arXiv subjects

Xiao-Xiong Zeng

Publications and source records attributed to Xiao-Xiong Zeng.

At least 37 records · Page 2Linked to original sources

arXiv2026

Repetitive Penrose Process in Accelerating Kerr Black Holes

This paper investigates the repetitive Penrose process in accelerating Kerr black holes and explores the influence of the acceleration factor on the repetitive Penrose process. After a brief review of accelerating Kerr black holes, we study the fundamental equations of the Penrose process in this spacetime, examine the stopping conditions required for the repetitive Penrose process, and obtain corresponding numerical results. The conclusions indicate that, apart from the third law of thermodynamics similar to previous cases, accelerating Kerr black holes exhibit stronger energy extraction capabilities compared to Kerr black holes during the repetitive Penrose process. Moreover, in prior studies, the energy utilization efficiency was difficult to exceed $50\%$. However, in accelerating Kerr black holes, when the decay radius is relatively small, the energy utilization efficiency can exceed $50\%$, indicating that the reduced extractable energy primarily transforms into extracted energy rather than irreducible mass. On the other hand, when the initial value of the acceleration factor is large, the extractable energy can decrease to nearly zero, which also differs from the case of Kerr black holes in previous studies.

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arXiv2026

Distinguishing Black Holes and Neutron Stars through Optical Images

This paper employs the backward ray tracing method to study the optical images of neutron stars under the conditions of a spherical light source and a thin accretion disk, considering a polynomial equation of state given by $p = K ρ^{1 + 1/n_c}$. By numerically solving the TOV equations, we obtain the interior solutions of neutron stars for different densities. The results indicate that as the polynomial index $n_c$ increases, the mass, radius, and compactness of the neutron star all increase, which has a significant impact on its optical properties. Under the assumption that the light is truncated at the surface of the neutron star, we find that for a spherical light source, an increase in $n_c$ leads to an enlargement of the Einstein ring radius. For a thin accretion disk, the light intensity always reaches its maximum at the surface of the neutron star. The increase in $n_c$ also causes the outline of the neutron star to grow. When the observer inclination angle $θ_o$ changes, the neutron star's outline deforms from a circular shape to a D shape, with the left side being significantly brighter than the right side. In addition, this paper also investigates the distribution characteristics of the redshift factor. At lower observer inclination angles, gravitational redshift dominates, while at higher inclination angles, the Doppler effect induces noticeable blueshift. Compared to the Schwarzschild black hole, the optical appearance of the neutron star shows significant differences. The study provides a theoretical basis for distinguishing neutron stars from black holes using high-resolution imaging and for constraining the equation of state.

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arXiv2025

Imprints of Dark Matter on the Shadow and Polarization Images of a Black Hole Illuminated by Various Thick Disks

Based on two distinct thick accretion flow disk models, such as a phenomenological RIAF-like model and an analytical Hou disk model, we investigate the impact of relevant parameters on the visual characteristics of the Schwarzschild black hole (BH) surrounded by perfect fluid dark matter (PFDM). We impose a general relativistic radiative transfer equation to determine the synchrotron emission from thermal electrons and generate horizon-scale images. In the RIAF-like model, we notice that the corresponding photon ring and central dark region are expanded with the aid of the PFDM parameter $η$, with brightness asymmetries originating at higher inclination angles and closely tied to flow dynamics and emission anisotropy. The fundamental difference between isotropic and anisotropic radiation is that anisotropy introduces vertical distortions in the higher-order images, resulting in an elliptical appearance. For the Hou disk model, the observed images produce narrower rings and dark interiors, while polarization patterns trace the brightness distribution and changes with the variations of the inclination angle and PFDM parameter $η$, which reflects the spacetime signature. All these results indicate that the observed intensity and polarization characteristics in the framework of thick disk models may serve as valuable probes of underlying spacetime geometry and the accretion-dynamics close to the horizon.

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arXiv2025

Hotspot Image Driven by Magnetic Reconnection in Kerr-anti-de Sitter Black Holes

Based on the Comisso-Asenjo mechanism, we investigate the kinematic images of plasma before and after magnetic reconnection in Kerr-Anti-de Sitter(Kerr-AdS) black holes. Following a brief review of the Comisso-Asenjo process in Kerr-AdS black holes, we introduce the hotspot model and the imaging method. Building upon these foundational theories, we obtain the trajectory of the plasma and the temporal evolution of the hotspot images. It is found that there are three flares within the observing time, which is driven by the Comisso-Asenjo mechanism. We also discuss the influence of the cosmological parameter on the hotspot imaging. The results indicate that the hotspot image enlarges as the absolute value of $Λ$ increases, demonstrating that the cosmological constant significantly affects the hotspot.

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arXiv2025

Optical Images of Mini Boson Stars in Palatini $f(R)$ Gravity

We investigate the optical properties of mini boson stars within the framework of Palatini $f(R)$ gravity, adopting a quadratic form $f(R) = R + ξR^2$, where $ξ$ is the gravitational coupling constant. By deriving the modified scalar Lagrangian and solving the field equations numerically, we explore photon trajectories and the resulting optical images under spherical light sources and thin accretion disks. Unlike Schwarzschild black holes (BHs), boson stars lack stable photon rings due to the positive second derivative of their effective potential. Consequently, their images are dominated by direct emissions from photons completing a single orbit. The study examines the dependence of the optical characteristics on the initial scalar field $ψ_0$ and the coupling parameter $ξ$. Numerical results include effective potentials, redshift maps, and detailed imaging of boson stars, providing insights into distinguishing boson stars from black holes using high-resolution astronomical observations.

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arXiv2025

Imaging and polarization patterns of various thick disks around Kerr-MOG black holes

We investigate the imaging and polarization properties of Kerr-MOG black holes surrounded by geometrically thick accretion flows. The MOG parameter $α$ introduces deviations from the Kerr metric, providing a means to test modified gravity in the strong field regime. Two representative accretion models are considered: the phenomenological radiatively inefficient accretion flow (RIAF) and the analytical ballistic approximation accretion flow (BAAF). Using general relativistic radiative transfer, we compute synchrotron emission and polarization maps under different spins, MOG parameters, inclinations, and observing frequencies. In both models, the photon ring and central dark region expand with increasing $α$, whereas frame dragging produces pronounced brightness asymmetry. The BAAF model predicts a narrower bright ring and distinct polarization morphology near the event horizon. By introducing the net polarization angle $χ_{\text{net}}$ and the second Fourier mode $\angleβ_2$, we quantify inclination- and frame-dragging-induced polarization features. Our results reveal that both $α$ and spin significantly influence the near-horizon polarization patterns, suggesting that high-resolution polarimetric imaging could serve as a promising probe of modified gravity in the strong field regime.

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arXiv2025

Hotspot Images Driven by Magnetic Reconnection in Kerr-Sen black hole

In the Kerr-Sen black hole, this study investigates the changes in hotspot images before and after the occurrence of magnetic reconnection. After reviewing the Comisso-Asenjo magnetic reconnection process and introducing the hotspot imaging method, we examine the temporal evolution of hotspot intensity, including when energy extraction occurs, when it does not occur, and when the observer's azimuthal angle is altered. We also discuss the influence of the black hole's expansion parameter and spin on hotspot imaging. The results indicate that the first flare may serve as a potential signature of ongoing energy extraction: changing the observer's azimuthal angle may alter the time interval between the first and second flares: a larger expansion parameter makes it more difficult to identify the energy extraction signal, and a higher spin also makes it more challenging to detect the energy extraction signal.

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arXiv2025

Probing Non-rotating Black Hole in Kalb-Ramond Gravity: Imaging and Polarized Signatures Surrounded by Different Thick Accretion Flows

In this work, we consider a spherically symmetric static black hole metric in Kalb-Ramond (KR) gravity, and investigate the impact of relevant parameters on the black hole shadow and polarization images. For black hole shadow images, we consider two geometrically thick accretion disk models such as a phenomenological RIAF-like model and an analytical HOU disk model. In each case, we observe a bright ring-like structure corresponding to the higher-order images with a surrounding region of non-zero intensity that represents the primary image. The increasing values of $\hatλ$ or $\hatγ$ results in decrease the size of the higher-order image, while increasing values of observer inclination $θ_{o}$ alter its shape and cause the horizons outline to be obscured. On the other hand, in HOU disk model, at high observer inclinations, the obscuration of the horizons outline by radiation from outside the equatorial plane is weakened. Consequently, the brightness of the primary image in the phenomenological model is significantly greater than that in the HOU disk model, indicating the strong gravitational lensing effect. For the polarized images, we use only the HOU disk model with anisotropic radiation, assuming an infalling accretion flow matter. The obtained results illustrate that the polarization intensity $P_{o}$ in the higher-order image region is significantly stronger than as compare to other regions, and it is rapidly decreases away from this region. The variation in $\hatλ$ and $\hatγ$ depicts the intrinsic structure of the space-time and $θ_o$ depends on the observers orientation, together they shape the polarization features.

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arXiv2025

Shadow and Polarization Images of Rotating Black Holes in Kalb-Ramond Gravity Illuminated by Several Thick Accretion Disks

Using ray-tracing techniques, this paper investigates the optical and polarization images of rotating black holes in Kalb-Ramond (KR) gravity illuminated by thick accretion disks. We examine two accretion disk models: the phenomenological radiatively inefficient accretion flow (RIAF) model and the analytical ballistic approximation accretion flow (BAAF) model. The RIAF model incorporates both isotropic and anisotropic radiation. In all models, the external bright rings corresponding to the high-order image and the internal dark region associated with the event horizon are observed. At high observational inclinations, the inner shadows are obscured by the radiation from the equatorial plane, which is significantly different from the thin accretion disk model. The primary distinction between isotropic and anisotropic radiation is that the latter causes distortion of the high-order image in the vertical direction, resulting in an elliptical structure. For the BAAF model, due to certain regions are geometrically thinner under the conical approximation, the high-order images are narrower compared to the RIAF model. Furthermore, we find that an increase in the rotational parameter $a$ leads to an asymmetry in the intensity distribution of the high-order image, while an increase in the spontaneous Lorentz violating parameters, $ς$ and $\varpi$, results in a decrease in the size of the high-order image. In the polarization image, the linear polarization is found to be significantly influenced by the intensity, while it is relatively less affected by the parameters $ς$ and $\varpi$. The electric vector position angle is mainly affected by the parameters $ς$ and $\varpi$.

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arXiv2025

Observational features of the Bardeen-boson star with thin disk accretion

In this work, we construct spherically symmetric solutions of Bardeen--boson stars within the framework of the Einstein--Klein--Gordon theory coupled to nonlinear electrodynamics by employing numerical methods. Considering a thin accretion disk in the equatorial plane as the light source, we systematically investigate the optical appearance of boson stars using the ray-tracing method and the stereographic projection technique. Particular attention is paid to the influence of the initial scalar field $ϕ_0$, the magnetic charge $\mathcal{G}$, and the observation angle $θ_o$, on the image structure. As compact horizonless objects, boson stars produce optical images dominated by direct emission, while their morphology undergoes significant distortions as $θ_o$ increases. Higher values of $ϕ_0$ and $θ_o$ can give rise to lensing images. For all the parameters, the image center exhibits a brightness depression similar to the inner shadow of black holes, which poses challenges for distinguishing between boson stars and black holes. To address this, we propose two possible approaches: (i) combining the analysis of lensing bands with the effective potential to determine the existence of photon rings; and (ii) examining the polarization effects under synchrotron emission mechanisms. These results provide theoretical support for future high-resolution imaging efforts aimed at discriminating boson stars from black holes.

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arXiv2025

Hawking tunneling radiation with thermodynamic pressure

Hawking radiation elucidates black holes as quantum thermodynamic systems, thereby establishing a conceptual bridge between general relativity and quantum mechanics through particle emission phenomena. While conventional theoretical frameworks predominantly focus on classical spacetime configurations, recent advancements in Extended Phase Space thermodynamics have redefined cosmological parameters (such as the $Λ$-term) as dynamic variables. Notably, the thermodynamics of Anti-de Sitter (AdS) black holes has been successfully extended to incorporate thermodynamic pressure $P$. Within this extended phase space framework, although numerous intriguing physical phenomena have been identified, the tunneling mechanism of particles incorporating pressure and volume remains unexplored. This study investigates Hawking radiation through particle tunneling in Schwarzschild Anti-de Sitter black holes within the extended phase space, where the thermodynamic pressure $P$ is introduced via a dynamic cosmological constant $Λ$. By employing semi-classical tunneling calculations with self-gravitation corrections, we demonstrate that emission probabilities exhibit a direct correlation with variations in Bekenstein-Hawking entropy. Significantly, the radiation spectrum deviates from pure thermality, aligning with unitary quantum evolution while maintaining consistency with standard phase space results. Moreover, through thermodynamic analysis, we have verified that the emission rate of particles is related to the difference in Bekenstein-Hawking entropy of the emitted particles before and after they tunnel through the potential barrier. These findings establish particle tunneling as a unified probe of quantum gravitational effects in black hole thermodynamics.

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arXiv2025

Energy extraction from the accelerating Kerr black hole via magnetic reconnection in the plunging region and circular orbit region

Based on the magnetic reconnection mechanism, this study investigates how to extract energy effectively from an accelerating Kerr black hole in the plunging region and circular orbit region. After introducing the properties of accelerating black holes, including the event horizon, ergosphere, circular orbits, and innermost stable circular orbit, we investigate the magnetic reconnection process in the plunging region. Specifically, we analyze variations of the azimuthal angle with respect to the acceleration, examine changes in energy per enthalpy of decelerated plasma, and plot energy extraction efficiency along with permissible energy extraction regions. Results show that in the plunging region, at larger radii of reconnection locations, the accelerating black hole exhibits higher energy extraction efficiency than a Kerr black hole. Away from extremality, the acceleration parameter impedes energy extraction, while near extremality, it enhances extraction. We also study energy extraction in circular orbit region by plotting energy extraction efficiency within permissible regions. We find that the permissible energy extraction area is reduced and the efficiency exceeds that of Kerr black holes due to the existence of acceleration parameter. Larger acceleration parameters yield more effective energy extraction regardless of extremality, which is different from that in the plunging region. Additionally, energy extraction efficiency in the plunging region surpasses that in the circular orbit region, aligning with prior conclusions.

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arXiv2025

Optical images of massive boson stars with nonlinear electrodynamics

This study investigates the optical imaging characteristics of massive boson stars based on a model with Einstein's nonlinear electrodynamics. Under asymptotically flat boundary conditions, the field equations are solved numerically to obtain the spacetime metric of the massive boson stars. Employing the ray-tracing method, we analyze the optical images of the massive boson stars under two illumination conditions: a celestial light source and a thin accretion disk. The research reveals that the configurations and optical images of the massive boson stars can be tuned via the initial parameter $ϕ_0$ and the coupling constant $Λ$. The absence of the event horizon in the massive boson stars results in distinct optical image characteristics compared to those of black holes.

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arXiv2025

Probing Horndeski Gravity via Kerr Black Hole: Insights from Thin Accretion Disks and Shadows with EHT Observations

In this study, we have considered the Kerr-like black hole (BH) model in Horndeski gravity and analyse the visual characteristics of shadow images under two illumination models, such as a celestial light source and a thin accretion disk. To capture the BH shadow images, we utilise a recent fisheye camera model and ray-tracing procedures. In this view, we carefully addressed the influence of the spin parameter $a$ and the hair parameter $h$ on the BH shadow images. The results indicate that for smaller values of $h$, the BH shadow contours shift noticeably towards the right side of the screen, while for larger values of $h$, the nearly circular shadow gradually deforms into a ``D'' shape profile. For a celestial light source, the larger values of $h$ lead to a reduction in the corresponding radius of the photon ring, while the space-dragging effect becomes more pronounced with increasing $a$. We further discuss the distinctive characteristics of images observed in both prograde and retrograde accretion disk scenarios. The results reveal that variations in $h$ significantly affect both the inner shadow and the resulting Einstein ring. Subsequently, we also discussed the distinct features of red-shift configurations on the disk for both direct and lensed images, which are closely related to the accretion flow and the relevant parameters. We also attempt to use the recent observational data from M$87^{\ast}$ and Sgr $A^{\ast}$ and constrain the hair parameter $h$, confirming the validity of Horndeski gravity.

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arXiv2025

The observation image of a soliton boson star illuminated by various accretions

In this paper, we explore the observable signatures of solitonic boson stars by employing ray-tracing simulations, with celestial spheres and thin accretion disks serving as illumination sources. By numerically fitting the metric form, we solve the geodesic equation for photons under the influence of the soliton potential, enabling us to simulate the optical appearance of the soliton boson star in two distinct regimes. In the weak coupling case (larger value of coupling parameter $α$) with an initial scalar field $ψ_0$, the images on the screen predominantly show direct and lensed images, where $ψ_0$ and $α$ modulate the image region size while the observation inclination $θ$ controls morphological asymmetry. In the case of strong coupling (small value of $α$), the images on the screen show a nested sub-annulus within the Einstein ring in the celestial model, whereas thin disk accretion models reveal higher-order lensing images indicative that photons are capable of orbiting the equatorial plane of the boson star multiple times. We also analyze how the effective potential and redshift factor depend on the correlation parameter. At low inclination($θ<30^{\circ})$, the redshift is the dominant effect, the image is characterized by a dim central cavity enclosed by a bright ring. At high inclination ($θ>60^{\circ})$, the Doppler effect becomes more pronounced, resulting in a substantial brightness disparity between the left and right sides of the optical image. These findings offer robust theoretical underpinnings for differentiating solitonic boson stars from black holes via high-resolution astronomical observations.

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arXiv2025

Energy Extraction From the Kerr-Bertotti-Robinson Black Hole via Magnetic Reconnection under Circular Plasma and Plunging Plasma

Recently, a class of exact solutions describing rotating black holes immersed in a uniform magnetic field has been proposed, and various properties of such black holes remain unclear. This paper aims to explore how to extract energy based on the magnetic reconnection mechanism in both the circular orbit region and the plunging region. After introducing the properties of this spacetime, we analyze physical quantities such as the size of the ergoregion, the event horizon, and the boundaries of the ergosphere. We then analyze the magnetic reconnection process within circular orbits. We plot energy extraction parameter diagrams, and analyze the power and efficiency of energy extraction. The results show that the magnetic field impedes energy extraction. Comparing with Kerr black holes and Kerr-Melvin black holes, we find that this black hole's energy extraction capability is stronger than that of the Kerr-Melvin black hole but weaker than that of the Kerr black hole. Furthermore, we study magnetic reconnection and energy extraction in the plunging region of this black hole. The results indicate that the magnetic field still impedes energy extraction. The black hole's energy extraction capability in this region also remains weaker than that of a Kerr black hole but stronger than that of a Kerr-Melvin black hole. Comparing the plunging region with the circular orbit region, we find that the energy extraction power in the plunging region is consistently stronger than in the circular orbit region.

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arXiv2025

Polarization Images of Solitonic Boson Stars

This study investigates the polarization characteristics of solitonic boson stars surrounded by a thin accretion disk. By comparing their polarization images with corresponding optical images, we find a positive correlation between the polarization intensity distribution in the polarization images and the brightness in the optical images. Consequently, the strongest polarization occurs at the location corresponding to the direct image. The influence of the coupling strength of the sixtic potential on the polarization intensity distribution is not monotonic, under strong coupling, the polarization will concentrated on the left side of the image as the coupling strength increases, whereas under weak coupling, it is more evenly distributed across the entire direct image as the coupling strength increases. Moreover, we find that as the initial scalar field increases, both the lensing image and photon ring become more prominent. However, the polarization intensity at these regions remains weak. Due to the absence of the event horizon in solitonic boson stars, the polarization vector can penetrate the stellar interior, unlike in black holes, where no polarization signals exist within the event horizon. Our numerical simulations clearly reveal this phenomenon, suggesting that polarization features may serve as an effective tool for distinguishing solitonic boson stars from black holes.

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arXiv2025

Observational signatures and polarized images of rotating charged black holes in Kalb-Ramond Gravity

This paper investigates the shadow images of rotating charged black holes in Kalb-Ramond (KR) gravity, using an accretion disk that is both optically and geometrically thin and located on the equatorial plane as the light source model. The results show that, compared to Sgr A*, the observational data of M87* impose stronger constraints on the charge parameter Q and the Lorentz-violating parameter G. Under the thin accretion disk model, a larger observer inclination theta_o deforms the inner shadow into a hat-like structure. The parameters (a, Q, G) mainly affect the size of the inner shadow and the brightness of the critical curve, where increasing these parameters reduces the shadow size and enhances the distinguishability of the critical curve. In the retrograde accretion disk case, the gravitational redshift significantly reduces the observed brightness of the image. In addition, we compute the distribution of the redshift factor on the projection screen. The results indicate that the Doppler effect induced by large theta_o enhances the blueshift in the image, while the light emitted by particles plunging into the black hole leads to strong redshift near the inner shadow. Finally, we study the polarization images under synchrotron radiation and find that the polarization intensity P_o reaches its maximum around the lensed image and higher-order images, whereas no polarization vectors appear within the inner shadow. This stands in sharp contrast to horizonless compact objects. These findings contribute to a deeper understanding of the shadow properties of charged black holes within Lorentz-violating gravity.

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