arXiv subjects

Yong-Qiang Wang

Publications and source records attributed to Yong-Qiang Wang.

At least 19 recordsLinked to original sources

arXiv2026

A Neutron Star Hidden Inside a Black Hole

We investigate dark matter admixed neutron stars in which the neutron star coexists with an anisotropic dark matter halo described by the Einasto density profile, a model recently shown to produce regular, singularity-free black hole solutions~[Phys. Rev. D \textbf{113}, 043011 (2026)]. Solving the modified Tolman--Oppenheimer--Volkoff equations with two different equations of state (BSk19 and SLy4), we find that the dark matter halo significantly alters the neutron star structure. Furthermore, for a specific range of halo parameters, $g_{rr}^{-1}$ changes sign outside the stellar surface, forming an event horizon with the neutron star persisting as a regular configuration inside it--- ``neutron stars in black holes''. This configuration appears in both equations of state and does not depend on a specific choice of the equation of state. The discovery of this configuration provides a new perspective and a concrete computable instance for the study of what lies inside a black hole.

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arXiv2026

Tidal Love numbers of multi-state Boson stars

In this paper, we calculate the tidal Love numbers of multi-state boson stars (MSBSs) composed of ground state and first excited state complex scalar fields. Under synchronized and nonsynchronized frequency conditions, the background solutions of MSBSs are classified into single-branch and double-branch types. The field functions, ADM mass, and binding energy of different solutions are discussed. We then calculate the quadrupolar ($\ell=2$) electric and magnetic tidal Love numbers for branches containing stable solutions. Our results show that the electric tidal Love numbers are initially positive and then suddenly transition to negative values. This phenomenon occurs when the parameters satisfy $\tilde{\mu}_1 > 0.891$ or $\tilde{\omega}_0 > 0.777$; for smaller values of these parameters, the electric Love numbers remain positive. The magnetic tidal Love numbers are always negative, with absolute values smaller than those of the electric tidal Love numbers.

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arXiv2026

Boson Stars in Teleparallel Gravity with a Nonminimally Coupled Field: The Violation of Energy Conditions and Gravitational Waveforms from EMRIs

In this work, we investigate boson star models within the framework of teleparallel gravity with non-minimal coupling, and obtain static, spherically symmetric solutions for both the ground state and excited states. The results indicate that the energy density of the excited-state solutions can become negative. For these solutions, the four commonly used energy conditions are no longer satisfied. In contrast, for all the ground-state solutions we have studied, the energy density remains positive and all four energy conditions are consistently satisfied. Moreover, considering the importance of astrophysical observations, the gravitational-wave signals from Extreme-Mass-Ratio Inspirals (EMRIs) composed of these boson stars are investigated. Our results reveal that the frequency-domain characteristic strain of these waveforms falls within the detectability range of LISA, which can provide potential evidence for distinguishing compact astrophysical objects.

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arXiv2026

Proca-Maxwell System in an Infinite Tower of Higher-Derivative Gravity

We numerically construct a five-dimensional Proca-Maxwell system coupled to an infinite tower of higher-derivative gravity, parameterized by the correction order and coupling constant. While the first-order correction case recovers standard Einstein gravity results, and the second-order correction (Gauss-Bonnet) case fails to resolve the central singularity in the vanishing frequency limit, we demonstrate that higher-order corrections effectively regularize the spacetime, yielding globally regular solutions. A key finding is the emergence of a ``frozen state'' in the supercritical regime: as the field frequency approaches zero, matter concentrates entirely within a critical radius, creating a regular core that externally mimics an extremal black hole. We further reveal that introducing the electric charge fundamentally alters this behavior; the electrostatic repulsion counteracts the gravitational collapse, effectively ``unfreezing'' the system and preventing the formation of the critical core. Significantly, unlike models relying on exotic matter, our solutions satisfy all standard energy conditions across the entire parameter space, establishing a physically viable pathway for constructing regular black hole mimickers.

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arXiv2026

Ellis--Bronnikov wormhole in Quasi-topological Gravity

We construct higher-dimensional traversable wormholes in quasi-topological gravity (QTG) supported by a phantom scalar field. Using a static, spherically symmetric ansatz, we numerically analyze how quasi-topological gravity corrections affect the geometry and physical properties of the wormhole solutions. The resulting wormhole solutions are symmetric about the throat. Negative mass can arise for certain choices of parameters. For certain parameter ranges, the scalar charge $\mathcal{D}$ of the phantom field rapidly decreases with increasing the higher-curvature coupling parameter $\alpha$ and approaches zero. Moreover, by changing $\alpha$, the overall level of the Kretschmann scalar is also lowered. Finally, for sufficiently large $\alpha$, $-g_{tt}$ becomes close to zero near the throat, exhibiting a ``horizon''-like structure.

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arXiv2026

Compact Stars Sourced by Perfect Fluid Dark Matter Halos

Recent studies have shown that dark matter halos can support regular black holes or compact stars by assuming an anisotropic energy-momentum tensor. In this paper, we extend the analysis to the dark matter halo as an isotropic perfect fluid. By employing galactic dark matter profiles-specifically the Einasto and Dehnen models-as the mass-energy density source, we numerically solve the Einstein field equations and find a class of non-singular, horizonless compact star solutions. Moreover, these configurations remain stable against axial perturbations while satisfying the dominant energy condition.

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arXiv2026

Static stable timelike circular orbits and Aschenbach effect in horizonless solutions of Einsteinian cubic gravity

In modified gravity theories, horizonless compact objects serve as a compelling alternative to black holes for testing strong-field gravity. Einsteinian cubic gravity (ECG) provides a gravitational framework for constructing these viable astrophysical models. We investigate the existence, stability, and observable signatures of static stable timelike circular orbits (SSTCOs) in static spherically symmetric ECG horizonless spacetimes. We derive timelike geodesic equations, construct the effective potential for circular orbits, and perform a numerical integration to verify orbital stability. We confirm that SSTCOs exist in both solution branches of ECG horizonless objects and that their radii coincide with the innermost stable circular orbit (ISCO). The Aschenbach effect manifests as a non-monotonic radial dependence of a zero angular momentum observer (ZAMO) measured velocity. Furthermore, we find that the stability of circular orbits exhibits a 'double stable region' structure. As the specific energy $E$ of a test particle transitions from the outer edge to the inner edge (i.e., the ISCO) of the inner stable region, its variation can exceed $1$ (i.e., $\Delta E > 1$), implying that during this process, the gravitational system can release an amount of energy exceeding the rest mass of the particle itself.

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arXiv2026

Compact Stars Sourced by Dark Matter Halos and Their Frozen States

Inspired by regular black holes (RBHs) sourced by dark matter halos, we generalize the anisotropic energy-momentum tensor by relaxing the $P_r = -\rho$ condition between radial pressure and density. We demonstrate that while RBHs are a unique special case, a broader class of relations yields horizonless compact stars. Under specific parameter limits, these objects approach a ``frozen state," mimicking black hole features without an event horizon. These compact star solutions could satisfy weak energy conditions and provide a robust mechanism for dark matter-sourced black hole mimickers.

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arXiv2025

Proca stars and their frozen states in an infinite tower of higher-derivative gravity

In this work, we investigate the five-dimensional Proca star under gravity with the infinite tower of higher curvature corrections. We find that when the coupling constant exceeds a critical value, solutions with a frequency approaching zero appear. In the finite-order corrections case $n=2$ (Gauss-Bonnet gravity), the matter field and energy density diverge near the origin as $\omega\to 0$. In contrast, for $n\geq 3$, the divergence is efficiently suppressed, both the field and the energy density remain finite everywhere, and both the matter field and energy density remain finite everywhere. In the limit $\omega \to 0$, a class of horizonless frozen star solutions emerges, which are referred to ``frozen stars". Importantly, frozen stars contain neither curvature singularities nor event horizons. These frozen stars develop a critical horizon at a finite radius $r_c$, where $-g_{tt}$ and $1/g_{rr}$ approach zero. The frozen star is indistinguishable from that of an extremal black hole outside $r_c$, and its compactness can reach the extremal black hole value.

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arXiv2025

Frozen Neutron Stars in Four-Dimensional Non-polynomial Gravities

This paper investigates the structure and properties of neutron stars in four-dimensional non-polynomial gravities. Solving the modified Tolman-Oppenheimer-Volkoff equations for three different equations of state (BSk19, SLy4, AP4), we confirm that neutron star solutions remain in existence. As the modification parameter $\alpha$ increases, neutron stars grow in both radius and mass. We find that, when the parameter $\alpha$ is sufficiently large, a frozen state emerges at the end of the neutron-star sequence. In this state, the metric functions approach zero extremely close to the stellar surface, forming a critical horizon, making it nearly indistinguishable from a black hole to an external observer. Such a frozen neutron star constitutes a universal endpoint of the neutron-star sequence in this theory, independent of the choice of the equation of state. Based on our results and current observational constraints, we derive bounds on the modification parameter $\alpha$ and show that frozen neutron stars remain allowed in the bounds.

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arXiv2025

Boson Stars in Bumblebee Gravity and Their Gravitational Waveforms from Extreme-Mass-Ratio Inspirals

We investigate the impact of Lorentz violation on the compactness of mini-boson stars and the resulting gravitational-wave signals from extreme-mass-ratio inspirals (EMRIs) within the framework of bumblebee gravity. Numerical solutions for static, spherically symmetric configurations reveal that a positive Lorentz-violating parameter $\ell$ suppresses repulsive pressure, thereby enhancing gravitational binding and yielding more compact boson stars. Conversely, a negative $\ell$ amplifies repulsive pressure and weakens gravitational binding, such that no static solutions exist beyond a critical negative value. These structural modifications imprint distinct features on EMRI dynamics, characterized by a monotonic decrease in both orbital eccentricity and radial range as $\ell$ gradually increases from negative to positive values. Unlike the intermittent bursts from grazing orbits that resemble black-hole signals, penetrating orbits that enter the boson-star core exhibit sustained, amplitude-modulated gravitational-wave signatures without quiet intervals. Their characteristic strain falls within the detectability range of LISA, providing a potential observable for constraining Lorentz violation.

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arXiv2025

Frozen solitonic Hayward-boson stars in Anti-de Sitter Spacetime

We construct solitonic Hayward-boson stars (SHBSs) in Anti-de Sitter (AdS) spacetime, which consists of the Einstein-Hayward model and a complex scalar field with a soliton potential. Our results reveal a critical magnetic charge $q_c$. For $q\geq q_c$ in the limit of $\omega \rightarrow 0$, the matter field is primarily distributed within the critical radius $r_c$, beyond which it decays rapidly, while the metric components $-g_{tt}$ and $1/g_{rr}$ become very small at $r_c$. These solutions are termed ``frozen solitonic Hayward-boson stars" (FSHBSs). Continuously decreasing $\Lambda$ disrupts the frozen state. However, we did not find a frozen solution when $q<q_c$. The value of $q_c$ depends both on the cosmological constant $\Lambda$ and the self-interaction coupling $\eta$. We also found that for high frequency solutions, increasing $\eta$ can yield a pure Hayward solution. However, for low frequency solutions, increasing $\eta$ reduces both $1/g_{rr}$ and $-g_{tt}$. Furthermore, we analyzed the effective potential of SHBSs and identified an extra pair of light rings in the second solution branch.

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arXiv2025

Axion boson stars with wormhole topology

We investigate a novel gravitational configuration formed by a massless real phantom field and an axion scalar field, minimally coupled to gravity. This system describes an Ellis-type wormhole situated at the center of an axion star. By normalizing the mass of the axion field to unity, the physical properties of the model are determined by three independent parameters: the potential's decay constant, the frequency of the axion field, and the wormhole's throat parameter. We assess the traversability of this wormhole by examining the curvature scalars and energy conditions of the static solution. Our analysis of the wormhole's embedding diagrams indicates that, although the wormhole typically exhibits a single-throat geometry, a double-throat configuration featuring an equatorial plane may arise under specific conditions. Finally, an analysis of the null-geodesics reveals the existence of at least one unstable light ring at the wormhole throat.

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arXiv2025

Frozen Neutron Stars

We investigate neutron stars with nonlinear magnetic monopoles in the framework of the Einstein-nonlinear electrodynamics model, specifically within the Bardeen and Hayward models. Solving the modified Tolman-Oppenheimer-Volkoff equations for three different equations of state, we find that upon reaching the critical magnetic charge $q_{c}$, neutron stars enter frozen states characterized by the critical horizon. This extends the concept of frozen states to compact objects composed of ordinary matter (non-field matter), thereby offering a new perspective for related research.

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arXiv2025

Scalarization of Bardeen spacetime

In this paper, we study the scalarization of the entire Bardeen spacetime which is constructed from a nonlinear magnetic monopole. We find that once the scalarization coupling parameter exceeds the scalarized threshold $a_t$, a scalarized Bardeen spacetime (SBS) solution exists for any magnetic charge $q$. However, the nature of the scalarization depends on the magnetic charge $q$. For $q$ less than a critical vaule $q_c$, when $a$ exceeds $a_t$, the scalar field emerges. As $a$ approaches $a_t$ from above (i.e., $a\to a_t^+$), the scalar field vanishes and the metric is reduced to a pure Bardeen spacetime. This behavior indicates that the solution exhibits the general ``spontaneous" scalarization phenomenon. Conversely, for $q \geq q_c$, in the limit $a \to a_t^+$, the scalar field is non-vanishing and the SBS approaches a ``frozen" SBS. Considering the importance of photon orbits in astronomical observations, we analyze the trajectories of photons around SBSs by analyzing null geodesics.

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arXiv2025

Light Rings, Accretion Disks and Shadows of Hayward Boson Stars

In this paper, we investigate the Einstein-Hayward gravity coupled to a complex scalar field without self-interaction. Using numerical methods, we construct a class of Hayward boson star solutions and examine their fundamental properties as well as the optical appearance of the accretion disk. Our results show that in the frozen state, both the quasi-horizon radius and the light ring radii increase with the magnetic monopole charge. Furthermore, using ray-tracing method, we find that for non-frozen states, the absence of an quasi-horizon results in the appearance of multiple photon rings within the shadow region of the accretion disks. In contrast, for frozen states, the presence of a quasi-horizon causes their images to resemble those of Schwarzschild black holes, with no additional photon rings appearing.

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arXiv2025

Proca stars in AdS Ellis wormholes

In this paper, we study Proca stars in asymptotically anti-de Sitter (AdS) Ellis wormholes. This study distinguishes itself from the analysis of the Proca stars in asymptotically flat spacetimes. In the AdS Ellis wormhole background, the mass of the wormhole solutions vanishes. Consequently, we employ numerical techniques to investigate in detail the impact of the cosmological constant on both the matter field and the wormhole geometry, while categorizing the solutions in accordance with the symmetries of the Proca field. The results show that when the cosmological constant decreases, not only does the characteristic spiral behavior of the Proca star solutions, namely the charge-frequency relation $Q$-$\omega$, gradually disappear, but the throat or both sides of the wormhole may also develop a ``horizon", presenting a ``black bounce" characteristic.

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arXiv2025

Spherically symmetric horizonless solutions and their frozen states in Bardeen spacetime with Proca field

In this paper, we construct a static spherical symmetric Bardeen-Proca star (BPS) model, which consists of the electromagnetic field and Proca field minimally coupled with gravity. The introduction of the Proca field disrupts the formation of event horizons, ensuring that these solutions are globally regular throughout the spacetime. We obtain families of BPS solutions under several magnetic charge conditions. Based on these results, we further investigate the ADM mass, Noether charge, and energy density distribution of them. We find that when the magnetic charge is sufficiently large, solutions with a critical horizon $r_{cH}$ emerge as $\omega \rightarrow 0$, and the time component of the metric approaches zero inside $r_{cH}$. To an observer at infinity, the collapse process of the matter near the critical horizon appears frozen. Consequently, we refer to the solution with $r_{cH}$ as the frozen Bardeen-Proca star (FBPS). Additionally, we also investigate the circular geodesic orbits of BPS. For the light ring, we find that the light rings always appear in pairs, located on both sides of the critical horizon and moving further apart as the frequency $\omega$ decreases. For timelike circular orbits, we investigate their distribution in the spacetime of BPSs and highlight four representative families of BPS solutions.

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