Abstract Quantum Improved Regular Kerr (QIRK) black hole is a rotating regular black hole model constructed based on the asymptotic safety method. The model eliminates the ring singularity and prevents the formation of closed timelike curves, while retaining well-defined thermodynamic properties. Given these properties, probing the observable features of the QIRK black hole is important. In this work, we numerically determine the region of parameter space in which the QIRK spacetime remains regular, admits an event horizon, and is free of closed timelike curves. Subsequently, we simulate images of a QIRK black hole surrounded by a thin accretion disk. We find the primary effect of the quantum correction parameter, $$\widetilde{\omega }$$ ω ~ , is a systematic reduction in the overall observed intensity, with only subtle effects on the image geometry. Using observational data from the Event Horizon Telescope (EHT) for Sgr A* and M87*, we further constrain the parameters of the QIRK black hole. Moreover, since there exist QIRK parameters that are free of singularities and can admit closed timelike curves, we investigate the images of CTCs under these conditions. These results reveal the distinctive observational features of the QIRK spacetime and provide a quantitative basis for assessing its viability as an astrophysical candidate.

In this work, we investigate a nonlinear electrodynamics model in the context of f(R,T) gravity. We begin by outlining the general features of the theory and analyzing the event horizon under conditions ensuring its real and positive definiteness. We then examine light trajectories, focusing on critical orbits, shadow radii, and geodesics of massless particles. The parameters α and β, associated with the nonlinear extension of the Reissner-Nordström spacetime, are constrained using observational data from the Event Horizon Telescope (EHT). Subsequently, we analyze the thermal aspects of the system, including Hawking temperature, entropy, and heat capacity. Quasinormal modes are computed for scalar, vector, tensor, and spinorial perturbations, with the corresponding time-domain profiles explored as well. Gravitational lensing is then studied in both weak and strong deflection limits, along with the stability of photon spheres. Finally, we examine additional topological aspects, including topological thermodynamics and the topological photon sphere.

Charged black holes in KR gravity: Weak deflection angle, shadow cast, quasinormal modes and neutrino annihilation

We investigate the influence of the generalized Compton wavelength (GCW), emerging from a three-dimensional dynamical quantum vacuum (3D DQV) on Schwarzschild-like black hole spacetimes. The GCW modifies the classical geometry through a deformation parameter ɛ , encoding quantum gravitational backreaction. We derive exact analytical expressions for the black hole shadow radius, photon sphere, and weak deflection angle, incorporating higher-order corrections and finite-distance effects of a black hole with generalized Compton effect (BHGCE). Using Event Horizon Telescope (EHT) data, constraints on ɛ are obtained: ɛ ∈ [ − 2 . 572 , 0 . 336 ] for Sgr. A* and ɛ ∈ [ − 2 . 070 , 0 . 620 ] for M87*, both consistent with general relativity yet allowing moderate deviations. Weak lensing analyses via the Keeton-Petters and Gauss–Bonnet formalisms further constrain ɛ ≈ 0 . 061 , aligning with solar system bounds. We compute the modified Hawking temperature, showing that positive ɛ suppresses black hole evaporation. Quasinormal mode frequencies in the eikonal limit are also derived, demonstrating that both the oscillation frequency and damping rate shift under GCW-induced corrections. Additionally, the gravitational redshift and scalar perturbation waveform exhibit deformations sensitive to ɛ . Our results highlight the GCW framework as a phenomenologically viable semiclassical model, offering testable predictions for upcoming gravitational wave and VLBI observations.

We present a Bayesian statistical analysis to evaluate the Nexus Paradigm (NP) of quantum gravity, using horizon-scale observations of supermassive black holes (SMBHs) Sagittarius A* (Sgr A*) and M87* from the Event Horizon Telescope (EHT). The NP predicts angular diameters for the dark depression, emission ring, and base diameter, which we compare to EHT measurements. Employing Gaussian likelihoods and priors informed by mass-to-distance ratio uncertainties, we compute the posterior distribution for the angular scale parameter θg, achieving a combined χ2≈0.0062 (four degrees of freedom) corresponding to a 4.37 σ (99.9972%) confidence level. Individual features show deviations <0.1 σ supporting the NP’s claim of 99th percentile agreement. Compared to General Relativity (GR), which predicts a shadow diameter inconsistent with the observed dark depression (χ2≈168, ~12.97 σ) the NP is favored with a Bayes factor of ~1036. These results validate the NP’s predictions and highlight its potential as a quantum gravity framework, though refined uncertainties and broader model comparisons are recommended.

A compact object illuminated by background radiation produces a dark silhouette. The edge of the silhouette or shadow (alternatively, the apparent boundary or the critical curve) is commonly determined by the presence of the photon sphere (or photon shell in the case of rotating spacetime), corresponding to the maximum of the effective potential for null geodesics. While this statement stands true for Kerr black holes, here we remark that the apparent boundary (as defined by Bardeen) forms under a more general condition. We demonstrate that a shadow forms if the effective potential of null geodesics has a positive finite upper bound and includes a region where photons are trapped or scattered. Our framework extends beyond conventional solutions, including but not limited to naked singularities. Furthermore, we clarify the difference between the apparent boundary of a dark shadow and the bright ring on the screen of a distant observer. These results provide a unified theoretical basis for interpreting observations from the Event Horizon Telescope (EHT) and guiding future efforts towards extreme-resolution observations of compact objects.

Abstract Recently, two new spherically symmetric black hole models with covariance have been proposed in effective quantum gravity. Based on these models, we use the modified Newman–Janis algorithm to generate two rotating quantum-corrected black hole solutions, characterized by three parameters, the mass M, the spin a, and the quantum parameter $$\zeta $$ ζ . To understand the effects of the quantum parameter $$\zeta $$ ζ on these two rotating black holes, we investigate in detail the horizons and static limit surfaces. By constraining the possible range of the parameters, we study the shadows cast by these rotating black holes. The results indicate that for both rotating BHs, the parameter $$\zeta $$ ζ mainly affects the shadow size in the non-extremal case, while it deforms the shadow shape by arising a cuspy edge in the near-extremal case. Through the presence of the cuspy edge in the shadow, we further discuss how to differentiate it from the shadows of other rotating quantum-corrected black holes. Utilizing the Event Horizon Telescope shadow observational results for M87* and Sgr A*, we set the black hole inclination angles to $$17^{\circ }$$ 17 ∘ , $$50^{\circ }$$ 50 ∘ , and $$90^{\circ }$$ 90 ∘ and subsequently calculate the angular diameter of the black hole shadows. Our analysis indicates that in the constrained parameter space for M87* and Sgr A*, the common parameter constraints obtained from the RBH-I are $$0.569246M< \zeta < 0.924954M$$ 0.569246 M < ζ < 0.924954 M . In contrast, the constraints from the RBH-II are $$0< \zeta < 3.018M$$ 0 < ζ < 3.018 M .

Abstract Recent observations of the supermassive black holes $$ M87^{*} $$ M 87 ∗ and Sgr $$\hbox {A}^{*}$$ A ∗ by the Event Horizon Telescope (EHT) have sparked intensified interest in studying the optical appearance of black holes (BHs). Inspired by this, we carry out a study on the optical features of Einstein–Euler–Heisenberg-Anti de Sitter/de Sitter (EEH-AdS/dS) BHs, including the trajectories of photons, shadow geometrical shape, energy emission rate, and deflection of light in this spacetime. Since, due to the nonlinear electrodynamics effects, photons propagate along null geodesics in an effective metric rather than the background metric, we first derive the effective metric of the EEH-AdS/dS BH. Then we study the null geodesics of the resulting effective metric and eventually compute the size of the EEH-AdS/dS BH shadow. To validate our results, we confront our results with the extracted information from EHT data of the supermassive BHs $$ M87^{*} $$ M 87 ∗ and estimate lower bounds for the shadow radius.

Abstract Event Horizon Telescope (EHT) observations of M87* provide a means of constraining the parameters of both the black hole and its surrounding plasma. However, the intrinsic variability of the emitting material introduces major sources of uncertainty, which complicates parameter inference. The precise nature of this variability remains uncertain, and previous studies have largely relied on general relativistic magnetohydrodynamic simulations to estimate its effects. Here, we fit a semianalytic, dual-cone model of the emitting plasma to multiple years of EHT observations to empirically assess the impact of intrinsic variability and improved array coverage on key measurements, including the black hole mass-to-distance ratio, spin, and viewing inclination. Despite substantial differences in the images of the two epochs, we find that the inferred mass-to-distance ratio remains stable and mutually consistent. The black hole spin is unconstrained for both observations, despite the improved baseline coverage in 2018. We show that intrinsic variability can contribute significantly to the inference error and that the inferred position angle and inclination of the black hole spin axis are discrepant between the two years. Our findings highlight both the promise and challenges of multiepoch EHT observations: while they can refine parameter constraints, they also reveal the limitations of simple parametric models in capturing the full source complexity. Our analysis—the first to fit semianalytic emission models to 2018 EHT observations—underscores the importance of quantifying data contributions from intrinsic variability in future high-resolution imaging studies of black hole environments and the role of repeated observations in quantifying these uncertainties.

Context. The Event Horizon Telescope (EHT) collaboration released in 2019 the first horizon-scale images of a black hole accretion flow, opening a novel route for plasma physics comprehension and gravitational tests. Although the present unresolved images deeply depend on the astrophysical properties of the accreted matter, general relativity predicts that they contain highly lensed observables, the so-called photon rings, embodying the effects of strong-field gravity. Aims. Focusing on the particular case of the supermassive black hole M87* and adopting a geometrically thin equatorial disc as a phenomenological configuration for the accreting matter, our goal is to study the degeneracy of space-time curvature and of physically motivated emission processes on plane-of-sky EHT-like images observed at 230 and 345 GHz. Methods. In a parametric framework, we simulated adaptively ray-traced images using the code GYOTO in various spherically symmetric space-time geometries for a comprehensive class of disc velocities and a library of realistic synchrotron emission profiles. We then extracted the width and the peak position of 1D intensity cross sections on the direct image and the first photon ring. Results. We show that among the investigated quantities, the most appropriate observables to probe the geometry are the peak positions of the first photon ring. Small geometric deviations can be unequivocally detected regardless of the motion of the disc, ranging from Keplerian rotation to radial infall, if the black hole mass-to-distance estimate is accurate up to around 2%, with the current uncertainty of 11% being just sufficient to access extreme deviations. Conlcusions. The equatorial set-up of this paper, which is favoured by present EHT observations of M87*, is adapted to modelling future measurements at higher observing frequencies, where absorption effects are negligible, and with higher resolution, indispensable to resolving the photon rings. Additional work is needed to investigate if our conclusions hold for more realistic disc configurations.

Abstract In this study, we investigate a Schwarzschild black hole surrounded by Dehnen-type dark matter. A comprehensive thermodynamic analysis of black holes is conducted, leading to the calculation of black hole remnants. We investigate the trajectory of light, establishing an upper limit for the parameters based on Event Horizon Telescope (EHT) observations of Sgr A*, ensuring that the black hole’s shadow resides within the allowed region. Furthermore, we derive the quasinormal modes (QNMs) for both scalar and electromagnetic perturbations. Utilizing a topological framework, we examine the stability of the photon sphere and classify the topology of the black hole in accordance with its thermodynamic potentials.

Abstract We investigate the generation of magnetic fields above black hole accretion disks due to the nonzero curl of the disk radiation field. By self-consistently computing the components of the radiation flux and their curl, we show that the rotational nature of the radiation field induces charge separation, leading to magnetic field generation in the plasma above the disk. Solving the magnetohydrodynamic equations, we derive the time evolution of these fields and demonstrate that they grow over astrophysically relevant timescales. For a standard Keplerian accretion disk, the produced magnetic fields remain weak, on the order of a few gauss, consistent with previous predictions. However, when a luminous corona is present in the inner disk region (r d < 3rg –10r g ), the generated fields reach dynamically significant strengths of up to 105 G, where the magnetic energy density approaches a few percentage of gas pressure. These fields develop within realistic growth timescales (such as the viscous timescale) and can be dynamically significant in governing disk and jet evolution. Our findings suggest that radiation-driven magnetic fields play a crucial role in accretion flow magnetization, influencing both disk dynamics and observational signatures. The predicted field strengths could affect the thermal emission, synchrotron radiation, and polarization properties of black hole accretion systems, with implications for X-ray binaries, active galactic nuclei, and jet formation. Future numerical simulations and high-resolution polarimetric observations, such as those from the Imaging X-ray Polarimetry Explorer, enhanced X-ray Timing and Polarimetry Mission, and the Event Horizon Telescope, may provide observational confirmation of our findings.

Abstract We consider a recently introduced extension of General Relativity based on the use of the Cotton tensor and dubbed as Cotton gravity, to estimate the size of a new constant γ appearing within a spherically symmetric, vacuum solution of such theory. Taking into account its non-asymptotically flat character, we use the inferred size of the central brightness depression of the supermassive object at the heart of the Milky Way galaxy (Sgr A*) by the Event Horizon Telescope to constrain at 2σ) the CG parameter as γ M ≈ 3.5 × 10 − 12 . We study the potential observational consequences from the smallness of such a value using exact and numerical expressions for the deflection angle, optical images from optically and geometrically thin accretion disks, isoradials, and instability scales (Lyapunov index) of nearly bound geodesics associated to photon rings. Our results point towards the impossibility to distinguish between these two geometries using current and foreseeable techniques in the field of interferometric detection of optical sources.

Abstract A brief discussion on the pseudo-complex General Relativity is presented. It is shown that this theory is a viable extension of GR, with deviations centered near to the event horizon. The theory introduces a dark energy accumulation, due to the coupling to the central mass. Predictions of this theory are resumed, as for example the structure in an accretion disk, with a dark ring followed by a bright ring further in. The current Event Horizon Telescope observation of M87 is not able to discriminate between GR and pcGR, due to a low resolution. Further predictions are also discussed, as the physics of neutron stars, the redshift at the surface of the star and Quasi Periodic Object.

In this study, we investigate the properties of black holes surrounded by Perfect Fluid Dark Matter (PFDM) within the framework of Scalar–Tensor–Vector Gravity (STVG), a prominent modified theory of gravity. By incorporating the effects of PFDM into a static and spherically symmetric black hole solution in STVG, we examine how the presence of dark matter and modifications to General Relativity affect observable astrophysical features. Special focus is given to the black hole shadow, a crucial optical signature shaped by the spacetime geometry around the event horizon. We analyze how parameters such as the PFDM density, magnetic charge, and STVG scalar fields influence the radius and shape of the shadow. Our results show that both the presence of PFDM and the modifications introduced by STVG lead to a significant enlargement of the photon sphere and observable deviations in the shadow's profile compared to standard GR predictions. These theoretical findings provide a deeper understanding of the interplay between alternative gravity models and dark matter environments and offer valuable insights for interpreting high-resolution black hole images from the Event Horizon Telescope (EHT).

Abstract The Event Horizon Telescope (EHT) imaged black holes M87 $$^*$$ ∗ (angular diameter $$\theta _d = 42 \pm 3\,\upmu \text {as},$$ θ d = 42 ± 3 μ as , mass $$\sim 6.5 \times 10^9\,M_\odot )$$ ∼ 6.5 × 10 9 M ⊙ ) and Sgr A $$^*$$ ∗ $$(\theta _d = 48.7 \pm 7\,\upmu \text {as},$$ ( θ d = 48.7 ± 7 μ as , shadow deviation $$\delta \approx -0.08^{+0.09}_{-0.09}$$ δ ≈ - 0 . 08 - 0.09 + 0.09 (VLTI), $$-0.04^{+0.09}_{-0.10})$$ - 0 . 04 - 0.10 + 0.09 ) (Keck). These observations enable tests of gravity in strong regimes. We propose a rotating non-commutative inspired Kiselev black hole (RNKBH), incorporating dark energy $$(\omega )$$ ( ω ) and non-commutative $$(\Theta )$$ ( Θ ) parameters, extending Kerr solutions. Our analysis reveals that $$\omega = -2/3$$ ω = - 2 / 3 allows larger $$\Theta $$ Θ but requires $$a \ge 0.16M$$ a ≥ 0.16 M to maintain horizons. Shadow calculations show significant deviations from Kerr predictions: for $$\Theta = 0.001-0.005M^2,$$ Θ = 0.001 - 0.005 M 2 , shadows shrink by 8-15% and distortion increases by 20-35% for rapidly spinning $$(a > 0.5M)$$ ( a > 0.5 M ) black holes. The $$\omega = -2/3$$ ω = - 2 / 3 exhibits shadow squeezing near the cosmological horizon. We compute shadow observables and compare them with EHT data. For Sgr A $$^*$$ ∗ $$(50^\circ $$ ( 50 ∘ inclination), the bounds are $$0.001497 \le \Theta \le 0.002868\,M^2$$ 0.001497 ≤ Θ ≤ 0.002868 M 2 at $$a =0.9146$$ a = 0.9146 $$(\omega = -2/3)$$ ( ω = - 2 / 3 ) . For M87 $$^*$$ ∗ $$(17^\circ )$$ ( 17 ∘ ) , $$0 \le \Theta \le 0.004993 \,M^2$$ 0 ≤ Θ ≤ 0.004993 M 2 at $$a=0.0.6167$$ a = 0.0 . 6167 $$(\omega = -2/3)$$ ( ω = - 2 / 3 ) . While EHT cannot yet distinguish RNKBH from Kerr BH, our results highlight its viability as an astrophysical candidate.

High-precision regression of physical parameters from black hole images generated by General Relativistic Ray Tracing (GRRT) is essential for investigating spacetime curvature and advancing black hole astrophysics. However, owing to limitations in observational resolution, high observational costs, and imbalanced distributions of positive and negative samples, black hole images often suffer from data scarcity, sparse parameter spaces, and complex structural characteristics. These factors pose significant challenges to conventional regression methods based on simplified physical models. To overcome these challenges, this study introduces the Multiscale Adaptive Network (MANet), a novel regression framework grounded in deep learning. MANet integrates an Adaptive Channel Attention (ACA) module to selectively enhance features in physically informative regions. Meanwhile, a Multiscale Enhancement Feature Pyramid (MEFP) is employed to capture fine-grained spatial structures, such as photon rings and accretion disks, while alleviating information loss due to downsampling. Experimental evaluations on GRRT-simulated datasets demonstrate that MANet substantially improves parameter estimation accuracy and generalization capability in high-dimensional parameter spaces, outperforming existing baseline approaches. This framework presents a promising avenue for high-precision parameter regression in Event Horizon Telescope (EHT) data analysis and broader astrophysical imaging applications characterized by sparse and noisy data.

When light meets sound, a new dimension of analysis unfolds. This work explores black hole observations through the lens of signal theory and acoustic wave mechanics, revealing a resonant bridge between electromagnetic and mechanical waves. Using Event Horizon Telescope EHT data, black hole imagery is treated as a three-dimensional digital signal, where the analytic Hilbert envelope and normalized Discrete Fourier Transform DFT expose hidden structures. The gravitational shadow is interpreted not as silence, but as a measurable energy dip—an imprint of absorption rather than absence. Euler’s identity is employed to map signal phase and symmetry into polar and complex domains, providing an intuitive mathematical pathway toward the event horizon. By applying foundational acoustic concepts such as resonance, interference, and entropy, the field surrounding the black hole is reinterpreted as a complex communication signal. This interdisciplinary framework unifies digital signal processing, electromagnetic theory, and acoustics into a novel methodology for astronomical analysis. Notably, when a full noise assessment is conducted, EHT images exhibit a significant enhancement in resolution and information transmission

Abstract Polarimetric light curves of Sagittarius A* (Sgr A*) sometimes exhibit loops in the Stokes Q and U plane over time, often interpreted as orbiting hotspot motion. In this work, we apply the differential geometry of planar curves to develop a new technique for estimating polarimetric rotation rates. Applying this technique to 230 GHz light curves of Sgr A*, we find evidence of clockwise motion not only during a postflare period on 2017 April 11, as previously discovered, but also during the quiescent days imaged by the Event Horizon Telescope (EHT). The data exhibit a clockwise fraction of 0.65 ± 0.09 and an overall Q−U rotation rate of − 2.6 ± 0.6 deg t g − 1 . We analyze a library of general relativistic magnetohydrodynamic simulations and find that face-on, clockwise-rotating models with strong magnetic fields are most likely to be consistent with the observations. These results are consistent with EHT and GRAVITY Collaboration studies and indirectly support an interpretation in which the polarized image of Sgr A* has been rotated by an external Faraday screen. This technique offers a novel probe of event-horizon-scale dynamics that complements dynamical reconstructions.

We investigate the motion of neutral test particles around a non-rotating regular black hole within the framework of asymptotically safe gravity, analyzing the impact of the black hole’s parameters on particle motion. This black hole solution is characterized by an additional parameter, η, which differentiates it from the standard Schwarzschild black hole solution. We obtain analytical formulations for the energy and angular momentum of equatorial circular orbits as functions of the black hole parameter. The stability of these circular orbits is examined by the effective potential. Additionally, we present a graphical study of the innermost stable equatorial circular orbits as functions of the black hole parameter and investigate the effective force exerted on circular orbits. We also derive the frequencies of radial and latitudinal harmonic oscillations as functions of the model parameters and discuss the key features of quasi-periodic oscillations of test particles near stable circular orbits in the black hole’s equatorial plane. The phenomenon of periastron precession is also considered. Additionally, we explore particle collisions near the black hole and demonstrate that such collisions can produce high energy near the event horizon. Our findings indicate that the black hole parameter significantly influences the motion of test particles around a regular black hole in the context of asymptotically safe gravity. We examine the shadow of black hole and using observational data from the Event Horizon Telescope (EHT) collaboration for Sgr A* and M87*, we have determined the range of η parameter that corresponds to the observations.