Newman-Janis Algorithm Research Articles

Shadow of a renormalization group improved rotating black hole

We present a study on quantum gravity effects on the shadow of a rotating black hole (BH) obtained in the setting of the asymptotically safe gravity. The rotating metric, which results from a static regular one recently presented in the literature, is generated by using the generalized Newman-Janis algorithm. The novelty of the static regular metric lies in the fact that it is the outcome of an effective Lagrangian which describes dust whose spherically symmetric collapse is non-singular as a consequence of the antiscreening character of gravity at small distances. The effective Lagrangian includes a multiplicative coupling, denoted as χ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi $$\\end{document}, with the Lagrangian of the collapsing fluid. The resulting exterior metric for large radii depends on a free parameter ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document} which captures the quantum gravity effects. The form of the coupling χ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\chi $$\\end{document} and its connection with the quantum parameter ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document} are determined by the running of the Newton coupling G(k) along a renormalization group trajectory that stops at the ultraviolet non-gaussian fixed point of the asymptotic safety theory for quantum gravity. Varying both the spin parameter a⋆\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$a_{\\star }$$\\end{document} and the quantum parameter ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document}, we explore the quantum gravity effects on several astronomical observables used to describe the morphology of the shadow cast by rotating BHs. In order to obtain constraints on the parameter ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document}, we confront our results with the recent Event Horizon Telescope (EHT) observations of the shadows of the supermassive BHs M87∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {M87}^*$$\\end{document} and Sgr A∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {A}^*$$\\end{document}. We find that the ranges of variation of all the studied shadow observables fall entirely within the ranges determined by the EHT collaboration. We then conclude that the current astronomical data do not rule out the renormalization group improved rotating BH.

Shadow of rotating black holes surrounded by dark fluid with Chaplygin-like equation of state and constraints from EHT results

Abstract The present work is devoted to testing spacetime properties around black holes surrounded by a dark fluid that are candidates for dark energy, which can be described by the Chaplygin-like equation of state through its shadow. To do this, we first study the horizon structure of the black hole and its shadow. Then, we obtain a rotating black hole solution in the presence of a dark fluid using the generalized Newman-Janis algorithm and study the effects of the black hole spin and the fluid parameters on the black hole horizons. Also, we obtain the shadow cast of the rotating black hole using celestial coordinates and have shown that the presence of the dark fluid causes an increase in shadow size. Moreover, we also obtain constraints on the spin and black hole charge together with the dark fluid parameters using the shadow size of supermassive black holes Sagittarius A* and M87* from Event Horizon Telescope observations. Finally, we also investigate the energy emission rate of the charged black hole surrounded by a Chaplygin-like dark fluid compared to both rotating and nonrotating cases.

Shadow and quasinormal modes of novel charged rotating black hole in Born–Infeld theory: Constraints from EHT results

This work investigates the shadow and quasinormal modes of a novel rotating Born–Infeld-type black hole. First, we study the effect of interaction between nonlinear electrodynamic fields and photons on shadow radius and show that it is less than the error of shadow measurements in Event Horizon Telescope (EHT) observations. Then, we obtain a rotating metric using the Newman–Janis algorithm, starting with the initial metric of the nonrotating novel Born–Infeld black hole solution. Next, we analyze the null geodesics to determine the celestial coordinates. The black hole’s shadow radius is determined by using celestial coordinates. The mass, spin, charge, and nonlinearity parameters all influence the shape and size of the black hole shadow. An increase in the spin parameter results in a gradual reduction in the size of black hole shadows and further distortion of the shadows. Furthermore, constraints on the spin and electric charge of black holes, as well as the nonlinearity parameter, are derived through the utilization of the shadow sizes of supermassive black holes Sagittarius (Sgr) A* and M87* obtained from observations of the EHT. Also, we investigate the correlation between the standard shadow radius and the equatorial and polar quasinormal modes for revolving black holes. Finally, we analyze the emission energy rate from the black hole based on different spacetime parameters. Our observation indicates that the emission energy rate decreases as the values of spin and nonlinearity parameters increase, keeping the charge-to-mass ratios of the Born–Infeld black hole constant.

Shadow and quasinormal modes of rotating charged black holes in Kalb–Ramond gravity and implications from EHT data

The recent observations of the Event Horizon Telescope for the supermassive black holes M87* and Sgr A* can potentially test gravity theories and acquire further knowledge on black holes and gravity parameters. An essential field of research focuses on the shadow and quasinormal modes of black holes. Therefore, we used the Newman–Janis algorithm to get a rotating charged black hole solution in Kalb–Ramond gravity. Then, we looked at the event horizon radius, the static limit, and the ergoregion of spacetime. We derive the null geodesic equations and the effective potential for photon radial motion. We also investigate how the black hole’s Kalb–Ramond, spin, and charge parameters affect the shadow size and distortion. It is obtained that an increase in the Kalb–Ramond parameter causes a decrease in the radius and increases the distortion of the shadow size. We obtain constraints on black hole charge and spin using the data obtained from Event Horizon Telescope observations of M87* and Sgr A* by testing the effects of the Kalb–Ramond gravity parameter. We examine the energy emission rate using Hawking radiation and demonstrate that a rise in the Kalb–Ramond parameter diminishes this rate. Finally, we calculated and analyzed the typical shadow radius, equatorial, and polar quasinormal modes using the geometric–optic connection between quasinormal mode parameters and conserved quantities along geodesics.

Analytical models of supermassive black holes in galaxies surrounded by dark matter halos

In this Letter, we present five analytical models in closed forms, each representing a supermassive black hole (SMBH) located at the center of a galaxy surrounded by dark matter (DM) halo. The density profile of the halo vanishes inside twice the Schwarzschild radius of the hole and satisfies the weak, strong, and dominant energy conditions. The spacetime are asymptotically flat, and the difference among the models lies in the slopes of the density profiles in the spike and regions far from the center of the galaxy. Three of them represent cusp models, whereas the other two represent core models. With the well-known (generalized) Newman-Janis algorithm, rotating SMBHs with DM halos can be easily constructed from these models.

Classical observables using exponentiated spin factors: electromagnetic scattering

In [1], the authors argued that the Newman-Janis algorithm on the space of classical solutions in general relativity and electromagnetism could be used in the space of scattering amplitudes to map an amplitude with external scalar states to an amplitude associated to the scattering of “infinite spin particles”. The minimal coupling of these particles to the gravitational or Maxwell field is equivalent to the classical coupling of the Kerr blackhole with linearized gravity or the so-called Kerr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{\ extrm{Kerr}} $$\\end{document} charged state with the electromagnetic field. The action of the Newman-Janis mapping on scattering amplitudes was then used to compute the linear impulse at first post-Minkowskian (1PM) order, via the Kosower, Maybee, O’Connell (KMOC) formalism. In this paper, we continue with the idea of using the Newman-Janis mapping on the space of scalar QED amplitudes to compute classical observables such as the radiative gauge field and the angular impulse. We show that for tree-level amplitudes, the Newman-Janis action can be reinterpreted as a dressing of the photon propagator. This turns out to be an efficient way to compute these classical observables. Along the way, we highlight a subtlety that arises in proving the conservation of angular momentum for scalar −Kerr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ -\\sqrt{\ extrm{Kerr}} $$\\end{document} scattering.

Motion of test particles in quasi anti-de Sitter regular black holes

In this paper, we explore the characteristics of two novel regular spacetimes that exhibit a nonzero vacuum energy term, under the form of a (quasi) anti-de Sitter phase. Specifically, the first metric is spherical, while the second, derived by applying the generalized Newman–Janis algorithm to the first, is axisymmetric. We show that the equations of state of the effective fluids associated with the two metrics asymptotically tend to negative values, resembling quintessence. In addition, we study test particle motions, illustrating the main discrepancies among our models and more conventional metrics exhibiting non-vanishing anti-de Sitter phase.

Kerr-scalaron Metric and Astronomical Consequences near the Galactic Center Black Hole

Astronomical tests of spacetime metric and gravitation theory near the Galactic center (GC) black hole, Sgr A*, have gained momentum with the observations of compact stellar orbits near the black hole and measurement of the black hole shadow. Deviation from the Kerr metric is a potential signature of modified gravity theory. In this work, we use the Newman–Janis algorithm to construct an axially symmetric and asymptotically flat metric in f(R) scalaron gravity theory. We call it the Kerr-scalaron metric. To study the astronomical consequences of the new metric, we use the compact stellar orbits and the black hole shadow. We use the observed size of the emission ring of the GC black hole shadow for estimating the deviation of the new metric from general relativity. It has been found that scalarons with masses within 10−17–10−16 eV are compatible with the observed emission ring size for a black hole spin of χ = 0.9. The Schwarzschild limit of the pericenter shift is estimated for compact stellar orbits near the black hole. General relativistic pericenter shift in wider orbits, including S stars such as S4716 and S2, has been reproduced with these scalarons. The parameter f SP measuring the deviation from Schwarzschild pericenter shift has been found to be f SP = 1.00–1.04 within stellar orbits with semimajor axes of 45–100 au. Scalarons have the capability to dominate Schwarzschild precession for orbits with semimajor axes much smaller than 45 au. Lense–Thirring (LT) precession with the new metric is estimated for the compact orbits. The massive scalarons produce LT precession with magnitude (12.25–24.5) μas yr−1 in the orbit of S2. The LT precession timescale is within 0.1% of the age of the S stars.

ЭФФЕКТ САНЬЯКА В ПРОСТРАНСТВЕ-ВРЕМЕНИ ВРАЩАЮЩЕЙСЯ ЧЕРНОЙ ДЫРЫ В ЧЕТЫРЕХМЕРНОЙ ТЕОРИИ ЭЙНШТЕЙНА–ГАУССА–БОННЕ

Recently, Glavan and Lin proposed a new four-dimensional theory of Einstein-Gauss-Bonnet (EGB) gravity by rescaling the Gauss-Bonnet (GB) coupling constant ߙ→ ܦ(/ߙ− 4) and adopting the ܦ→ 4 constraint at the level of the equations of motion. The GB coupling constant contributes to the field equations and thus bypasses Lovelock's theorem. This theory preserves the number of degrees of freedom and avoids Ostrogradsky instability. Glavan and Lin obtained an exact solution for nonsingular static and spherically symmetric black holes in the four-dimensional EGB theory of gravity. Later, Kumar and Ghosh applied the Newman-Janis algorithm to the static solution and constructed a solution for a rotating black hole in the 4-dimensional EGB theory of gravity. Since the theory solves many observational problems in astrophysics and is promising for research, the influence of the GB coupling parameter on astrophysical effects is actively studied in the literature. However, this theory has not been tested previously using time effects. The work examines one of the most interesting time effects – the Sagnac effect. Thus, the goal of the work is to study the Sagnac effect in the space-time of a rotating black hole in the four-dimensional EGB theory and obtain a constraint on the GB coupling parameter using observational data of the Sagnac effect. In the most general case, the Sagnac effect is understood as the difference in the time it takes light beams moving in opposite directions to pass through a closed circle. The effect has many applications, is observable on Earth and is taken into account in satellite navigation systems (GPS, GLONASS) when synchronizing time signals.

Rotating regular black holes and other compact objects with a Tolman-type potential as a regular interior for the Kerr metric

In this paper, we obtain a new class of stationary axisymmetric spacetimes by using the Gürses–Gürsey metric with an appropriate mass function in order to generate a rotating core of matter that may be smoothly matched to the exterior Kerr metric. The same stationary spacetimes may be obtained by applying a slightly modified version of the Newman–Janis algorithm to a nonrotating spherically symmetric seed metric. The starting spherically symmetric configuration represents a nonisotropic de Sitter-type fluid whose radial pressure [Formula: see text] satisfies an state equation of the form [Formula: see text], where the energy density [Formula: see text] is chosen to be the Tolman-type-VII energy density [R. C. Tolman, Phys. Rev. 55, 364 (1939)]. The resulting rotating metric is then smoothly matched to the exterior Kerr metric, and the main properties of the obtained geometries are investigated. All the solutions considered in this study are regular in the sense they are free of curvature singularities. Depending on the relative values of the total mass m and rotation parameter a, the resulting stationary spacetimes represent different kinds of rotating compact objects such as regular black holes, extremal regular black holes, and regular starlike configurations.

Rotating black holes surrounded by a dark matter halo in the galactic center of M87 and Sgr A[formula omitted]

In this paper, we construct the solutions of modified Schwarzschild spacetime with three kinds of dark matter profiles, i.e. the Beta profiles, Moore profiles and Fermionic halos profiles. In order to make the models more realistic, by using the modified Newman–Janis algorithm (MNJA), we extend our solution to obtain three rotating black holes surrounded by dark matter halo and discuss the black hole properties such as the event horizon, ergoregion, geodesics of null particles and the black hole shadow. We consider two specific examples, i.e. using the present values for the parameters of the dark matter density profiles for the Sgr A∗ black hole and M87 central black hole, it shows that the effects of the dark matter on the size of shadows are almost negligible compared to the shadow of Kerr black hole. Besides, in specific condition having dark matter with very high density, we show that the shadow images of Beta model rotating black hole increase, but the shadow images of Moore model and Fermionic model rotating black holes decrease, furthermore, the Moore profile has the largest deviation, the Beta profile has the least deviation and the deviation of Fermionic halo model is in the middle. These phenomena may provides us with a possibility: the effect of dark matter on the apparent shadow shape could shed some light on mass density profiles of dark matter halo of a galaxy, by studying black holes in dark matter, it may provide an inspiration for us to indirectly detect dark matter.

Modeling the black holes surrounded by a dark matter halo in the galactic center of M87

In this paper, the structure of a dark matter halo can be well described by the mass model of M87 and the Einasto profile for the cold dark matter model, i.e., ρeina(r)=ρeexp(-2α-1((r/re)α-1))\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rho _{\ ext {eina}} (r)=\\rho _\ ext {e} \\exp ( -2 \\alpha ^{-1} ((r/r_\ ext {e})^\\alpha -1 ) )$$\\end{document}(Wang et al. in Nature 585:39–42, 2020). Under these conditions, we construct a solution of a static spherically symmetric black hole in a dark matter halo. Then, using the Newman–Janis algorithm, we extend this static solution to the case of rotation, and obtain a solution for the Kerr-like black hole. We prove that this solution of the Kerr-like black hole is indeed a solution to the Einstein field equations. Finally, taking M87 as an example, we study and analyze some physical properties of this Kerr-like black hole, and then compare them with the Kerr black hole. Particularly, from the perspective of the black hole shadow and the fact that the Kerr-like black hole and the Kerr black hole is distinguishable, we give the upper limit of the shape parameter of the Einasto density profile, that is approximately α<0.22\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha <0.22$$\\end{document}, which may provide a new method to further improve and perfect the density profile of dark matter model. These research results for the black hole in a dark matter halo may indirectly provide an effective method for detecting the existence of dark matter.

A stationary axisymmetric vacuum solution for pure 2 gravity

The closed-form expression for pure vacuum solution obtained in Phys. Rev. D 107, 104 008 (2023) lends itself to a generalization to axisymmetric setup via the modified Newman–Janis algorithm. We adopt the procedure put forth in Phys. Rev. D 90, 064 041 (2014) bypassing the complexification of the radial coordinate. The procedure presumes the existence of Boyer-Lindquist coordinates. Using the Event Horizon Telescope Collaboration results, we model the central black hole M87* by the thus obtained exact rotating metric, depending on the mass, rotation parameter and a third dimensionless parameter. The latter is constrained upon investigating the shadow angular size assuming mass and rotation parameters are those of M87*. Stability is investigated.

Constraints on the rotating self-dual black hole with quasi-periodic oscillations

An impressive feature of loop quantum gravity (LQG) is that it can elegantly resolve both the big bang and black hole singularities. By using the Newman-Janis algorithm, a regular and effective rotating self-dual black hole (SDBH) metric could be constructed, which alters the Kerr geometry with a polymeric function P from the quantum effects of LQG geometry. In this paper, we investigate its impact on the frequency characteristics of the X-ray quasi-periodic oscillations (QPOs) from 5 X-ray binaries and contrast it with the existing results of the orbital, periastron precession and nodal precession frequencies within the relativistic precession model. We apply a Monte Carlo Markov Chain (MCMC) simulation to examine the possible LQG effects on the X-ray QPOs. We found that the best constraint result for the rotating self-dual geometry from LQG came from the QPOs of X-ray binary GRO J1655-40, which establish an upper bound on the polymeric function P less than 6.15 × 10-3 at 95% confidence level. This bound leads to a restriction on the polymeric parameter δ of LQG to be 0.66.

Quantum Gravity Evolution of the Kalb–Ramond like black hole

In this study, we use the Newman–Janis algorithm to investigate the Kalb–Ramond spacetime solutions and the spin parameter is added by the Newman–Janis algorithm. Firstly, we compute the temperature of Kalb–Ramond like black hole with spin parameter to use a quantum tunneling technique, which is obtained by using the surface gravity. We examine the thermodynamics like Hawking temperature and entropy of a Kalb–Ramond like black hole with first order quantum corrections. We first obtain the corrected Hawking temperature through a semi-classical technique by using the Lagrangian equation in the WKB approximation. Moreover, we examine logarithmic corrected entropy for spinning black hole and graphically analyze the effects of spin, correction and Lorentz parameters on the corrected temperature and logarithmic corrected entropy.

4D Einstein-Gauss-Bonnet black hole in Power-Yang-Mills field: a shadow study

We consider a static black hole immersed in the Power-Yang-Mills field in four-dimensional Einstein-Gauss-Bonnet gravity and investigate the effect of various parameters on the radius of the photon sphere. The modified form of the Newman-Janis algorithm is used for obtaining a rotating black hole solution in this gravity. Further, we try to explore the influence of the Yang-Mills magnetic charge Q with power q, Gauss-Bonnet parameter α, and spin a on the horizon radius. The geodesic equations are constructed by incorporating the Hamilton-Jacobi formalism. The radial component of the geodesic equations gives the effective potential which is further used in deriving the mathematical structure for the shadows by using Bardeen's procedure for a fixed observer at infinity. The shadows are calculated and plotted in terms of two celestial coordinates for an equatorial observer. It is observed that all the parameters have a very significant effect on the shadow and related physical observables. We also obtain the constraint values for the spin, magnetic charge, and Gauss-Bonnet parameters, using the shadow size of supermassive black holes Sagittarius A* and M87* from the EHT observations for the cases of q = 0.6 and q = 0.9. It is shown that there are upper and lower bounds for the charge and spin of M87* at q = 0.6, while only the upper bounds for the charge and spin of Sagittarius A*. Finally, we investigate the energy emission rate in the Hawking radiation around the 4D Einstein-Gauss-Bonnet black hole in the Power-Yang-Mills field.

Shadows of Kerr–Vaidya-like black holes

In this work, we study the shadow boundary curves of rotating time-dependent black hole solutions which have well-defined Kerr and Vaidya limits. These solutions are constructed by applying the Newman–Janis algorithm to a spherically symmetric seed metric conformal to the Vaidya solution with a mass function that is linear in Eddington–Finkelstein coordinates. Equipped with a conformal Killing vector field, this class of solution exhibits separability of null geodesics, thus allowing one to develop an analytic formula for the boundary curve of its shadow. We find a simple power law describing the dependence of the mean radius and asymmetry factor of the shadow on the accretion rate. Applicability of our model to recent Event Horizon Telescope observations of M87 and Sgr A is also discussed.

Spherical Particle Orbits around a Rotating Black Hole in Massive Gravity

In this paper, we present a rotating de Rham–Gabadadze–Tolley black hole with a positive cosmological constant under massive gravity, achieved by applying a modified Newman–Janis algorithm. The black hole exhibited stable orbits of constant radii, prompting a numerical study of the behavior of the solutions to a nonic equation governing the radii of planar orbits around the black hole. Additionally, we investigated the stability of orbits near the event horizon and provide a comprehensive analytical examination of the solutions to the angular equations of motion. This was followed by a simulation of some spherical particle orbits around the black hole.

Shadows of rotating Einstein-Maxwell-dilaton black holes surrounded by a plasma

We consider rotating charged black holes with a scalar dilaton field and surrounded by plasma, with the purpose of studying their shadows. The corresponding metric has been previously obtained in the literature from the static solution by using the Newman-Janis algorithm. Assuming a well known form for the pressureless and nonmagnetized plasma distribution, which is suitable for the separation of the Hamilton-Jacobi equation for light, we derive an expression that determines the shape of the shadow. We present some examples of contours and we analyze their observable properties as functions of the charge and the dilaton coupling. We find that the presence of plasma introduces a dependency on the frequency, with the shadow becoming smaller as the frequency decreases.

Newman-Janis algorithm’s application to regular black hole models

We examine the Newman-Janis algorithm's application to an exact regular static solution sustained by a minimally coupled scalar field with a nonstandard kinetic term. Although coordinate complexification leads to a regular Kerr-like black hole, we are facing discrepancies in Einstein's equations in a fairly small domain, for which the regularizing parameter is responsible. Outside this most intriguing region, the geometry is nothing but the standard Kerr spacetime.