree Das, Debabrata Deb, Surajit Kalita, Fridolin Weber and Banibrata Mukhopadhyay

Universe 2026, 12(5), 130; https://doi.org/10.3390/universe12050130 - 30 Apr 2026

Viewed by 348

Abstract

Strong magnetic fields and anisotropic stresses can substantially modify the structure and observable properties of compact stars. In this review, we present a unified treatment of magnetically induced anisotropy across neutron stars, hybrid stars, and white dwarfs, connecting the microphysical equation of state effects to macroscopic structure and multimessenger observables. We demonstrate that magnetic-field geometry plays a decisive role: toroidally oriented (transverse) fields enhance the maximum mass by providing additional perpendicular pressure support, whereas radially oriented fields primarily increase central compression with comparatively small mass gain. In neutron stars, anisotropy and magnetic stresses can shift phase-transition thresholds in hybrid models and enable configurations in the lower mass gap with significantly smaller magnetic energy compared to the gravitational binding energy. We further show that continuous gravitational wave emission from magnetically deformed neutron stars provides a complementary probe of internal field geometry through ellipticity-driven strain evolution. In magnetized white dwarfs, super-Chandrasekhar masses arise from the spatial redistribution of magnetic stresses rather than from globally strong magnetic energy. Taken together, these results highlight that magnetic-field geometry and matter anisotropy are as important as field strength in determining mass–radius relations, tidal deformability, gravitational wave detectability, and the emergence of extreme compact-star configurations. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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29 pages, 2565 KB

Open AccessReview

Dense Matter and Compact Stars in Strong Magnetic Fields

by Monika Sinha and Vivek Baruah Thapa

Universe 2026, 12(5), 122; https://doi.org/10.3390/universe12050122 - 25 Apr 2026

Viewed by 229

Abstract

Compact stars serve as natural systems where matter exists at densities far beyond those achievable in laboratory experiments. Among them, magnetars are expected to possess interior magnetic fields that may reach values of the order of

10^{17}

–

10^{18}

G. These [...] Read more.

10^{17}

–

10^{18}

G. These extreme conditions are expected to alter the microscopic and macroscopic properties of dense matter. In this review, we examine how strong magnetic fields affect fermionic matter through mechanisms such as Landau quantization and anomalous magnetic moment interactions. We further discuss the behavior of magnetized hadronic matter within relativistic mean-field approaches and consider the possible emergence of additional degrees of freedom, including hyperons,

Δ

resonances, meson condensates, and quark matter. The consequences of these effects for neutron star structure and observational constraints are also briefly outlined. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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24 pages, 985 KB

Open AccessReview

Neutrino Production Mechanisms in Strongly Magnetized Quark Matter: Current Status and Open Questions

by Igor A. Shovkovy and Ritesh Ghosh

Universe 2026, 12(3), 61; https://doi.org/10.3390/universe12030061 - 25 Feb 2026

Cited by 1 | Viewed by 649

Abstract

We review the main neutrino emission mechanisms operating in dense quark matter under strong magnetic fields, with particular emphasis on conditions expected in the interiors of compact stars. We discuss the direct Urca and neutrino synchrotron processes in unpaired quark matter, incorporating the effects of Landau-level quantization. For the direct Urca process, the quantization of the electron energy spectrum plays a critical role, whereas quark quantization can often be neglected at sufficiently high baryon densities. The resulting field-dependent neutrino emissivity is anisotropic and exhibits an oscillatory behavior as a function of magnetic-field strength. We explore the implications of these effects for magnetar cooling and for possible anisotropic neutrino emission that could contribute to pulsar kicks. In addition, we review the

ν \bar{ν}

synchrotron emission process, which, although subdominant, provides valuable insights into the interplay between magnetic fields and weak interactions in dense quark matter. Overall, our analysis highlights the nontrivial influence of strong magnetic fields on neutrino production in magnetized quark cores, with potential consequences for the thermal and dynamical evolution of compact stars. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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Other

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17 pages, 952 KB

Open AccessPerspective

Magnetized Matter in Neutron Star Dynamics: Challenges Ahead

by J. E. Horvath

Universe 2026, 12(5), 147; https://doi.org/10.3390/universe12050147 - 18 May 2026

Viewed by 196

Abstract

Matter at ultra-high densities finds a physical realization inside neutron stars. It is generally acknowledged that huge magnetic fields are present in these stellar objects, and if not for the presence of the magnetic fields, neutron stars would be much more “silent” and practically invisible. However, a series of questions still remain concerning the role of magnetic fields in the neutron star structure and internal dynamics. We present an overview of these topics, pointing out the importance of a new set of observations (glitches and related events, and precession) and old questions that must be accommodated by a more complete theory of neutron star physics. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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