arXiv subjects

Ke-Jian He

Publications and source records attributed to Ke-Jian He.

At least 19 recordsLinked to original sources

arXiv2026

Energy Extraction via Magnetic Reconnection from Rotating ModMax Black Holes

Magnetic reconnection has been widely recognized as an important mechanism for extracting energy from rapidly rotating black holes. We find that magnetic reconnection can also efficiently extract energy from slowly rotating ModMax black holes. In this paper, we investigate the magnetic reconnection process of ModMax black holes in both circular and plunging orbits. First, we analyze the fundamental characteristic quantities of the ModMax black hole, including the event horizon, ergosphere radius, and circular photon orbits. The results indicate that the charge parameter $Q$ and screening factor $γ$ exhibit a competing effect on the fundamental characteristic quantities of the ModMax black hole. Furthermore, for the extremal ModMax black hole, a larger $Q$ and a smaller $γ$ lower the minimum allowed spin parameter $a$. Subsequently, we analyze the parameter space $(r,a)$ for energy extraction in circular and plunging orbits. A larger $Q$ and a smaller $γ$ reduce the region for energy extraction and decrease the minimum spin parameter $a$ required for energy extraction. For circular orbits, the minimum allowed spin parameter is $a\simeq 0.50071$, while for plunging orbits, this threshold further decreases to $a\simeq0.22407$. This proves that energy extraction from lower spin ModMax black holes is theoretically feasible. Finally, we compare the energy extraction region, power, and efficiency between circular and plunging orbits. It is found that these quantities in plunging orbits are always higher than those in circular orbits, indicating that energy extraction from plunging orbits via magnetic reconnection may be more efficient.

gr-qc↗

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

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.

gr-qc↗

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.

gr-qc↗

arXiv2026

Unveiling Inner Shadows and Polarization Signatures of Rotating Einstein-Gauss-Bonnet Black Holes

Based on the backward ray-tracing method, this paper numerically investigates the shadow and polarization images of rotating Einstein-Gauss-Bonnet (EGB) black hole within the framework of a thin disk model. We systematically analyze the effects of the main model parameters and the observation inclination angle $θ_o$ on both types of images. The results show that, as an intrinsic property of the black hole, the inner shadow undergoes significant deformation with increasing $θ_o$. The increase of the GB coupling constant $ξ$ only reduces the size of the inner shadow, while the spin parameter a does not alter its size but also its shape. And, the photon ring is more sensitive to variations in $θ_o$, while it is less affected by $ξ$ and $a$. For polarization images, the influence of $ξ$ on the polarization intensity is generally consistent with that observed in the accretion disk images. However, the polarization direction near the region of the inner shadow and photon ring changes significantly with $ξ$. This feature can provide an additional and effective observational tool for extracting information about the spacetime structure in Einstein-Gauss-Bonnet (EGB) gravity. Finally, we conclude that, compared to previous reliance on either accretion disk or polarization images alone, the simultaneous combination and synergistic analysis of both can more profoundly reveal the optical properties of rotating EGB black holes, providing a stronger theoretical basis for identifying such black holes through future high-resolution observations.

gr-qc↗

arXiv2026

Probing Observable Features of Lorentz violation in Low-Energy Hořava Gravity with Accretion Disk Images of Black Hole

In this paper, we study the observable signatures of Lorentz violation (LV) in low-energy Horava gravity by simulating the images and polarization features of rotating LV black holes using a backward ray-tracing method. Within a thin-disk accretion model and the ZAMO framework, we numerically solve the geodesics equation of photon and simulate the corresponding thin-disk images and polarization patterns. The results show that the LV parameter l strongly affects the inner shadow, brightness asymmetry, and polarization properties of the thin disk. The decrease of l leads to a more elliptical and untilted inner shadow, while increasing l produces a pronounced leftward D-shaped structure of the critical curve. In addition, the variation of l alters the distribution of polarized intensity and polarization direction, especially near the critical curve. Moreover, it also shows that a positive l enhances the black hole's angular velocity, while a negative one suppresses it, indicating that the sign of l determines the trend direction of the LV effect. These findings suggest that future high-resolution EHT observations combining the thin-disk images and polarization patterns could provide valuable tests of the LV effect.

gr-qc↗

arXiv2026

Energy Extraction from Rotating Charged Black Holes in Kalb-Ramond Gravity

This work presents a comprehensive study of energy extraction via the Comisso-Asenjo magnetic reconnection mechanism from rotating charged black holes in the context of Kalb-Ramond (KR) gravity. We systematically investigate the influence of various parameters on the energy extraction process, comparing the results in two distinct regions: the circular orbit region and the plunging region. {The results reveal that the Lorentz-violating parameter has a significant impact on energy extraction, affecting not only the parameter space where energy extraction is possible, but also the energy extraction power and efficiency.} It is found that the energy extraction process in the circular orbit region can offer a promising avenue for constraining KR gravity. In contrast, although energy extraction from the plunging region remains feasible even for black holes with relatively low spins and takes place nearer to the event horizon, its sensitivity to the Lorentz-violating parameter is significantly reduced. Overall, the Comisso-Asenjo magnetic reconnection mechanism can serve as a probe of the KR field, particularly through the energy extraction process in the circular orbit region.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2025

Observational features of massive boson stars with thin disk accretion

In this paper, based on the action of a complex scalar field minimally coupled to a gravitational field, we numerically obtain a series of massive boson star solutions in a spherically symmetric background with a quartic-order self-interaction potential. Then, considering a thin accretion flow with a certain four-velocity, we further investigate the observable appearance of the boson star using the ray-tracing method and stereographic projection technique. As a horizonless compact object, the boson star's thin disk images clearly exhibit multiple light rings and a dark central region, with up to five bright rings. As the observer's position changes, the light rings of some boson stars deform into a symmetrical "horseshoe" or "crescent" shape. When the emitted profile varies, the images may display distinct observational signatures of a "Central Emission Region". Meanwhile, it shows that the corresponding polarized images not only reveal the spacetime features of boson stars but also reflect the properties of the accretion disk and its magnetic field structure. By comparing with black hole, we find that both the polarized signatures and thin disk images can effectively provide a possible basis for distinguishing boson stars from black holes. However, within the current resolution limits of the Event Horizon Telescope (EHT), boson stars may still closely mimic the appearance of black holes, making them challenging to distinguish at this stage.

gr-qc↗

arXiv2025

A Novel Jet Model for the Novikov-Thorne Disk and its Observable Impact

Recent high-resolution observations have established a strong link between black hole jets and accretion disk structures, particularly in the 3.5 mm wavelength band [Nature. 616, 686 (2023)]. In this work, we propose a ``jet-modified Novikov-Thorne disk model'' that explicitly incorporates jet luminosity into the accretion disk radiation framework. By integrating synchrotron radiation from relativistic electrons in the jet, we derive a modified luminosity function that accounts for both the accretion disk and jet contributions. Our analysis demonstrates that the inclusion of jet luminosity enhances the total accretion disk luminosity by approximately 33.5\%, as derived from the integration of radiative flux. Furthermore, we compare our modified model with the standard Novikov-Thorne model and find that the jet contribution remains significant across different observational inclinations. These results highlight the necessity of incorporating jet effects when estimating the observable flux of black hole accretion systems, which has direct implications for future astronomical observations.

astro-ph.HE↗

arXiv2025

Holographic image features of an AdS black hole in Einstein-power-Yang-Mills gravity

By utilizing the AdS/CFT correspondence, we investigate the holographic image of an AdS black hole in Einstein-power-Yang-Mills gravity. The AdS boundary hosts a Gaussian oscillation source, which induces a lensed response on the opposite side of the boundary during propagation through bulk spacetime. The optical system assists observers at the north pole to continuously capture holographic images that show an axisymmetric bright ring known as the Einstein ring. As the observation position shifted, the bright ring gradually transformed into a luminous arc and eventually transitioned into a light point. Simultaneously, we examine the impact of variations in relevant physical quantity on the ring, and present the corresponding brightness curve. The results indicate that as the temperature $T$ and nonlinear Yang Mills charge parameter $q$ increase, the ring radius also increases, while an increase in the chemical potential $u$ leads to a decrease. However, the peak brightness curve of the ring invariably decreases as the values of $T$, $u$, and $q$ increase, albeit to varying degrees. Upon comparing the outcomes of geometric optics, it can be observed that the position of the ring in holography images is consistent with that of the photon ring.

hep-th↗

arXiv2025

The shadow and accretion disk images of the rotation loop quantum black bounce

In this paper, we study the shadow and observational image of the Kerr-like Loop Quantum Gravity (LQG) inspired black bounce with the help of the celestial light source and the thin disk source by employing the backward ray-tracing method. The results indicate that both the LQG parameter alpha and the rotation parameter a contribute to a reduction in the shadow size; however, the influence of a is predominant, while the effect of alpha circular orbit. One can find that the correlation parameter (a, alpha), along with the observer's inclination angle, affect the image's asymmetry and the distortion of the inner shadow. As the inclination increases, the direct and lensed images diverge, creating a structure resembling a hat. Meanwhile, we also investigate the redshift distribution of the direct lensed images of the accretion disk under different parameters and observation angle. The results show that the distribution of redshift and observed intensity is obviously related to the behavior of accretion flow. These results may provide a potential approach to limit black hole parameters, detect quantum gravity effects, and distinguish the LQG black hole from other black hole models.

gr-qc↗

arXiv2025

Holographic Einstein Ring of Quantum Corrected AdS-Reissner-Nordstrom Black Holes in Kiselev Spacetime

This study, grounded in AdS/CFT correspondence, utilizes wave optics theory to explore the Einstein ring of a quantum-corrected AdS-Reissner-Nordström black hole (BH) in Kiselev spacetime. By fixing the wave source on the AdS boundary, the corresponding response function generated on the antipodal side of the boundary is successfully obtained. Using a virtual optical system with a convex lens, the holographic image of the Einstein ring of the BH is captured on a screen. The study also investigates the impact of various physical parameters and the observer's position on the characteristics of the Einstein ring. The results indicate that changes in the observer's position cause the image to transition from an axisymmetric ring to an arc, ultimately converging to a single luminous point. Additionally, the Einstein ring radius decreases with increasing values of the quantum correction parameter $a$, the equation of state parameter $Ω$, temperature $T$, and chemical potential $μ$ , respectively. In contrast, the ring radius increases as the cosmological fluid parameter $c$ increases. Furthermore, the ring radius becomes more distinct as the wave source frequency $ω$ increases. From the perspective of geometric optics, the photon ring of the quantum-corrected AdS-Reissner-Nordström BH in Kiselev spacetime is further studied. Numerical results suggest that the incident angle of the photon ring aligns with that of the Einstein ring.

gr-qc↗

arXiv2024

Holographic Einstein Ring of Deformed AdS-Schwarzschild Black Holes

In this work, the wave optics is employed to investigate the Einstein ring of a deformed AdS-Schwarzschild black hole (BH). When the source is fixed on the AdS boundary, one can obtain the corresponding response function generated on the antipodal side of the boundary. By utilizing a virtual optical system equipped with a convex lens, we are able to capture an image of the BH's holographic Einstein ring on the screen. The influence of the relevant physical parameters and the observer's position on the characteristics of the Einstein ring is also investigated, revealing that variations in the observer's position result in a transition of the displayed image from an axisymmetric ring to an arc, ultimately converging into a solitary point of luminosity. In addition, variations in the relevant physical parameters naturally exert influences on the Einstein ring. The photon ring of the BH was also investigated from a geometric optics perspective, and the numerical results indicate that the incident angle of the photon ring aligns with that of the Einstein ring. In the context of modified gravity theories, the investigation of Einstein rings formed by deformed AdS-Schwarzschild BH is expected to not only contribute to advancing the development of gravitational theories but also facilitate a more comprehensive understanding of spacetime geometry and the physical properties of BHs, thereby distinguishing them from Schwarzschild BH.

hep-th↗