Abstract Since the Event Horizon Telescope (EHT) collaboration released horizon-scale images of the supermassive black holes Sgr~A* and M87*, a new observational window for probing black hole spacetimes in the strong-gravity regime has opened. As an important class of Kerr black hole mimickers, rotating Simpson-Visser (SV) black holes exhibit a degeneracy with Kerr black holes at the level of shadow size, making it difficult to distinguish them using shadow observations alone.Motivated by this issue, we present a systematic investigation of the radiative properties and optical appearance of rotating SV black holes surrounded by a thin accretion disks, and mainly analyze the influence of the regularization parameter $g$ on related observables. The results show that although the kinetic quantities and the location of the innermost stable circular orbit (ISCO) depend on the regularization parameter $g$, the radiative efficiency of the rotating SV black hole is the same as its Kerr counterpart. Within the Novikov-Thorne thin-disk model, the radiative flux, effective temperature, and spectral luminosity are studied, and by adopting observational parameters relevant to Sgr~A* and M87*, concrete examples of the rotating SV black holes are calculated and compared with that of the Kerr black holes. The results show that the parameter $g$ suppresses the maximum values of these quantities. In addition, using a backward ray-tracing technique, we numerically simulate the optical appearance of rotating SV black holes and analyze the corresponding intensity images,
redshift and observed flux distributions. Our results show that these quantities are affected by $g$. In particular, as $g$ increases, the observed intensity is significantly suppressed and the photon ring region has remarkable increase in its width. Our findings suggest that accretion-disk-related observables may provide important avenues to distinguish rotating SV black holes and Kerr black holes, and offer theoretical guidance for future high-resolution observations.

Using general relativistic radiative transfer (GRRT) simulations, we investigate the bright ring features and polarization structures in images of the Kerr-Sen black hole associated with Sgr A*, as illuminated by 230 GHz thermal synchrotron emission from radiatively inefficient accretion flows (RIAF). Our findings reveal that an increase in the dilaton parameter leads to a shrinking of the bright ring, accompanied by enhancements in both its width and brightness. As the disk thickness grows, the bright ring's diameter and width both decrease. The brightness enhancement induced by the disk thickness is less prominent than that driven by the dilaton parameter. Comparing with the Event Horizon Telescope (EHT) observational data of SgrA*, we present the allowed ranges of black hole parameters, and find that effects of the disk thickness on the allowed parameter space are stronger than those of the observer's inclination. Furthermore, we analyze the coefficient β 2, associated with the two-fold rotational symmetry of the electric vector position angles (EVPA), to probe the polarization structure of the black hole images, and reveal that effects of the disk thickness on β 2 are much weaker than those from the dilaton parameter.

Abstract We study observational signatures of Einstein-Maxwell-Dilaton (EMD) black holes embedded in a cold, isotropic, dispersive plasma. Within geometric optics, we incorporate homogeneous and stratified plasma profiles to assess their impact on photon sphere location, shadow size, and weak gravitational lensing. The shadow radius shrinks monotonically with increasing plasma strength despite an outward photon sphere shift; at fixed dilaton coupling, larger charge suppresses the shadow, whereas stronger dilaton coupling enlarges it. Comparing predictions with Event Horizon Telescope (EHT) measurements of M87* and Sgr A*, we infer contiguous, model dependent ranges of plasma strength consistent with current bounds on ring size and asymmetry. In the weak field limit, dispersion enhances light deflection and increases the effective Einstein angle, total magnification, and image parity fractions. These shadow lensing trends delineate EMD parameter space compatible with present data and motivate targeted, multi frequency tests.

This paper examines gravitational lensing in Schwarzschild-like black holes (BHs) within Kalb–Ramond (KR) gravity incorporating cosmic string (CS) and cloud of strings (CoS) topological configurations. We present the first comprehensive analysis of photon deflection angles and magnification in both Schwarzschild-like BHs in KR gravity pierced by CSs (SKRCS) and Schwarzschild-like BHs in KR gravity with CoS (SKRCoS) spacetimes through dual approaches: perturbative expansions yielding approximate solutions and exact elliptic integral formulations providing complete mathematical descriptions. For CS configurations characterized by Lorentz violation (LV) and CS parameters, deflection angles exhibit systematic modifications through composite geometric factors, while CoS geometries demonstrate distinct characteristics through the composite parameter [Formula: see text] that fundamentally alter light propagation. Magnification analysis reveals distinctive critical curve positioning and amplitude scaling relationships enabling observational discrimination between exotic and conventional BH scenarios. Strong field analysis employing the Bozza–Tsukamoto formalism establishes logarithmic divergence coefficients characterizing scaling behavior near photon spheres. We demonstrate asymptotic local flatness of these spacetimes, ensuring validity of lensing calculations. Observational constraints from Solar System precision tests and Event Horizon Telescope data restrict LV parameters within stringent bounds, while galactic-scale observations permit expanded parameter ranges for CS and CoS configurations.

Abstract Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring’s peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin a * . We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor ∣ a * ∣ ≲ 0.2. This is consistent with the Blandford–Znajek model for M87’s jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

Abstract Observations of supermassive black holes by the Event Horizon Telescope reveal significant inhomogeneities, most likely related to density and magnetic field perturbations. To model these features, we conduct high-resolution 2D general-relativistic magnetohydrodynamic simulations of a Fishbone–Moncrief torus around a Kerr black hole using the black hole accretion code BHAC . We compare unperturbed accretion with a case featuring plasma density bubbles with pressure-balanced magnetic islands of different amplitudes. Power spectrum analysis of accretion time series, performed via the Blackman–Tukey method, shows that the perturbed case exhibits (1) steeper spectral indices compared to the unperturbed case, deviating from the characteristic 1/ ω noise spectrum, and (2) increased correlation times, providing evidence for absorption of macrostructures at the event horizon. Spatial autocorrelation analysis of near-horizon turbulence confirms larger energy-containing coherent structures in the perturbed case, altering the accretion rate. These results provide new insights for interpreting observations of supermassive black hole environments, where near-horizon turbulence may play a key role in the accretion process.

In this work, we investigate the shadow properties of the Kerr–Newman–Anti-de Sitter black hole coupled to nonlinear electrodynamics. The shadow is constructed by employing the celestial coordinate approach for an observer located at a finite distance, which is required due to the non-asymptotically flat structure of the spacetime. The size, distortion, area, and oblateness of the shadow are analyzed in terms of the black hole parameters, namely, the spin, the effective charge, and the nonlinearity parameter. We show that the nonlinear electrodynamics significantly modifies the photon region and therefore changes the shadow observables, while the rotation mainly controls the deformation of the silhouette. We further confront the theoretical results with the Event Horizon Telescope observations of M87* and Sgr A* in order to constrain the parameter space of the model. The allowed ranges of the effective charge depend sensitively on the nonlinearity parameter, and the combination of both sources leads to tighter and physically more consistent bounds. In addition, we study the energy emission rate derived from the shadow radius and the Hawking temperature and discuss how it is affected by the rotation and the nonlinear electromagnetic field. Our analysis shows that the considered black hole solution provides a consistent extension of the Kerr geometry in a non-asymptotically flat background and that the shadow observables can be used as an efficient tool to test the effects of nonlinear electrodynamics in strong gravity.

Abstract Sagittarius A* (Sgr A*) is a compact radio source at the Galactic center. Observations have confirmed that its mass is approximately (4.1 ± 0.4) × 10 6 M ⊙ , and Sgr A* is generally believed to be powered by gas accretion onto a supermassive black hole. Multifrequency radio observations of the pulsar J1745−2900, about 0.12 pc away from Sgr A*, reveal an unusually large Faraday rotation. Combined with X-ray observations, this indicates that there is a strong magnetic field (greater than 8 mG) leading to a low β plasma at large scales. We show that the gas starts to be captured by the black hole below tens of thousands of the Schwarzschild radii r S , where the gas pressure starts to dominate. Assuming that the accretion rate along magnetic fields at large scales decreases with the distance to the black hole following a power law, it is shown that, with an accretion disk below tens of r S , as revealed with the Event Horizon Telescope observations, there should be a supersonic wind above such a small accretion disk, and the accretion flow may be convection-dominated from tens of r S to tens of thousands of r S . Detailed modeling is warranted.

Abstract In this paper, we study the polymer black hole solution surrounded by a quintessence field. The influence of quintessence on the polymer black hole is investigated through its thermodynamic properties, such as the Hawking temperature, entropy, and specific heat, which allow us to address the question of thermodynamic stability. We then calculate bounds on the electromagnetic greybody factors and photon emission rates of the black hole, highlighting the interplay between quintessence and quantum gravity effects in determining these phenomena. We also examine the effects of quintessence and quantum gravity on the geodesics and shadows of massless particles around the black hole. Our results are further compared with observational data of both the Sagittarius A* and M87* black holes from the Event Horizon Telescope (EHT) collaboration.

We investigate the properties of black hole shadows in the renormalization group (RG) improved Bonanno–Reuter spacetime, incorporating quantum gravitational corrections via the scale-dependent parameter ( ω ˜ ) in a plasma medium. Light propagation in a non-uniform, pressureless plasma with a radial density profile is analyzed through modified equations of motion. The black hole shadow angular radius is computed, and its dependence on ω ˜ and the plasma index is analyzed. The analysis of specific limiting cases indicates systematic deviations of the black hole shadow relative to the classical Schwarzschild limit. Using Event Horizon Telescope (EHT) observations of Sgr A*, we place constraints on ω ˜ . Furthermore, within the considered parameter range, plasma and quantum-gravity effects exhibit an observational degeneracy, which future high-resolution measurements with the next-generation EHT are expected to break, thereby providing tighter constraints on the model parameters.

Abstract We present independent imaging analyses of Event Horizon Telescope (EHT) observations of the active galactic nuclei in radio galaxy Centaurus A and quasar 3C 279 using Generative Deep learning Image Reconstruction with Closure Terms (G en DIR e CT), a recently developed machine-learning framework built on conditional diffusion models that uses interferometric closure invariants as primary observables. For Centaurus A, our reconstruction reveals two prominent emission ridges (≃ 80 μas each) along the jet sheath with a brightness ratio of 1.4 ± 0.1 and an opening angle of 12.3 ± 0.3 deg. For 3C 279, we identify three distinct components in the image, with the southern jet ejecta on sub-parsec scale exhibiting a proper motion of 4.6 ± 1.0 μas over ≈ 5.39 days away from the northern components, corresponding to an apparent superluminal velocity of ≃ 10 ± 2 times light speed. These measurements are consistent with those reported by the EHT Collaboration. The results are significant because we demonstrate that: (1) imaging from interferometric aperture synthesis data, especially in VLBI and most acutely in extremely sparse arrays like the EHT, remains a severely ill-posed and challenging inverse problem, yet closure invariants preserve robust morphological information that can strongly constrain structural features, and (2) more importantly, closure-invariant imaging largely avoids calibration systematics, thus providing a fundamentally independent view of spatial structure with very high angular resolution. The generative nature of G en DIR e CT further allows us to sample and characterise clusters of plausible image solutions for each dataset. As a calibration-independent, generative imaging approach, G en DIR e CT offers a robust and truly independent blind-imaging tool for current and future VLBI experiments.

Abstract We investigate a rotating black hole embedded in a five-dimensional flat bulk by extending the Kerr metric through the Gürses-Gürsey line element. Employing Boyer-Lindquist coordinates, we reinterpret the black hole as a charged-like object in five dimensions and analyze its horizon structure and shadow morphology. Our results reveal that the shadow is shaped by an axially symmetric gravitational field modulated by an extrinsic curvature term arising from the higher-dimensional embedding. Simulations demonstrate that the visibility amplitude and shadow profile of the Gürses-Gürsey black hole align with Event Horizon Telescope observations of $$\hbox {M87}^{*}$$ M87 ∗ in the Kerr limit, while also allowing measurable deviations that could be probed by future high-resolution experiments.

Abstract We investigate how deviations from the Bekenstein–Hawking entropy modify black-hole spacetimes through the recently proposed entropy-geometry correspondence. For four representative modified entropies, namely Barrow, Rényi, Kaniadakis, and logarithmic, we derive the corresponding effective metrics and analyze their thermodynamic and topological classification using the off-shell free energy and winding numbers. We show that Barrow and Rényi entropies yield a single unstable sector with global charge $$W=-1$$ W = - 1 , while logarithmic and Kaniadakis corrections produce canceling defects with $$W=0$$ W = 0 , revealing topological structures absent in the Schwarzschild case. Using the modified metrics, we further calculate the photon-sphere radius and shadow size, showing that each modified entropy relation induces characteristic optical shifts. Thus, by comparing with Event Horizon Telescope observations of Sgr A $$^*$$ ∗ , we extract new bounds on all entropy-deformation parameters. Our results demonstrate that thermodynamic topology, together with photon-sphere phenomenology, offers a viable way to test generalized entropy frameworks and probe departures from the Bekenstein-Hawking area law.

Abstract The main goal of this work is to investigate how relevant quantum gravity corrections can be, at an effective level, in the geometry describing the exterior of a black hole, and whether such corrections can be tested observationally. For this purpose, we employ Bozza's method to calculate the deflection angle of light in presence of the strong gravitational field generated by an improved Schwarzschild-like black hole whose metric, regular throughout the entire spacetime, was derived using the improved Generalized Uncertainty Principle (GUP). This framework incorporates effective quantum gravity corrections that resolve the physical singularity inside the black hole, quantified by a model parameter | Qc |. In addition, the event horizon, the photon sphere, and the shadow radius receive modifications characterized by a second model parameter Qb . Using observational properties of the supermassive black holes Messier 87* and Sagittarius A * reported by the Event Horizon Telescope, we derive constraints on the parameter | Qb |, namely 0 ≤ | Qb | ≤ 0.3. To the best of our knowledge, these are the first constraints reported in the literature for this improved GUP parameter. Since | Qc | does not play a significant role in the correction of the shadow radius, it was not possible to impose restrictions on its allowed values, however, it is important to consider a non-zero | Qc | in order to avoid a black hole singularity.

We investigate the influence of a scalar field on particle dynamics and electromagnetic phenomena in the spacetime of a gravitational compact object described by the Fisher-Janis-Newman-Winicour (FJNW) solution. The motion of test particles is analyzed using the Hamilton-Jacobi formalism, with particular attention to the innermost stable circular orbit (ISCO). We further explore photon motion and black hole shadows, deriving analytic expressions for the photon sphere radius and critical impact parameter. Applying these results to Event Horizon Telescope (EHT) observations of M87 and Sgr A, we show that the predicted shadow sizes remain consistent with the measured values within the 1σ confidence interval, thereby constraining the scalar parameter. In addition, we obtain the analytical solution of Maxwell’s equations for the vector potential produced by a stationary current loop in the FJNW background. The resulting magnetic field exhibits a uniform structure in the interior region, reminiscent of the Wald solution, while in the exterior region it behaves like a dipolar field. We find that the external magnetic field strength decreases with increasing scalar parameter, in contrast to the tidal charge scenario of relativistic stars.

Black holes are among the most compelling predictions of general relativity (GR) and are now strongly supported by observations from gravitational-wave detectors and the Event Horizon Telescope (EHT). While standard black hole solutions suffer from central singularities, regular black holes avoid this issue by introducing a nonsingular core. In this work, we extend the Dymnikova regular black hole to higher dimensions using a smooth matter distribution. The resulting spacetime features a de Sitter-like core and two horizons. We analyze photon motion and show that circular photon orbits remain unstable, giving rise to a well-defined black hole shadow. Our results indicate that the shadow size grows with the black hole scale but decreases slightly as the number of dimensions increases. We also investigate thermodynamic properties, including Hawking temperature and energy emission, and find a strong dependence on dimensionality. Finally, we compare our model with EHT observations to place constraints on the parameters and highlight potential observational signatures of higher-dimensional regular black holes.

Abstract Very long baseline interferometry (VLBI) achieves the highest angular resolution in astronomy. VLBI measures corrupted Fourier components, known as visibilities. Reconstructing on-sky images from these visibilities is a challenging inverse problem, particularly for sparse arrays such as the Event Horizon Telescope (EHT) and the Very Long Baseline Array, where incomplete sampling and severe calibration errors introduce significant uncertainty in the image. To help guide convergence and control the uncertainty in image reconstructions, regularization on the space of images is utilized, such as enforcing smoothness or similarity to a fiducial image. Coupled with this regularization is the introduction of a new set of parameters that modulate its strength. We present a hierarchical Bayesian imaging approach (hierarchical interferometric Bayesian Imaging, HIBI) that enables the quantification of uncertainty for all parameters. Incorporating instrumental effects within HIBI is straightforward, allowing for simultaneous imaging and calibration of data. To showcase HIBI’s effectiveness and flexibility, we build a simple imaging model based on Markov random fields and demonstrate how different physical components can be included, e.g., black hole shadow size, and their uncertainties can be inferred. For example, while the original EHT publications were unable to constrain the ring width of M87*, HIBI measures a width of 9.3 ± 1.3 μ as. We apply HIBI to image and calibrate EHT synthetic data, real EHT observations of M87*, and multifrequency observations of OJ 287. Across these tests, HIBI accurately recovers a wide variety of image structures and quantifies their uncertainties. HIBI is publicly available in the Comrade VLBI software repository.

We investigate weak gravitational lensing and shadow properties of the Konoplya-Zhidenko black hole surrounded by perfect fluid dark matter and a plasma medium. Considering both homogeneous and non-homogeneous (singular isothermal sphere) plasma distributions, we analyze how the dark matter, the deformation parameter, and plasma frequency modify the deflection angle of light rays and black hole’s shadow radius. Our results show that plasma enhances light bending and reduces the shadow size, with the largest shadow in vacuum, followed by non-homogeneous plasma, and the smallest in homogeneous plasma. Comparison with Event Horizon Telescope observations of Sgr A* constrains the allowed parameter space, linking theoretical predictions to measured shadow radii. Additionally, we study Hawking temperature, specific heat, and Gibbs free energy, revealing the roles of perfect fluid dark matter and spacetime deformation in black hole stability and phase behavior. This analysis provides insights into the observational signatures and thermodynamic properties of deformed black holes embedded in realistic galactic environments.

Abstract In this paper we study the shadow and quasi-normal modes (QNMs) of a black hole (BH) surrounded by a dark matter halo with Hernquist-type density distribution, which was reported in Cardoso et al. (Phys. Rev. D 105(6):L061501, 2022). In astrophysical scenarios, we find that the shadow radius enlarges as the compactness of halo increases. Therefore, we obtain an upper bound for the compactness $$\mathcal{C}\le 0.092$$ C ≤ 0.092 with the Event Horizon Telescope (EHT) observations. We calculate axial gravitational QNMs of the galactic BH up to $$\mathcal{C}\sim \mathcal{O}(1)$$ C ∼ O ( 1 ) , and fit the redshift relative to Schwarzschild QNMs up to second order in the compactness (for $$\mathcal{C}\le 0.3)$$ C ≤ 0.3 ) . These highly redshifted QNMs, resulting from large compactness, are key to modeling the dark matter halo.

Abstract We study synchrotron polarization in spatially resolved horizon-scale images, such as those produced by the Event Horizon Telescope (EHT). In both general relativistic magnetohydrodynamic (GRMHD) simulations and simplified models of the black hole magnetosphere, the polarization angle, quantified by the complex observable ∠β 2 , depends strongly and systematically on the black hole spin. This relationship arises from the coupling between spin and the structure of the magnetic field in the emission region, and it can be computed analytically in the force-free limit. To explore this connection further, we develop a semianalytic inflow framework that solves the time-stationary axisymmetric equations of GRMHD in the black hole’s equatorial plane; this model can interpolate between the force-free and inertial regimes by varying the magnetization of the inflow. Our model demonstrates how finite inertia modifies the structure of the electromagnetic field and can be used to quantitatively predict the observed polarization pattern. By comparing reduced models, GRMHD simulations, and analytic limits, we show that the observed synchrotron polarization can serve as a robust diagnostic of spin under assumptions about Faraday rotation and the emission geometry. Applied to EHT data, the model disfavors high-spin configurations for both M87 ∗ and Sgr A ∗ , highlighting the potential of polarimetric imaging as a probe of both black hole spin and near-horizon plasma physics.