arXiv subjects

Chen-Yu Yang

Publications and source records attributed to Chen-Yu Yang.

At least 19 recordsLinked to original sources

arXiv2026

Polarization Images of Neutron Stars Illuminated by a Thin Accretion Disk: A Comparison with Black Holes

We investigate linear polarization images of static, spherically symmetric neutron stars illuminated by a geometrically and optically thin accretion disk. Neutron star equilibrium configurations are constructed with a polytropic equation of state, and the null geodesic equations and the parallel transport equation for the linear polarization vector are solved in the geometric optics approximation. The numerical results show that the total polarized intensity is generally positively correlated with the total intensity and reaches its maximum near the neutron star surface. As the observer inclination increases, the symmetry of the polarization images is progressively broken. In addition, the magnetic field configuration mainly affects the direction of the polarization vectors, while its influence on the overall polarized intensity distribution is comparatively limited. To further characterize the spatial structure of the polarization direction, we introduce the net electric vector position angle $\chi_{\mathrm{net}}$ and the second azimuthal Fourier mode $\angle\beta_2$. A comparison with polarization images of a Schwarzschild black hole reveals clear differences between the two types of compact objects in the locations of strongly polarized regions and the size of the central region without a polarization signal. These results show that linear polarization images provide information beyond total intensity images for distinguishing neutron stars from black holes.

gr-qc↗

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 $B_\theta$ 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

Horizon-scale intensity and polarization images of rotating Konoplya-Zhidenko black holes with thick accretion flows

We investigate the shadow and polarization images of a Konoplya-Zhidenko rotating non-Kerr black hole surrounded by a geometrically thick and optically thin accretion flow. The accretion flow is described by an analytical ballistic approximation accretion flow model. The numerical results show that the shadow image exhibits two main features, an outer bright ring and an inner dark region. The former corresponds to higher order images, while the latter is produced by the black hole event horizon. Increasing the deformation parameter $\eta$ does not significantly change the overall shape of the higher order images, but it enlarges their size. Increasing the spin parameter $a$ and the observer inclination angle $\theta_o$ enhances the asymmetry of the higher order images and makes the intensity on the left side much larger than that on the right side. This behavior is associated with frame dragging and the relativistic Doppler effect. In the polarization images, the degree of linear polarization is much smaller in the higher-order image region than in other regions, and the polarization vectors extend over the whole image plane. These results indicate that the thick disk model produces features in both intensity and polarization images that differ markedly from those in thin disk models. Within the framework used in this work, the observed intensity and polarization signatures can serve as effective probes of the underlying spacetime geometry and near horizon accretion dynamics.

gr-qc↗

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

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 $\theta_o$ 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 $\theta_o$. The increase of the GB coupling constant $\xi$ 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 $\theta_o$, while it is less affected by $\xi$ and $a$. For polarization images, the influence of $\xi$ 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 $\xi$. 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.

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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 $\theta$ 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 $\theta$. 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

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 $\theta_o$ 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 $\theta_o$ 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.

gr-qc↗

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 $p = K \rho^{1 + 1/n_c}$. 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 $\theta_o$ 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.

gr-qc↗

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 $\eta$, 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 $\eta$, 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.

gr-qc↗

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 $\hat{\lambda}$ or $\hat{\gamma}$ results in decrease the size of the higher-order image, while increasing values of observer inclination $\theta_{o}$ 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 $\hat{\lambda}$ and $\hat{\gamma}$ depicts the intrinsic structure of the space-time and $\theta_o$ depends on the observers orientation, together they shape the polarization features.

gr-qc↗

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, $\varsigma$ 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 $\varsigma$ and $\varpi$. The electric vector position angle is mainly affected by the parameters $\varsigma$ and $\varpi$.

gr-qc↗

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 $\phi_0$, the magnetic charge $\mathcal{G}$, and the observation angle $\theta_o$, on the image structure. As compact horizonless objects, boson stars produce optical images dominated by direct emission, while their morphology undergoes significant distortions as $\theta_o$ increases. Higher values of $\phi_0$ and $\theta_o$ 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.

gr-qc↗

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.

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

Optical Characteristics of the Kerr-Bertotti-Robinson Black Hole

The Kerr-Bertotti-Robinson (Kerr-BR) black hole, a theoretical model of a rotating black hole immersed in a uniform magnetic field, has been proposed recently by Podolsky and Ovcharenko. This study investigates the optical characteristics of the Kerr-BR black hole based on the exact solution. We analyze the optical image under two illumination models: a celestial light source and a geometrically thin accretion disk. We reveal distinct roles for the fundamental parameters in the model. Specifically, it is found that under both illumination models, the effect of the rotation parameter on the optical image of the Kerr-BR black hole is significantly different from that of the magnetic field. As the magnetic field increases, the radii of both the shadow and the Einstein ring enlarge. We also attempt to use the data from M87* and Sgr A* to constrain the magnetic field. These results enhance our understanding of the optical characteristics of the Kerr-BR black hole and establish a theoretical foundation for interpreting future observations on the optical image of the black hole immersed in a uniform magnetic field. Finally, we point out that with advances in the resolution of black hole images, it is possible to detect potential BR-like magnetic fields around black holes.

gr-qc↗

arXiv2025

Kerr-like Black Hole Surrounded by Cold Dark Matter Halo: The Shadow Images and EHT Constraints

Here we provide shadow images of a Kerr-like black hole (BH) in cold dark matter (CDM) halo illuminated with a celestial light source and a thin accretion disk. The impact of spin parameter, critical density and the scale radius on the observed images of BHs is carefully addressed. The results indicate that as spin parameter increases, the circular orbits are shifted rightwards, while the larger values of critical density and the scale radius are the cause to enhance the radius of circular orbits of the BH shadow. In the case of the celestial light source, the impact of critical density on shadow distortion is negligible, but this influence is relatively smaller and becomes appreciable when the scale radius has larger values. Next, we discuss the intensity and the size of the inner shadow, which are gradually increasing with the increase of both parameters. On the other hand, in the case of retrograde flow, the intensity of the shadow images significantly decreases, and a crescent moon emerges on the upper right side of the screen. Subsequently, the distinctive features of red-shift factors for direct and lensed images with prograde and retrograde flows are discussed. The outcomes indicate that the distribution of red-shift factors and the optical appearance are closely related to the behavior of accreting flow as well as to relevant parameters.

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 $\alpha$) with an initial scalar field $\psi_0$, the images on the screen predominantly show direct and lensed images, where $\psi_0$ and $\alpha$ modulate the image region size while the observation inclination $\theta$ controls morphological asymmetry. In the case of strong coupling (small value of $\alpha$), 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($\theta<30^{\circ})$, the redshift is the dominant effect, the image is characterized by a dim central cavity enclosed by a bright ring. At high inclination ($\theta>60^{\circ})$, 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↗