arXiv subjects

Zhenglong Ban

Publications and source records attributed to Zhenglong Ban.

11 recordsLinked to original sources

arXiv2026

Periodic orbits and gravitational waveforms around a Schwarzschild black hole with a cloud of strings embedded in perfect fluid dark matter

In this study, we explore the dynamics of particle orbits and their corresponding gravitational wave signatures in the vicinity of a Schwarzschild black hole (BH) surrounded by a cloud of strings and embedded in a perfect fluid dark matter medium. The model is characterized by two parameters: $a$, associated with the string cloud, and $\alpha$, representing the dark matter distribution. We systematically analyze how the marginally bound orbit (MBO) and the innermost stable circular orbit (ISCO) depend on these parameters. Our findings reveal that while both the orbital radius and angular momentum increase with increasing $a$, they decrease as $\alpha$ increases; notably, the energy exhibits the opposite trend, decreasing with $a$ and increasing with $\alpha$. Furthermore, we examine periodic orbits indexed by rational numbers $q$ and the gravitational waveforms they generate. The results demonstrate that an increase in the string cloud parameter $a$ induces a significant phase delay in the waveform. Specifically, waveforms with lower values of $a$ oscillate over shorter time intervals, whereas those with higher values extend to longer time scales. These distinct features, including noticeable differences in amplitude, allow the waveforms to be clearly distinguished from those in a pure Schwarzschild spacetime.

gr-qc↗

arXiv2026

Connection Between the Shadow Radius and Quasinormal Frequencies for Black Holes in STVG with Perfect Fluid Dark Matter

We investigate the connection between black hole shadow and quasinormal mode (QNM) spectra in the context of scalar-tensor-vector gravity (STVG) coupled to perfect fluid dark matter (PFDM), characterized by the MOG parameter $\alpha$ and the dark matter intensity $\lambda$. Employing complementary methods -- namely the sixth-order WKB approximation, Pad\'e resummation, and time-domain numerical integration -- we compute QNM frequencies for scalar ($s=0$), electromagnetic ($s=1$), and axial gravitational ($s=2$) perturbations. Both the real part of the QNM frequencies and the peak height of the effective potential display a consistent parametric dependence: they increase with $\lambda$ yet decrease with growing $\alpha$. In the eikonal limit ($l \gg 1$), we derive an exact analytical link between the shadow radius $R_{\mathrm{sh}}$ and the QNM frequency $\omega_R$. Noting that $R_{\mathrm{sh}}$ is determined by the critical impact parameter $b_c = r_{\mathrm{ph}}/\sqrt{f(r_{\mathrm{ph}})}$, while $\omega_R = \Omega l$ with photon angular velocity $\Omega = \sqrt{f(r_{\mathrm{ph}})}/r_{\mathrm{ph}}$, we obtain the precise relation $\omega_R = l / b_c$, identifying $R_{\mathrm{sh}} \equiv b_c$ for an asymptotically flat observer. This prediction is robustly validated by numerical results across all three computational approaches at large multipole numbers. Our findings reveal that the black hole shadow and gravitational ringdown are not independent phenomena, but dual observational signatures of the same underlying structure -- the unstable photon orbit -- thereby offering a unified multi-messenger framework to simultaneously constrain modified gravity and dark matter in the strong-field regime.

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arXiv2026

Observational Signatures of Rotating Ay\'{o}n-Beato-Garc\'{i}a Black Holes: Shadows, Accretion Disks and Images

We investigate the shadows, accretion disks, and observational images of rotating Ay\'on-Beato--Garc\'{\i}a (ABG) black holes with mass $M$, spin $a$, and nonlinear-electrodynamics (NLED) charge parameter $\zeta$, treating photons as neutral test particles on null geodesics of the background metric so as to obtain the purely geometric shadow. The shadow shrinks with increasing $\zeta$ and develops a ``D''-shaped morphology for near-extremal spin. The thermal disk properties follow from the Novikov--Thorne model with inner edge at the innermost stable circular orbit (ISCO), whereas the images use a separate, phenomenological optically thin emission model extending to the horizon. The image asymmetry and redshift maps depend strongly on $(a, \zeta)$ and the inclination. Comparing the geometric shadow diameters with Event Horizon Telescope observations of M87$^{*}$ and Sgr A$^{*}$ gives the indicative estimates $\zeta \lesssim 0.21\,M$ and $\zeta \lesssim 0.43\,M$, respectively; since $\zeta$ characterizes each black hole individually, these are quoted separately rather than combined, and the Kerr limit $\zeta = 0$ remains fully consistent with both.

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arXiv2026

Observational Signatures of Accretion Disks around a Schwarzschild Black Hole in a Hernquist Dark Matter Halo

We investigate how a Hernquist type dark matter (DM) halo, parametrized by its core radius $r_{s}$ and central density $\rho_{s}$, influences both the gravitational wave (GW) emission from timelike periodic orbits and the electromagnetic appearance of a thin accretion disk around a Schwarzschild black hole (BH). By analyzing the effective potential for timelike geodesics, we show that the DM halo shifts the marginally bound orbit (MBO) and the innermost stable circular orbit (ISCO) outward, reflecting its modification of the spacetime geometry and the energy-angular momentum structure of particle motion. Employing a semi-analytical method, we compute orbital trajectories and the associated GW waveforms, revealing that the DM halo alters the characteristic zoom-whirl dynamics and induces measurable changes in waveform morphology. Furthermore, we generate direct and secondary images of the accretion disk across various observer inclinations and find that increasing $r_{s}$ or $\rho_{s}$ results in cooler, dimmer disks with modified flux distributions. Our results demonstrate that the presence of a DM halo imprints distinct signatures in both gravitational and electromagnetic observables, offering a multimessenger pathway to probe DM environments near BHs.

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arXiv2025

Spherically symmetric charged (anti-)de Sitter black hole in $f(R,T)$ gravity coupled with nonlinear electrodynamics

We investigate black hole solutions in quadratic $f(R,T)=R+kT^2$ gravity coupled with nonlinear electrodynamics $\mathcal{L}(F)=\alpha - F/(4\pi) + \gamma F^2$. Solving the field equations yields an exact magnetically charged (anti-)de Sitter black hole metric featuring novel $r^{-6}$ and $r^{-14}$ correction terms. The horizon structure can exhibit up to four horizons depending on the parameters. Thermodynamic analysis reveals that the heat capacity exhibits critical phase transitions, with the critical point significantly shifted by the coupled corrections. Using the effective metric formalism, we study the photon sphere and black hole shadow, finding that the magnetic charge $Q$ shrinks the shadow while the nonlinear parameter $\gamma$ enlarges it. Constraints from Event Horizon Telescope observations of M87* and Sgr A* place the parameter $\gamma$ at the order of $10^3$ at $1\sigma$ and $2\sigma$ confidence levels. Our results demonstrate that quadratic $f(R,T)$ gravity combined with nonlinear electrodynamics provides a consistent framework for strong-field gravity phenomenology.

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arXiv2025

Regular hairy black holes through gravitational decoupling method

Within a framework requiring a well-defined event horizon and matter obeying the weak energy condition, we employ gravitational decoupling method to construct non-singular hairy black holes: spherically or axially symmetric. These solutions arise from a deformation of the Minkowski vacuum, where the maximum deformation can yield the Schwarzschild metric for the static case, and the Kerr geometry for the stationary case, respectively.

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arXiv2025

Rotating Charged Black Holes with Scalar Hair Constructed via the Newman-Janis Algorithm: Accretion Disk Structure and Shadow Characteristics

In this paper, we generate a rotating charged black hole (BH) with scalar hair via the Newman--Janis algorithm (NJA) and study its thin accretion disk and shadow. The structure of the event horizon and ergosurface is analyzed in detail, revealing how the charge parameter $Q$ and scalar hair parameter $s$ influence the spacetime geometry. We analyze the energy flux and temperature distribution of the accretion disk, finding that increasing either $Q$ or $s$ leads to higher energy flux and peak temperature. The BH shadow is also examined, showing that its apparent size decreases monotonically with increasing $Q$ or $s$. Notably, in the near-extremal regime, the shadow develops a distinctive cuspy edge, indicative of strong light bending in the scalarized and charged spacetime. By comparing the theoretically predicted shadow diameter with Event Horizon Telescope (EHT) observations of Sgr A$^*$, we derive observational constraints on the model parameters. For inclination angles of $17^\circ$ and $90^\circ$, a joint analysis constrains the charge parameter to $0<Q<0.522745$ (at fixed $s=0.1$) and the scalar hair parameter to $0<s<0.283373$ (at fixed $Q=0.3$). Our results demonstrate how scalar hair and electric charge leave imprints on accretion disk emissions and black hole shadows, offering new observational signatures for testing gravity theories beyond general relativity.

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arXiv2025

Shadow and thin accretion disk around Ay\'{o}n-Beato-Garc\'{i}a black hole coupled with cloud of strings

In this paper, we investigate the shadow and thin accretion disk around Ay\'{o}n-Beato-Garc\'{i}a (ABG) black hole (BH) coupled with a cloud of strings (CS), characterized by the nonlinear electrodynamics (NLED) parameter $g$, and the CS parameter $a$. By comparing shadow diameters with Event Horizon Telescope (EHT) observations of M87$^{*}$ and Sgr A$^*$, we have established constraints on the BH parameters $g$ and $a$. Additionally, we analyze the BH shadow, lensing ring, and photon ring features for the ABG BH coupled with CS. Our results indicate that the shadow radius increases monotonically with the CS parameter $a$, while it decreases with increasing $g$. Finally, the study explores the physical properties and observational signatures of thin accretion disks around ABG BH with CS. The results show that an increase in parameter $g$ leads to a hotter and more luminous disk, while an increase in parameter $a$ results in a cooler and less luminous disk.

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arXiv2025

Probing Einstein-Maxwell-Scalar Black hole via Thin Accretion Disks and Shadows with EHT Observations of M87* and Sgr A*

We investigated the shadows and thin accretion disks of Einstein-Maxwell-Scalar (EMS) black hole. Firstly, we investigated the influence of EMS parameters on the black hole shadow using the null geodesic method and constrained these parameters based on EHT observations of M87* and Sgr A*. Furthermore, we analyzed the direct emission, lensing ring, and photon ring structures in EMS black hole. Comparing our results with the Schwarzschild and Reissner-Nordstr$\ddot{\mathrm{o}}$m (RN) black holes, we found that the Schwarzschild black hole exhibits the largest shadow radius and the highest observed intensity.

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arXiv2025

Thin accretion disk around Schwarzschild-like black hole in bumblebee gravity

The physical properties and optical appearance of a thin accretion disk surrounding a Schwarzschild-like black hole (BH) are investigated within the framework of bumblebee gravity. To understand how the Lorentz symmetry breaking (LSB) parameter $l$ affects the disk's behavior, we analyze main characteristics such as energy flux, temperature distribution, and emission spectrum. In addition, direct and secondary images of the accretion disk are generated and examined to explore how both the observational inclination angle and the LSB parameter $l$ shape the visual profile. Furthermore, we compute the redshift and observed flux distributions of the disk from the perspective of distant observers at various inclination angles. Our results indicate that the redshift factor grows as $l$ decreases. When the parameter $l$ assumes negative values, the BH exhibits enhanced luminosity with decreasing $l$. These findings highlight the crucial influence of the LSB parameter $l$ on the observable features of BHs.

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arXiv2024

Shadows of rotating black holes in effective quantum gravity

Recently, two new spherically symmetric black hole models with covariance have been proposed in effective quantum gravity. Based on these models, we use the modified Newman-Janis algorithm to generate two rotating quantum-corrected black hole solutions, characterized by three parameters, the mass $M$, the spin $a$, and the quantum parameter $\zeta$. To understand the effects of the quantum parameter $\zeta$ on these two rotating black holes, we investigate in detail the horizons and static limit surfaces. By constraining the possible range of the parameters, we study the shadows cast by these rotating black holes. The results indicate that for both rotating BHs, the parameter $\zeta$ mainly affects the shadow size in the non-extremal case, while it deforms the shadow shape by arising a cuspy edge in the near-extremal case.

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