arXiv subjects

Xiao-Xiong Zeng

Publications and source records attributed to Xiao-Xiong Zeng.

At least 19 recordsLinked to original sources

arXiv2026

Energy extraction from Kerr--Bertotti--Robinson black holes: polar region collapse and the capture of negative energy protons

The Kerr-Bertotti-Robinson black hole is an exact Einstein-Maxwell solution describing a rotating black hole in an asymptotically uniform magnetic field. It differs from the Kerr black hole in the Wald solution: the magnetic field lines are no longer cylindrical, with the constant-$A_ϕ$ lines saturating at large distances, and the interface $\vec{E}\cdot\vec{B}=0$ is no longer a cone but contracts with radius and eventually disappears. Determining the acceleration of charged particles pointwise, we find that protons are driven outward and electrons inward in the polar region, and vice versa in the equatorial region; the polar region shrinks as the magnetic field grows. Captured protons carry negative energy and angular momentum, whereas captured electrons carry positive energy and angular momentum. Fixing the capture domain by the gauge-invariant flux criterion and integrating over the $(r,θ)$ plane, we find that in weak magnetic fields the black hole gains mass, while in strong magnetic fields energy extraction occurs. Energy extraction can be achieved under the usual spherically symmetric density, without introducing the anisotropic density distribution required in the Wald solution for a Kerr black hole.

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arXiv2026

Superradiant Bose--Einstein condensates around Kerr black holes

Ultralight bosonic dark matter can accumulate around a rotating black hole, where superradiance amplifies the field until a macroscopic cloud forms. Whether such a cloud behaves as a Bose--Einstein condensate depends on the self-interaction, which earlier work has either retained on static backgrounds or dropped on the Kerr metric. Here we treat the two together. Starting from the Klein--Gordon equation with a quartic potential, we separate the linear problem into spheroidal and radial equations and solve them self-consistently, obtain the superradiant growth rate from the conserved Noether current, project the nonlinear term onto a single mode, and integrate the resulting Gross--Pitaevskii equation with a bordered Newton method at fixed particle number. Rotation modulates the self-interaction geometrically: the effective coupling carries a factor $Δ(r)$ and therefore switches off at the horizon. Once the field is rescaled, the whole solution family depends on the single dimensionless parameter $\mathcal{N}=λN$. We recover the hydrogenic spectrum of the gravitational atom and the $α^{4\ell+5}$ scaling of the growth rate, and we obtain the exact relation $J_{z}=\hbar mN$, which receives no correction from the self-interaction. We find that the condensate is a torus rather than a spherical shell, with its density vanishing identically on the rotation axis, and that the cloud is modified appreciably only for $\mathcal{N}\gtrsim10^{3}$.

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arXiv2026

Extracting Energy from a Non-Kerr Rotating Spacetime with an Anomalous Quadrupole Moment via Magnetic Reconnection

This paper investigates how to extract energy from a non-Kerr rotating spacetime with an anomalous quadrupole moment via the magnetic reconnection mechanism. Unlike many other rotating spacetimes, this spacetime possesses closed timelike curves, and the corresponding spacetime regions must be excluded when extracting energy. After introducing the event horizon, ergosphere, and closed timelike curves of this spacetime, we deeply analyze the energy per unit enthalpy at infinity for accelerated and decelerated plasmas, the allowed region for energy extraction, and the power and efficiency of energy extraction. The results show that energy extraction is possible for both positive and negative anomalous quadrupole moments, but a positive and small anomalous quadrupole moment corresponds to higher power and efficiency of energy extraction.

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arXiv2026

Optical appearance of the Konoplya-Zhidenko rotating non-Kerr black hole surrounded by a thin accretion disk

In this study, we investigate the optical appearance of rotating Konoplya-Zhidenko non-Kerr black holes in the presence of thin accretion disks, with the aim of examining whether the information of deformation parameters manifest in observable signatures. By employing a fisheye camera model in conjunction with backward ray-tracing techniques, we simulate images for both prograde and retrograde accretion scenarios. The results indicate that the deformation parameter $ξ$ can partially mitigate the shadow deformation induced by the rotation parameter $a$. The inner shadow displays characteristic morphological transformations at varying observation angles, transitioning from an axisymmetric circular form at low angles to a hat-like configuration at higher angles. Furthermore, at high observational inclination angles, the direct image and the lensed image become distinctly discernible, and an increase in the deformation parameter $ξ$ enhances the observed intensity of the image. Interestingly, the motion behavior of the accretion flow influences the observed intensity distribution on the screen, a finding that is consistent with the redshift distribution. Hence, variations in the deformation parameter $ξ$, the observation angles, and the motion behavior of the accretion flow collectively influence the observable appearance of the black hole. We expect this work to provide valuable references for identifying observable signatures of spacetime deviations from general relativity.

astro-ph.HE↗

arXiv2026

Particle decay and energy conservation in the Kerr-Newman black hole

In this paper, we study the decay of a particle in Kerr--Newman spacetime. Both theoretical analysis and numerical simulations show that when a particle splits in Kerr--Newman spacetime, its mass is inevitably reduced, and this mass deficit is transformed into kinetic energy in the center-of-mass frame. We denote the parent particle as $O$ and the daughter particles as $A$ and $B$. We also find that as the charge parameter $Q$ increases, the specific angular momenta of the parent particle $O$ and the daughter particle $B$ become smaller, the masses of the two daughter particles become larger, the absolute values of their specific energies become smaller, the relative Lorentz factors among the three particles become smaller, and their four-velocities also become smaller. These trends are opposite to those observed when increasing the cosmological constant in Kerr--de Sitter spacetime.

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arXiv2026

Polarization images of non-topological soliton Bardeen boson stars

In this study, we investigate the polarized images of non-topological soliton Bardeen boson stars by solving the coupled Einstein nonlinear electrodynamics complex scalar field equations, based on the thin accretion disk model surrounding these compact objects. We focus on the influence of key parameters, including the initial scalar field, magnetic charge, observer inclination angle, and magnetic field configuration, on the resulting polarization characteristics. The results show that the geometry of the magnetic field, particularly the relative strength between the radial $B_r$ and angular $B_θ$ components, plays a crucial role in determining the polarization pattern. Additionally, variations in the scalar field amplitude and magnetic charge significantly affect both the intensity and spatial distribution of the polarization. These results show that the polarization morphology is sensitive to the spacetime geometry and magnetic field configuration, and provide a qualitative basis for comparing boson stars with black holes.

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arXiv2026

Hotspot Images from Magnetic Reconnection Processes in the plunging Region of a Kerr Black Hole

Employing the hotspot imaging technique, this work investigates the plasma motion trajectories prior to and following the Comisso-Asenjo mechanism within the plunging region. After a concise overview of the magnetic reconnection process in the plunging region of a Kerr black hole, we present the hotspot model and the associated imaging methodology. Through numerical simulations, we separately examine the hotspot images in the plunging region under three conditions: no magnetic reconnection, with magnetic reconnection, and when the escape condition fails. These outcomes are also contrasted with hotspot images in the circular orbit zone. Our findings reveal that for hotspot images without magnetic reconnection, when the plasma follows plunging orbits, the flare strength gradually declines; conversely, for circular orbits, the flare strength remains approximately constant. Additionally, we observe that the signal indicative of energy extraction is less conspicuous in the plunging region compared to the circular orbit region.

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arXiv2026

Distinguishing Black Holes and Neutron Stars via Optical Images Illuminated by Thick Accretion Disks

This paper investigates the optical images of neutron stars within the framework of the radiatively inefficient accretion flow model, taking into account a polytropic equation of state. After obtaining the numerical solutions of the neutron star, we solved numerically the geodesic equations together with the radiative transfer equation. We mainly examine the effects of the polytropic index $N$ and the observer inclination angle $θ_o$ on the image morphology. The obtained images are also compared with the shadow of a Schwarzschild black hole. It is shown that, under the assumption that photon trajectories are terminated at the neutron star surface, the image exhibits a bright higher order structure surrounding an inner dark region. As $N$ increases, the size of the higher-order image gradually expands. As $θ_o$ increases, the obscuration of the neutron star silhouette by radiation originating outside the equatorial plane becomes more pronounced. Compared with the black hole shadow obtained under the same parameter configuration, the neutron star exhibits a larger higher order image and a more extended obscured inner dark region, whereas the higher order image of the black hole is more readily distinguishable. These results indicate significant differences in the optical appearance of neutron stars and black holes, and thus provide a theoretical basis for distinguishing between them through high resolution imaging.

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arXiv2026

Imprints of Black Hole Shadows and Polarization Patterns of Various Thick Disks: Bumblebee gravity

The main objective of this study is to explore the shadow and polarization patterns of a Kerr-Sen-like BH induced from Bumblebee gravity, which, among other alternative theories of gravity beyond Einstein gravity, stands out as a promising candidate for explaining certain high-energy astrophysical phenomena. Specifically, we would like to probe the influence of the rate of LSB parameter $\ell$ and the Bumblebee charge $Q$ on the resulting image morphology at $230\mathrm{GHz}$. We adopt a phenomenological RIAF-like model and an analytical BAAF disk model. Both models depict that the bright ring is encircled by two central dark regions, each of which gradually shrinks with increasing $\ell$. Consequently, frame-dragging gives rise to a pronounced brightness asymmetry, which is more enhanced with increasing $Q$. A notable feature in the anisotropic emission case is the emergence of a vertically stretched, elliptical ring structure. Compared with the RIAF framework, the bright ring in the BAAF disk images appears geometrically thinner, and the separation between the primary and higher-order images becomes more pronounced. Finally, the polarization patterns trace the brightness distribution and vary with both $\ell$ and $Q$, reflecting the spacetime structure. These results demonstrate that intensity and polarization in thick disk models provide probes of Kerr-Sen-like BHs and near-horizon accretion physics

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arXiv2026

Repetitive Penrose process in Konoplya-Zhidenko rotating non-Kerr black holes

This paper investigates the repetitive Penrose process in Konoplya-Zhidenko rotating non-Kerr black hole, exploring the influence of the deformation parameter on the repetitive Penrose process. After a brief review of the Konoplya-Zhidenko rotating non-Kerr black hole, we study the fundamental equations of the Penrose process in this spacetime, examine the iterative stopping conditions required for the repetitive Penrose process, and obtain the corresponding numerical results. It is concluded that, in addition to previously observed phenomena, under the same decay radius, a larger initial dimensionless deformation parameter $\hatη$ leads to greater values of the energy return on investment and energy utilization efficiency, particularly at higher decay radii. Furthermore, a smaller initial $\hatη$ results in a larger maximum value of the energy return on investment. For energy utilization efficiency, the initial $\hatη$ should take an intermediate value to maximize its peak. Additionally, we find that a larger initial $\hatη$ corresponds to a smaller maximum value of the extracted energy.

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arXiv2026

Imaging and Polarimetric Signatures of Konoplya-Zhidenko Black Holes with Various Thick Disk

We investigate the imaging properties of spherically symmetric Konoplya-Zhidenko (KZ) black holes surrounded by geometrically thick accretion flows, adopting a phenomenological radiatively inefficient accretion flow (RIAF) model and an analytical ballistic approximation accretion flow (BAAF) model. General relativistic radiative transfer is employed to compute synchrotron emission from thermal electrons and generate horizon-scale images. For the RIAF model, we analyze the dependence of image morphology on the deformation parameter, observing frequency, and flow dynamics. The photon ring and central dark region expand with increasing deformation parameter, with brightness asymmetries arising at high inclinations and depending on flow dynamics and emission anisotropy. The BAAF disk produces narrower rings and darker centers, while polarization patterns trace the brightness distribution and vary with viewing angle and deformation, revealing spacetime structure. These results demonstrate that intensity and polarization in thick-disk models provide probes of KZ black holes and near-horizon accretion physics.

astro-ph.HE↗

arXiv2026

Visual Characteristics of a Rotating Black Hole in $4$D Einstein-Gauss-Bonnet Gravity with Thin Accretion Disk Under EHT Constraints

This study investigates the visual characteristics of a rotating black hole (BH) within the fabric of $4$D Einstein-Gauss-Bonnet gravity illuminated with two illumination models, such as a celestial light sphere and a thin accretion disk. To visualize the BH shadow images, we use a recent fisheye camera model and ray-tracing method. And then, we focus on investigating the impact of the coupling parameter $α$ and the spin parameter $a$ on the shadow images. The results exhibit that the shadow radius decreases, while the shadow deviation increases with the aid of $α$. However, with respect to $a$, the shadow radius is slightly increased compared to the corresponding shadow deviation. For a celestial light sphere, the increasing values of $α$, lead to a decrease in the corresponding photon ring, while the space-dragging effect becomes more prominent with increasing $a$. For a thin accretion disk, we enhance its inner edge to the BH event horizon, and the particle motion is different in the regions inside and outside the innermost stable circular orbit. The result demonstrates that the shadow becomes progressively asymmetric with $a$, while the overall size of the inner shadow gradually decreases with the variations of $α$. Subsequently, we also investigated the distinct features of red-shift configurations on the disk for both direct and lensed images. Additionally, we used the latest observational data from M87* and Sgr A* to impose certain parameter constraints on $α$; the results depict the consistency of our considering the BH model.

astro-ph.HE↗

arXiv2026

Gravitational lensing and observational features of a dynamic black hole

In this work, we investigate the gravitational lensing effects and the dynamic evolution of the shadow of Vaidya black holes by employing backward ray-tracing techniques. Within the celestial sphere framework, the black hole shadow exhibits a complete evolutionary sequence, transitioning from an initial stable configuration through continuous expansion to a final static state. Notably, during and after the active accretion phase, a distinct lensing ring emerges outside the shadow. Extending this analysis to the thin accretion disk model reveals richer observational signatures. A bright ring, formed by the superposition of the photon ring and lensing ring, appears outside the shadow but persists only during the initial and final stages of accretion, vanishing entirely when accretion becomes active. Interestingly, as the accretion process progresses, an additional ring-like structure, which is caused by the dynamical redshift effect, emerges in the image. This ring-like structure not only contracts inward but also brightens continuously as accretion proceeds. Under varying observational inclinations, the Doppler effect and the dynamical redshift effect jointly modulate the brightness distribution of the image, resulting in significant asymmetry in the inner shadow, bright ring, and additional ring. Our findings uncover dynamical redshift as a novel observable phenomenon intrinsic to evolving spacetimes, offering a potential discriminant for identifying accreting black holes and providing observational access to the imprints of temporal spacetime evolution on black hole images.

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arXiv2026

Optical images of Kerr-Sen black hole illuminated by thick accretion disks

This paper investigates the shadow and polarization images of a Kerr-Sen black hole illuminated by geometrically thick and optically thin accretion disks. We adopt two classes of accretion models, namely the phenomenological radiatively inefficient accretion flow (RIAF) model and the analytical ballistic approximation accretion flow (BAAF) model. Based on radiative transfer theory, we examine the effects of the spin parameter $a$, black hole charge $Q$, and observer inclination angle $θ$ on the shadow images. Both models show that, as the charge $Q$ increases, the photon rings and the central dark regions shrink simultaneously. Meanwhile, frame dragging gives rise to a pronounced brightness asymmetry, which becomes more significant with increasing $a$ and $θ$. The main difference between isotropic and anisotropic radiation is that, in the latter case, the higher order images are brighter in the upper and lower polar regions. For the BAAF model, because the conical approximation renders certain regions geometrically thinner, the spatial extent of the higher order images is narrower than that in the RIAF model, and the separation between the direct image and the higher order images is more distinct. In the polarization images, the spatial distribution of the polarization vector directions is mainly determined by gravitational lensing and frame dragging, whereas the intensity near the photon ring and the scale of the higher order images are significantly influenced by $Q$.

astro-ph.HE↗

arXiv2026

Polarized Radiative Transfer of Kerr-Newman Black Hole

In this analysis, we investigate the polarization radiation imaging of Kerr-Newman black holes, with a particular focus on the impact of black hole charge on photon propagation and polarization characteristics. By extending the traditional Walker-Penrose method, which is limited by its reliance on specific symmetric structures and Killing tensors, we overcome these limitations by constructing an ordinary differential equations (ODEs) numerical framework that combines the photon orbit equation with the polarization parallel transport equation. This allows for the self-consistent evolution of photon trajectories and polarization states in any spacetime backgrounds without relying on specific symmetries. Using this framework, we analyze the effects of black hole spin and charge on the polarization characteristics of radiation from both prograde and retrograde accretion disks. Our results show that black hole charge can significantly modify photon trajectories and polarization patterns: increasing charge compresses and distorts the EVPA structure on photon-ring scales, inducing localized rotations and asymmetries that may provide a potential diagnostic of a nonzero black hole charge.

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arXiv2026

Extracting Energy from Magnetized Rotating Black Holes in Horndeski Gravity via the Magnetic Penrose Process

In Horndeski gravity, we investigate how to extract energy from a rotating black hole immersed in a uniform magnetic field $B$ based on the Magnetic Penrose Process. We map the ergosphere and negative energy regions of this spacetime, and analyze the relationship between the energy extraction efficiency and the hair parameter through both theoretical analysis and numerical simulations. The results show that the larger the hair parameter $h$, the smaller the ergosphere and negative energy regions of the black hole. For the same decay radius, in the case of $\hat{q} B \geq 0$, if the decay radius $r_x > 2$, the efficiency decreases as $h$ increases; if $r_x < 2$, the efficiency increases as $h$ increases; if $r_x = 2$, the efficiency is independent of $h$. However, when $\hat{q}B < 0$, except for the special case $r_x = 2$ where the efficiency is independent of $h$, the variation of efficiency with $h$ depends on the specific values of $r_x$ and $\hat{q}B$, and may exhibit either monotonic decrease or an initial increase followed by a decrease. We also find that in the absence of a magnetic field, the efficiency is negative and meaningless when $r_x > 2$, and such cases are excluded. In addition, when $\hat{q} B \geq 0$, the larger the $h$, the lower the maximum efficiency; when $\hat{q} B < 0$, in the case of a small magnetic field, the efficiency is negative and meaningless, while in the case of a large magnetic field, the efficiency of the black hole with hair is positive at high decay radius and reaches a high value, whereas the efficiency of the Kerr black hole remains negative.

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arXiv2026

Shadows and Polarization Images of a Four-dimensional Gauss-Bonnet Black Hole Irradiated by a Thick Accretion Disk

We adopt a general relativistic ray-tracing approach to study the shadows and polarization images of spherically symmetric Gauss-Bonnet (GB) black holes enveloped by geometrically thick accretion flows. Specifically, we adopt a phenomenological RIAF-like model and an analytical Hou disk model. In the RIAF-like model, increasing the GB coupling parameter $λ$ reduces both the size and brightness of the higher-order image, while increasing $θ$ alters the shape of the higher-order image and obscures the horizon's outline. The main difference between isotropic and anisotropic emission is that the latter produce distortion of the high-order image in the vertical direction, leading to an elliptical morphology. For the Hou disk model, due to specific regions being geometrically thinner with the conical approximation, the high-order images are narrower with the increase in $λ$ than the RIAF model. While increasing $θ$ enhances the brightness of the direct images outside the higher-order images, but hardly changes the size of the higher-order images, which is in sharp contrast to the RIAF model. Meanwhile, the Hou disk produces polarization patterns that trace the brightness configuration and are affected by $λ$ and $θ$, reflecting the intrinsic structure of spacetime. These results illustrate that intensity and polarization in thick-disk models provide probes of GB black holes and near-horizon accretion dynamics.

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arXiv2026

Repetitive Penrose process in Kerr-de Sitter black holes

Recently, references [1,2] found that the repetitive Penrose process cannot extract all the extractable rotational energy of a Kerr black hole, and reference [3] found that the repetitive electric Penrose process cannot extract all the electrical energy of a Reissner-Nordström (RN) black hole. This suggests that a law analogous to the third law of thermodynamics exists for the repetitive Penrose process. In this paper, we intend to study the repetitive Penrose process in the Kerr-de Sitter (Kerr-dS) black hole. We will explore influences of the cosmological parameter on the repetitive Penrose process. The results show that, in addition to a similar third law of thermodynamics, the Kerr-dS black hole yields a higher energy return on investment (EROI) and single-extraction energy capability compared to the Kerr black hole. Specifically, the larger the cosmological parameter, the stronger the EROI and the single-extraction energy capability. Furthermore, we also find that at a lower decay radius, the Kerr black hole exhibits a higher energy utilization efficiency (EUE) and more extracted energy after the repetitive Penrose process is completed. However, at a higher decay radius, the situation is reversed, i.e., the Kerr-dS black hole exhibits a higher EUE and more extracted energy, which is due to the existence of stopping condition of the iteration.

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