Black Hole Shadow Research Articles

Constraining a one-dimensional wave-type gravitational wave parameter through the shadow of M87* via Event Horizon Telescope

During the glorious success of the EHT in providing the first image of a black hole, numerous papers have been published about the effect of different astrophysical environments on black hole geometry. Motivated by the work on how gravitational wave affects the shadow of a Schwarzschild black hole [Eur. Phys. J. C 10.1140/epjc/s10052-021-09287-2], we extend it by considering a quantum correction on the black hole through the extended uncertainty principle (EUP). Along with this correction, we probe the gravitational wave’s effect on the null geodesics and photonsphere and find constraints to the gravitational wave parameter ϵ using the black hole shadow of M87* for some given test value for the gravitational wave frequency σ. Not only were some nodes found in the light trajectory, but the general behavior of paths changes periodically as the time t progresses. These patterns then confirm the chaotic formation of the shadow seen by some remote observer. Finally, the constraint that we find for ϵ is 10−10 orders of magnitude for the effect of the gravitational wave to be seen at a distance of D=16.8 Mpc. As a consequence of such a value for ϵ, another result reveals that while there is are gravitational wave effect on the shadow perceived at D, the deviations on the photonsphere are nearly non-existent. Apart from Earth-based detectors for gravitational waves, the study implies the possibility of an alternative detection method, especially if a gravitational wave source is near a lone black hole.

Observational appearance of Kaluza–Klein black holes

The optical properties of rotating black holes in Kaluza–Klein theory described by the total mass, spin, and electric and magnetic charges are investigated in detail. Using a developed general relativistic ray-tracing code to calculate the motion of photons, shadows of Kaluza–Klein black holes are generated. The properties of the shadow and the light deflection angle around these black holes are also studied in order to put constraints on the parameters of Kaluza–Klein black holes using M87* shadow observations. The possibility of imposing constraints on Kaluza–Klein black holes using shadow observations is investigated. Moreover, we find that small charges (electric and magnetic) of the black hole can meet these constraints. We conclude that with the current precision of the M87* black hole shadow image observation by the EHT collaboration, the shadow observations of Kaluza–Klein black holes are indistinguishable from that of the Kerr black hole. Much better observational accuracy than the current capabilities of the EHT collaboration are required in order to place verified constraints on the parameters of modified theories of gravity in the strong field regime.

Shadow Geometry of Kerr Naked Singularities

Direct imaging of supermassive black holes (SMBHs) at event horizon-scale resolutions, as recently done by the Event Horizon Telescope, allows for testing alternative models to SMBHs such as Kerr naked singularities (KNSs). We demonstrate that the KNS shadow can be closed, open, or vanishing, depending on the spins and observational inclination angles. We study the critical parameters where the KNS shadow opens a gap, a distinctive phenomenon that does not happen with the black hole shadow. We show that the KNS shadow can only be closed for dimensionless spin a ≲ 1.18 and vanishing for a ≳ 1.18 for certain ranges of inclination angles. We further analyze the effective angular momentum of photon orbits to demonstrate the fundamental connections between light geodesics and the KNS shadow geometry. We also perform numerical general relativistic ray-tracing calculations, which reproduce the analytical topological change in the KNS shadow, and illustrate other observational features within the shadow due to the lack of an event horizon. By comparing the geometric features of the KNS shadow with black hole shadow observations, the topological change in the shadow of KNSs can be used to test the cosmic censorship hypothesis and KNSs as alternative models to SMBHs.

Quasinormal modes and optical properties of 4-D black holes in Einstein Power-Yang–Mills gravity

This paper explores the impact of the Yang–Mills charge parameter and the exponent term on a 4D black hole solution in the Einstein Power-Yang–Mills theory. Through an investigation of the massless scalar quasinormal mode spectrum, black hole shadow, and emission rate, we have determined that the effects of these two parameters are opposite. Specifically, the Yang–Mills charge parameter causes an increase in the real quasinormal frequencies with a correspondingly smaller damping rate. It also results in a smaller black hole shadow and a lower evaporation rate.

Shadows and photon rings of regular black holes and geonic horizonless compact objects

Abstract The optical appearance of a body compact enough to feature an unstable bound orbit, when surrounded by an accretion disk, is expected to be dominated by a luminous ring of radiation enclosing a central brightness depression typically known as the shadow. Despite observational limitations, the rough details of this picture have been now confirmed by the results of the Event Horizon Telescope (EHT) Collaboration on the imaging of the M87 and Milky Way supermassive central objects. However, the precise characterization of both features—ring and shadow—depends on the interaction between the background geometry and the accretion disk, thus being a fertile playground to test our theories on the nature of compact objects and the gravitational field itself in the strong-field regime. In this work we use both features in order to test a continuous family of solutions interpolating between regular black holes and horizonless compact objects, which arise within the Eddington-inspired Born–Infeld theory of gravity, a viable extension of Einstein’s general relativity (GR). To this end we consider seven distinctive classes of such configurations (five black holes and two traversable wormholes) and study their optical appearances under illumination by a geometrically and optically thin accretion disk, emitting monochromatically with three analytic intensity profiles previously suggested in the literature. We build such images and consider the sub-ring structure created by light rays crossing the disk more than once and existing on top of the main ring of radiation. We discuss in detail the modifications as compared to their GR counterparts, the Lyapunov exponents of unstable nearly-bound orbits, as well as the differences between black hole and traversable wormholes for the three intensity profiles. In addition we use the claim by the EHT Collaboration on the radius of the bright ring acting (under proper calibrations) as a proxy for the radius of the shadow itself to explore the parameter space of our solutions compatible with such a result.

Thermodynamics and shadows of GUP-corrected black holes with topological defects in Bumblebee gravity

In this work we investigate a Schwarzschild-type black hole that is corrected by the Generalized Uncertainty Principle (GUP) and possesses topological defects within the framework of Bumblebee gravity. Our focus is on the thermodynamic characteristics of the black hole, such as temperature, entropy and heat capacity, which vary as functions of the horizon radius, and also on shadow as an optical feature. Our investigation reveals significant changes in the thermodynamic behavior of the black hole due to violations of Lorentz symmetry, GUP corrections, and the presence of monopoles. However, the shadow of the black hole is unaffected by violations of Lorentz symmetry. In addition, we provide a limit on the parameters of Lorentz symmetry violation, GUP and topological defects based on a classical test involving the precession of planetary orbits and the advancement of perihelion in the solar system.

Constraining an Einstein-Maxwell-dilaton-axion black hole at the Galactic Center with the orbit of the S2 star

We derive new constraints on the dilaton parameter appearing in the spherically-symmetric black hole solution of Einstein-Maxwell-dilaton-axion gravity, by studying the geodesic motion of the S2 star in the Galactic Center. Einstein-Maxwell-dilaton-axion black holes represent a compelling alternative to the standard black hole paradigm in General Relativity. This theory emerges from the low energy effective action of the heterotic string theory and has been proven to predict peculiar observational features from the direct imaging of black hole shadows. At a fundamental level, Einstein-Maxwell-dilaton-axion includes additional electromagnetic, dilatonic and axionic fields coupled to the space-time metric. When considering charged non-rotating black hole solutions, the additional fields endow the metric with one extra parameter b, called dilaton parameter, that is theoretically bound to 0 < b < M. Using publicly available astrometric data for S2 we derive an upper bound on b ≲ 12M at 95% confidence level and we demonstrate that only including the measurement of the relativistic orbital precession for S2 is sufficient to reduce this bound to b ≲ 1.4M at the same confidence level. Additionally, using a mock data mimicking future observations of S2 with the GRAVITY interferometer, we show that improved astrometric precision can help further narrow down the allowed dilaton parameter range to b ≲ 0.033M after monitoring the S2 orbit for one and a half period.

Observational constraints on Yukawa cosmology and connection with black hole shadows

We confront Yukawa modified cosmology, proposed in Jusufi et al. [arXiv:2304.11492], with data from Supernovae Type Ia (SNe Ia) and Hubble parameter (OHD) observations. Yukawa cosmology is obtained from a Yukawa-like gravitational potential, with coupling parameter α and wavelength parameter λ, which gives rise to modified Friedmann equations. We show that the agreement with observations is very efficient, and within 1σ confidence level we find the best-fit parameters λ=2693−1262+1191Mpc, α=0.416−0.326+1.137, and a graviton mass of mg=2.374−0.728+2.095×10−42GeV. Additionally, we establish a connection between the effective dark matter and dark energy density parameters and the angular radius of the black hole shadow of the SgrA and M87 black holes in the low-redshift limit, consistent with the Event Horizon Telescope findings.

Investigating the connection between generalized uncertainty principle and asymptotically safe gravity in black hole signatures through shadow and quasinormal modes

The links between the deformation parameter beta of the generalized uncertainty principle (GUP) to the two free parameters hat{omega } and gamma of the running Newtonian coupling constant of the Asymptotic Safe gravity (ASG) program, has been conducted recently in [Phys. Rev. D 105 (2022) 12, 124054. https://doi.org/10.1103/PhysRevD.105.124054] In this paper, we test these findings by calculating and examining the shadow and quasinormal modes of black holes, and demonstrate that the approach provides a theoretical framework for exploring the interplay between quantum gravity and GUP. Our results confirm the consistency of ASG and GUP, and offer new insights into the nature of black holes and their signatures. The implications of these findings for future studies in quantum gravity are also discussed.

Some observable physical properties of the higher dimensional dS/AdS black holes in Einstein-bumblebee gravity theory

We study the greybody factors, quasinormal modes, and shadow of the higher dimensional de-Sitter (dS)/anti de-Sitter (AdS) black hole spacetimes derived from the Einstein-bumblebee gravity theory within the Lorentz symmetry breaking (LSB) framework. We specifically apply the semi-analytical WKB method and the time domain approach to study the scalar and Dirac perturbations of the black hole. In-depth researches are done on the effects of the LSB and dimensionality on the bosonic/fermionic greybody factors, quasinormal modes, and shadow of the higher dimensional bumblebee black hole. The results obtained are discussed, tabulated, and illustrated graphically.

Thermodynamics and Shadows of quantum-corrected Reissner–Nordström black hole surrounded by quintessence

In this work, we consider the quantum-corrected Reissner–Nordström black hole surrounded by quintessence matter and examine its thermodynamics and shadow. To this end, we handle the quantum-corrected metric of the black hole, which is given by Wu and Lui in 2022, and couple the quintessence matter field effects to the lapse function. Then, we obtain the mass, Hawking temperature, specific heat, and equation of state functions. In all these functions, we discuss two cases in which the quantum corrections are greater or smaller than the quintessence matter effects. We observe that within these two scenarios, the black hole has different characteristic thermal behaviors. Moreover, we find that the quintessence matter plays an important role in leading to a black hole remnant. Furthermore, we observe that black hole behaves like Van der Waals fluid. Finally, we investigate the black hole shadow by Carter’s approach. We find that quintessence matter and quantum corrections modify the shadow of the black hole.

Shadow and stability of quantum-corrected black holes

Recently the quantum Oppenheimer–Snyder gravitational collapse model has been proposed in loop quantum gravity, providing quantum-corrected Schwarzschild spacetimes as the exterior of the collapsing dust ball. In this paper, the quantum gravity effects on the black hole shadows in this model are studied, and the stability of the quantum-corrected black holes is also analyzed by calculating the quasinormal modes. It turns out that the quantum correction always shrinks the radius of shadows, and the quantum-corrected black holes are stable against the scalar and vector perturbations.

Asymptotically locally flat and AdS higher-dimensional black holes of Einstein–Horndeski–Maxwell gravity in the light of EHT observations: shadow behavior and deflection angle

Unification of gravity with other interactions, achieving the ultimate framework of quantum gravity, and fundamental problems in particle physics and cosmology motivate to consider extra spatial dimensions. The impact of these extra dimensions on the modified theories of gravity has attracted a lot of attention. One way to examine how extra dimensions affect the modified gravitational theories is to analytically investigate astrophysical phenomena, such as black hole shadows. In this study, we aim to investigate the behavior of the shadow shapes of higher-dimensional charged black hole solutions including asymptotically locally flat (ALF) and asymptotically locally AdS (ALAdS) in Einstein–Horndeski–Maxwell (EHM) gravitational theory. We utilize the Hamilton–Jacobi method to find photon orbits around these black holes as well as the Carter approach to formulate the geodesic equations. We examine how extra dimensions, negative cosmological constant, electric charge, and coupling constants of the EHM gravity affect the shadow size of the black hole. Then, we constrain these parameters by comparing the shadow radius of these black holes with the shadow size of M87* supermassive black hole captured by the Event Horizon Telescope (EHT) collaborations. We discover that generally the presence of extra dimensions within the EHM gravity results in reducing the shadow size of higher-dimensional ALF and ALAdS charged black holes, whereas the impact of electric charge on the shadow of these black holes is suppressible. Interestingly, we observe that decreasing the negative cosmological constant, i.e., increasing its absolute value, leads to increase the shadow size of the ALAdS charged higher-dimensional black hole in the EHM gravity. Surprisingly, based on the constraints from EHT observations, we discover that only the shadow size of the four dimensional ALF charged black hole lies in the confidence levels of EHT data, whereas owing to the presence of the negative cosmological constant, the shadow radius of the four, five, and seven dimensional ALAdS charged black holes lie within the EHT data confidence levels.

Shadows of quintessential dark energy black holes in the domain of outer communication

The rotating black holes in the quintessential dark energy correspond to three horizons: inner, outer, and quintessential horizon. The domain of outer communication is the region between outer and quintessential horizon. Here in this work, we study the photon region and shadows of the quintessential dark energy black holes when the observer stays statically in the domain of outer communication. The quintessential dark energy black holes shadow characterizes by its mass (M), spin parameter (a), quintessential dark energy parameter (ωq), and normalization factor (γ). The dark energy parameter ωq can take values between −1.1<ωq<−1/3 and follows the equation of state ωq = pressure(p)/energy density(ρq). This state parameter significantly affects the shape and the size of the black hole shadow. We generalize all the geodesic equations of motion for ωq and obtain relation to visualizing the black hole shadow by a static observer at any arbitrary distance in the domain of outer communication. We estimate the black hole shadow observables: radius Rs, distortion parameter δs, and the shadow area A. Using the numerical values of shadow radius Rs and area A, we obtain the angular diameter (θd) of the black hole shadow. The angular diameter of the M87 and Sgr A∗ black holes shadow are 42±3μas and 48.7±7μas respectively as observe by Event Horizon Telescope (EHT). In this case, the angular diameter of the black hole shadow varies from θd≈21o±4 with γ from 0.08/M to 0.12/M for the state parameter ωq=−2/3 when the static observer at ro=5M in the domain of outer communication.

Deflection angle analysis under the influence of non-plasma medium and plasma medium for regular black hole with cosmic string

By applying the Keeton and Petters technique, we find the deflection angle as a series expansion with a specific factor of mass. We investigate the shadow structure of a regular black hole with cosmic string with a non-magnetized, pressure-free plasma around it. In order to separate the Hamilton equation and locate the photon areas, certain plasma distributions are taken into consideration. Moreover, by utilizing a new ray-tracing technique, we analyze the photon’s path around a regular black hole with cosmic string in the presence of plasma whose density depends on the radius coordinate. We also analyze the plasma effects on the shadow of black hole. We obtain an analytical formula that describes the boundary curve of the shadow for such a black hole in an expanding universe for an observer at any finite location outside the horizon. In the presence and absence of plasma, we detect deviations that are further investigated by analyzing the geometry of shadow angle at a particular value of plasma frequency.

Using multiobjective optimization to reconstruct interferometric data. Part I

Context. Imaging in radioastronomy is an ill-posed inverse problem. However, with increasing sensitivity and capabilities of telescopes, several strategies have been developed in order to solve this challenging problem. In particular, novel algorithms have recently been proposed using (constrained) nonlinear optimization and Bayesian inference. Aims. The Event Horizon Telescope (EHT) Collaboration convincingly investigated the fidelity of their image reconstructions with large surveys, solving the image reconstruction problem with different optimization parameters. This strategy faces a limitation for the existing methods when imaging active galactic nuclei: Large and expensive surveys solving the problem with different optimization parameters are time-consuming. We present a novel nonconvex, multiobjective optimization modeling approach that gives a different type of claim and may provide a pathway to overcome this limitation. Methods. To this end, we use a multiobjective version of the genetic algorithm (GA): the Multiobjective Evolutionary Algorithm Based on Decomposition, or MOEA/D. The GA strategies explore the objective function by evolutionary operations to find the different local minima and to avoid becoming trapped in saddle points. Results. First, we tested our algorithm (MOEA/D) using synthetic data based on the 2017 EHT array and a possible EHT plus next-generation EHT configuration. We successfully recover a fully evolved Pareto front of nondominated solutions for these examples. The Pareto front divides into clusters of image morphologies representing the full set of locally optimal solutions. We discuss approaches to find the most natural guess among these solutions and demonstrate its performance on synthetic data. Finally, we apply MOEA/D to observations of the black hole shadow in Messier 87 with the EHT data in 2017. Conclusions. The MOEA/D is very flexible and faster than any other Bayesian method, and it explores more solutions than regularized maximum likelihood methods. We have written two papers to present this new algorithm. In the first, we explain the basic idea behind multiobjective optimization and MOEA/D, and we use MOEA/D to recover static images. In the second paper, we extend the algorithm to allow dynamic and (static and dynamic) polarimetric reconstructions.

Observing axions through photon ring dimming of black holes

It is known that magnetic fields exist near black holes and photons can go around black holes due to strong gravity. Utilizing these facts, we can probe hypothetical pseudoscalar particles, so-called axions. In fact, photons can be converted into axions when they propagate in a magnetic field. The conversion of such photons into axions leads to a dimming of the photon ring around the black hole shadow. We show that photon ring dimming can occur efficiently for supermassive black holes. Remarkably, it turns out that the maximal dimming rate of the photon ring is 25%. In the case of M87*, the dimming of 10% will be observed in the X-ray and gamma-ray bands if the angular resolution of $10^{-5}$ arcsec is achieved. The frequency band and the magnitude of the dimming depend on the axion-photon coupling and axion mass. Hence, the distorted spectrum of the photon ring provides a novel tool for detecting axions.

Shadow of a regular black hole in scalar-tensor-vector gravity theory

We investigate the shadow cast by a regular black hole in scalar-tensor-vector modified gravity theory. This black hole differs from a Schwarzschild-Kerr black hole by the dimensionless parameter $\ensuremath{\beta}$. The size of the shadow depends on this parameter. Increasing the value of the parameter $\ensuremath{\beta}$ shrinks the shadow. A critical value of the parameter $\ensuremath{\beta}$ is found to be ${\ensuremath{\beta}}_{\mathrm{crit}}=0.40263$. The shadow for the horizonless dark compact object has been analyzed for the static, spherically symmetric case and compared with M87* and Sgr A* data. Shadow observables have been determined in the context of the regular black hole and used for obtaining the energy emission rate. The peak of the energy emission rate shifts to lower frequency for the increasing value of the parameter $\ensuremath{\beta}$.

Shadows of rotating black holes in plasma environments with aberration effects

The shadows of black holes encode significant information about the properties of black holes and the spacetime surrounding them. So far, the effects of dispersive media, such as plasma, and relativistic aberration on the propagation of light around compact objects have been treated separately in the literature. In this paper, we will employ the Konoplya, Stuchl\'{\i}k, and Zhidenko family of stationary, axially symmetric, and asymptotically flat metrics to describe the spacetime around rotating black holes. We will study how the parameters of the black hole, the chromatic effects resulting from the presence of a nonmagnetized, pressureless plasma environment, and the effects of relativistic aberration of a moving observer modify the morphology of the shadow.

Surveying Flux Density in Galaxies with Apparent Large Black Holes at Millimeter/Submillimeter Wavelengths

We present millimeter and submillimeter continuum observations for 36 sources with potentially large black hole shadows at 230 and 345 GHz using the Submillimeter Array. The sources are selected based on the criterion of the large diameter of the black hole shadows. Our motivation is to explore the nature of the accretion flow of the potential candidates of low-luminosity active galactic nuclei (LLAGNs) through photometry at millimeter/submillimeter wavelengths. The detected result serves as the pathfinder of future high-angular resolution observations such as the Event Horizon Telescope. As a result, we successfully detected 17 and eight sources at 230 and 345 GHz, respectively. We reveal that three of the detected sources (IC 310, NGC 1277, and NGC 5846) show significant excess at millimeter/submillimeter wavelengths in comparison with the extrapolation both from low-frequency radio and infrared, which are considered to be attributed to the extended jet and dust component, respectively. One possible explanation is that these excesses are associated with the hot accretion flow onto the supermassive black holes of the LLAGNs. By adopting the advection-dominated accretion flow model's semi-analytic model, we obtained the upper bound of the mass accretion rate. Those are less than 10−2 , where is the Eddington mass accretion rate computed via the Eddington luminosity. This is in good agreement with the expected range of the LLAGNs. The ∼200 mJy flux densities and negative spectral index of IC 1459 within millimeter and submillimeter wavelengths make it a promising candidate for future submillimeter high-angular resolution experiments for imaging the black hole shadow.