Einstein Maxwell Scalar Theory Research Articles

Plasma impact on black hole shadows and gravitational weak lensing in the Einstein–Maxwell-scalar theory

In this paper, we investigate the optical properties of a non-rotating charged black hole (BH) in the Einstein–Maxwell-scalar (EMS) theory, together with a plasma medium. We first consider the photon sphere and shadow radius under the impact of the plasma medium existing in the environment surrounding the BH in the EMS theory. We show that the radius of the photon sphere and the BH shadow decrease under the influence of the parameter β. We further study gravitational weak lensing in detail by adapting general methods and derive the light ray’s deflection angle around the BH together with the plasma environment. It is found that for uniform plasma, the deflection angle increases with the rise of the plasma parameter, whereas it decreases with the increase of the plasma parameter for non-uniform plasma. Besides, we also study the magnification of image brightness.

Thin accretion disk around black hole in Einstein–Maxwell-scalar theory

We examine the accretion process in a thin disk surrounding a supermassive black hole within the framework of Einstein–Maxwell-scalar (EMS) gravity. Our investigation aims to elucidate how variations in model parameters affect different physical properties of the disk. When keeping EMS parameters β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document} and q constant, we observe a reduction in radiation flux and temperature as α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} increases. However, the luminosity and radiative efficiency exhibit relatively minor variation. Conversely, under fixed α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} and q, an escalation in β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document} leads to heightened levels of radiation flux, temperature, luminosity, and radiative efficiency. These results underscore the diverse influences of model parameters on observable metrics, providing valuable insights for the astronomical study of distinct black holes.

Restoring strong cosmic censorship in Reissner–Nordström–de Sitter black holes via non-minimal electromagnetic-scalar couplings

We investigate whether the Strong Cosmic Censorship (SCC) Conjecture can be reinstated at the classical level in Reissner–Nordström–de Sitter (RNdS) black holes by introducing non-minimal couplings between the electromagnetic and scalar fields in Einstein–Maxwell-scalar (EMS) theory. By conducting numerical calculations, we find that the SCC can be restored within a specific range of the coupling constant. Notably, for a given value of the cosmological constant, there exists a critical coupling constant above which the EMS theory satisfies the SCC. These findings suggest that the non-minimal couplings between the electromagnetic and scalar fields may play a crucial role in the restoration of the SCC in RNdS spacetime. Additionally, under negative coupling constants, the low-lying modes of nearly extremal black holes exhibit β>1,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta > 1,$$\\end{document} enabling C2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C^2$$\\end{document} extensions beyond the Cauchy horizon and intensifying the violation of the SCC.

Charged particles and Penrose process near charged black holes in Einstein–Maxwell-scalar theory

We study the dynamics of charged test particles around an electrically charged black hole in Einstein–Maxwell-scalar (EMS) gravity. The event horizon properties of the spacetime around the black hole are explored and the upper limit for the EMS theory parameters corresponding to extreme charge and minimal value of the event horizon are found. The effective potential for the radial motion of the charged particles at the equatorial plane is investigated. Specific energy and angular momentum of the particles corresponding to circular stable orbits are also studied. We also investigate the effects of the EMS parameter and the black hole charge on innermost stable circular orbits (ISCOs). We also investigate synchrotron radiation of charged particles in the spacetime of the charged black hole in EMS gravity. We also explore electric Penrose and Bañados–Silk–West processes near the black hole horizon, where we analyse in detail the effects of EMS parameters on energy efficiency in the Penrose process and critical angular momentum that allows colliding particles near the horizon, together with the center of mass energy in charged particles collisions.

Spontaneous scalarization of dyonic black hole in Einstein–Maxwell-scalar theory

In this paper, we study the scalarization of the static and spherically symmetric dyonic Reissner–Nordstrom (RN) black holes in the Einstein–Maxwell-scalar theory where the scalar field is coupled to an electromagnetic Chern–Simons term. When both electric and magnetic charges are present, there exists an unstable region of parametric space for the dyonic RN black holes where the scalarization of black holes should occur. That is to say, mixing electric and magnetic charges can reduce the scalarization in this theory. Firstly, we calculate the perturbation field equations under the dyonic RN black hole background and obtain the corresponding asymptotic-flat perturbation solutions, which are the bifurcation points at the dyonic RN branch. The results show that the perturbation scalarization demands a lower bound of the coupling constant. Then, we calculate the scalarized black hole solutions bifurcating from the dyonic RN solutions. We find that there exist a lot of discrete branches of the scalarized solutions. Contract to the dyonic RN solutions, these scalarized solutions can be overcharged and their mass could even approach zero. After illustrating the behavior of the entropy for the scalarized black holes, we demonstrate that the scalarized configurations might be thermodynamically more stable than GR configurations. Moreover, we also show that for each scalarized branch, the black hole cannot reach the extremal limit with vanishing temperature.

Quasiperiodic oscillation around charged black holes in Einstein–Maxwell-scalar theory

In the present paper, first, we study the event horizon properties of charged black holes (BHs) in Einstein Maxwell-scalar (EMS) gravity. Then, we investigate the circular motion of test particles’ around the BH in the EMS gravity. We also analyze the effects of the EMS parameters on the position of innermost circular orbits (ISCOs), energy, and angular momentum of the test particles corresponding to circular orbits. We provide detailed studies of the efficiency of energy release from EMS BHs based on the Hartle–Thorne model and fundamental frequencies of oscillations of particles along their circular stable orbits. Moreover, we have explored possible values of upper and lower frequencies of twin-peak quasiperiodic oscillations (QPOs) around the BHs. Finally, we obtain relationships between the BH charge and the EMS parameters using observational data from the QPOs detected in the microquasars: GRS 1905+105, GRO J 1655-40, H 1745+322, and XTE 1550-564.

Spin-charge induced spontaneous scalarization of Kerr-Newman black holes

We investigate the tachyonic instability of Kerr-Newman (KN) black holes in the Einstein-Maxwell-scalar (EMS) theory with a positive coupling parameter $\alpha$. This corresponds to exploring the onset of spontaneous scalarization for KN black holes. For this purpose, we use the hyperboloidal foliation method (HFM) to solve the linearized scalar equation numerically. We obtain a 3D graph [$\log_{10}\alpha(a,Q)$] which indicates the onset surface for spontaneous scalarization of KN black holes in the EMS theory. We find that there is no lower bound on the rotational parameter $a$, but its upper bound is given by $M^2-Q^2 \ge a^2$. Also, we confirm that the high rotation enhances spontaneous scalarization of KN black holes in the EMS theory.

Spin-induced scalarization of Kerr-Newman black holes in Einstein-Maxwell-scalar theory

In this paper, we consider the tachyonic instability of Kerr-Newman (KN) black holes in the Einstein-Maxwell-scalar (EMS) theory with nonminimal negative scalar coupling to Maxwell term, and then obtain a bound $(\frac{a}{r_+}\geq0.4142)$ for spin parameter $a$ of KN black hole in the limit of coupling constant $\alpha\rightarrow-\infty$ by using analytical method. In addition, we perform a $(2+1)$-dimensional time evolution of linearized scalar field perturbation on the KN black hole background by implementing the hyperboloidal foliation method numerically in the EMS theory. It recovers the \textit{spontaneous scalarization} phenomenon for KN black hole in some certain regions of the parameter spaces. We further plot corresponding threshold curves $\alpha(a)$ which describe a boundary between bald KN black holes and scalarized spinning black holes in the EMS theory.

Dynamics and collisions of magnetized particles around charged black holes in Einstein–Maxwell-scalar theory

In this paper, we aim to investigate the dynamics of magnetized particles around magnetically and electrically charged static black holes (BHs) in Einstein–Maxwell-scalar theory. First, we explore the possible values of EMS parameters for which the spacetime geometry represents a BH spacetime. Since there is no interaction between the electric field of the electrically charged BH and the proper magnetic field of the magnetized test particle. Therefore, we consider BH immersed in an external asymptotically uniform magnetic field. We explore the properties of an external magnetic field around a charged EMS BH. Moreover, we also explored the effects of BH charge and EMS theory parameters on particle’s energy and angular momentum in the circular stable orbits, together with the radius of innermost stable circular orbits. All the obtained results are compared with the acquired results of the Reissner–Nordström BH. Finally, we investigate the behaviour of the center of mass energy of colliding magnetized and electrically charged particles around the EMS BH.

Black Hole and Wormhole Solutions in Einstein–Maxwell Scalar Theory

We classified and studied the charged black hole and wormhole solutions in the Einstein–Maxwell system in the presence of a massless, real scalar field. The possible existence of charged black holes in general scalar–tensor theories was studied in Bronnikov et al., 1999; black holes and wormholes exist for a negative kinetic term for the scalar field. Using a conformal transformation, the static, spherically symmetric possible structures in the minimal coupled system are described. Besides wormholes and naked singularities, only a restricted class of black hole exists, exhibiting a horizon with an infinite surface and a timelike central singularity. The black holes and wormholes defined in the Einstein frame have some specificities with respect to the non-minimal coupling original frame, which are discussed in the text.

Предельно короткие оптические импульсы в присутствии дилатонов

The propagation of an extremely short optical pulse is analyzed based on a numerical solution of the Maxwell`s equation related to the dilaton field in a flat space-time. The dynamics of the pulse turned out to be unstable and the pulse collapses. The influence of the parameter α of the Lagrangian is analyzed for the cases of the Einstein-Maxwell scalar theory, low-energy action of string theory, Kaluza-Klein field equations obtained from dimensional reduction of Einstein’s five-dimensional theory.

Slowly rotating black holes in the novel Einstein\u2013Maxwell-scalar theory

We investigate a slowly rotating black hole solution in a novel Einstein–Maxwell-scalar theory, which is prompted by the classification of general Einstein–Maxwell-scalar theory. The gyromagnetic ratio of this black hole is calculated, and it increases as the second free parameter beta increases, but decreases with the increasing parameter gamma equiv frac{2 alpha ^{2}}{1+alpha ^2}. In the Einstein–Maxwell-dilaton (EMD) theory, the parameter beta vanishes but the free parameter alpha governing the strength of the coupling between the dilaton and the Maxwell field remains. The gyromagnetic ratio is always less than 2, the well-known value for a Kerr–Newman (KN) black hole as well as for a Dirac electron. Scalar hairs reduce the magnetic dipole moment in dilaton theory, resulting in a drop in the gyromagnetic ratio. However, we find that the gyromagnetic ratio of two can be realized in this Einstein–Maxwell-scalar theory by increasing beta and the charge-to-mass ratio Q/M simultaneously (recall that the gyromagnetic ratio of KN black holes is independent of Q/M). The same situation also applies to the angular velocity of a locally non-rotating observer. Moreover, we analyze the period correction for circular orbits in terms of charge-to-mass ratio, as well as the correction of the radius of the innermost stable circular orbits. It is found the correction increases with beta but decreases with Q/M. Finally, the total radiative efficiency is investigated, and it can vanish once the effect of rotation is considered.

Constructing black holes in Einstein–Maxwell-scalar theory

Exact black hole solutions in the Einstein–Maxwell-scalar theory are constructed. They are the extensions of dilaton black holes in de Sitter or anti de Sitter universe. As a result, except for a scalar potential, a coupling function between the scalar field and the Maxwell invariant is present. Then the corresponding Smarr formula and the first law of thermodynamics are investigated.

Test particle motion around a black hole in Einstein-Maxwell-scalar theory

In this paper, we explore the test particle motion around a black hole in Einstein-Maxwell-scalar (EMS) theory using three different black hole solutions within this theory. We have first analyzed the spacetime curvature structure of these solutions and shown the existence of two singularities and the first one is at the center $r=0$. In black hole spacetime, there are two regions divided by the critical value of the cosmological parameter $\lambda_0$. The photon sphere around the black hole in EMS theory has also been studied and found that it does not depend on cosmological parameter $\lambda$. We have analyzed the innermost stable circular orbits (ISCO) around the black hole and shown that for all solutions ISCO radius for neutral particle decreases with the increase of black hole charge. We have also studied the charged particle motion around the black hole where charged particle motion is considered in the presence of the gravitational field and the Coulomb potential. It is shown that ISCO radius for charged particles increases depending on the selected value of the coupling parameter which is in contradiction with observations of the inner edge of the accretion disks of the astrophysical black holes and can be used as a powerful tool to rule out the EMS theory from consideration for the gravitational field theory. It is also studied the fundamental frequencies governed by test particle orbiting around the black hole in EMS theory. Finally, as a test of black hole solution in EMS theory ISCO radii is compared with that in Kerr black hole and found that the spin parameter of Kerr can be mimic up to $a/M\simeq 0.936$.

Scalarized charged black holes with scalar mass term

We study the scalarized charged black holes in the Einstein-Maxwell-Scalar (EMS) theory with scalar mass term. In this work, the scalar mass term is chosen to be $m^2_\phi=\alpha/\beta$, where $\alpha$ is a coupling parameter and $\beta$ is a mass-like parameter. It turns out that any scalarized charged black holes are not allowed for the case of $\beta \le 4.4$ with $q=0.7(M=0.5,Q=0.35)$ because this case implies the stable Reissner-Nodstr\"{o}m (RN) black holes. In the massless limit of $\beta\to \infty$, one recovers the case of the EMS theory. We note that the unstable RN black hole implies the appearance of scalarized charged black holes. The other unstable case of $\beta>4.4$ with $q=0.7$ allows us to obtain the $n=0,1,2,\cdots$ scalarized charged black holes for $\alpha(\beta)\ge \alpha_{\rm th}(\beta)$ where $\alpha_{\rm th}(\beta)$ represents the threshold of instability for the RN black hole. Furthermore, it is shown that the $n=0$ black hole is stable against radial perturbations, while the $n=1$ black hole is unstable. This stability result is independent of the mass parameter $\beta$.

Master equations and stability of Einstein-Maxwell-scalar black holes

We derive master equations for linear perturbations in Einstein-Maxwell scalar theory, for any spacetime dimension D and any background with a maximally symmetric n = (D - 2)-dimensional spatial component. This is done by expressing all fluctuations analytically in terms of several master scalars. The resulting master equations are Klein­ Gordon equations, with non-derivative couplings given by a potential matrix of size 3, 2 and 1 for the scalar, vector and tensor sectors respectively. Furthermore, these potential matrices turn out to be symmetric, and positivity of the eigenvalues is sufficient (though not necessary) for linear stability of the background under consideration. In general these equations cannot be fully decoupled, only in specific cases such as Reissner-Nordstrom, where we reproduce the Kodama-Ishibashi master equations. Finally we use this to prove stability in the vector sector of the GMGHS black hole and of Einstein-scalar theories in general.

Stability of scalarized charged black holes in the Einstein\u2013Maxwell\u2013Scalar theory

We analyze the stability of scalarized charged black holes in the Einstein–Maxwell–Scalar (EMS) theory with quadratic coupling. These black holes are labelled by the number of n=0,1,2,ldots , where n=0 is called the fundamental black hole and n=1,2,ldots denote the n-excited black holes. We show that the n=0 black hole is stable against full perturbations, whereas the n=1,2 excited black holes are unstable against the s(l=0)-mode scalar perturbation. This is consistent with the EMS theory with exponential coupling, but it contrasts to the n=0 scalarized black hole in the Einstein–Gauss–Bonnet–Scalar theory with quadratic coupling. This implies that the endpoint of unstable Reissner-Nordström black holes with alpha >8.019 is the n=0 black hole with the same q. Furthermore, we study the scalarized charged black holes in the EMS theory with scalar mass m^2_phi =alpha /beta .

Quasinormal modes of scalarized black holes in the Einstein–Maxwell–Scalar theory

We perform the stability analysis on scalarized charged black holes in the Einstein–Maxwell–Scalar (EMS) theory by computing quasinormal mode spectrum. It is noted that the appearance of these black holes with scalar hair is closely related to the instability of Reissner–Nordström black holes without scalar hair in the EMS theory. The scalarized charged black hole solutions are classified by the order number of n=0,1,2,⋯, where n=0 is called the fundamental branch and n=1,2,⋯ denote the n excited branches. Here, we show that the n=1,2 excited black holes are unstable against the s(l=0)-mode scalar perturbation, while the n=0 black hole is stable against all scalar–vector–tensor perturbations. This is consistent with other scalarized black holes without charge found in the Einstein–Scalar–Gauss–Bonnet theory.

Einstein–Born–Infeld black holes with a scalar hair in three dimensions

We present black hole solutions in (2+1)-dimensional Einstein’s theory of gravity coupled with Born–Infeld (BI) nonlinear electrodynamic and a massless self-interacting scalar field. The model has five free parameters: mass [Formula: see text], cosmological constant [Formula: see text], electric [Formula: see text] and scalar [Formula: see text] charges and BI parameter [Formula: see text]. To attain exact solution for such a highly nonlinear system we adjust, i.e. finely tune, the parameters of the theory with the integration constants. In the limit [Formula: see text], we recover the results of Einstein–Maxwell–Scalar theory, obtained before. The self-interacting potential admits finite minima apt for the vacuum contribution. Hawking temperature of the model is investigated versus properly tuned parameters. By employing this tuned-solution as basis, we obtain also a dynamic solution which in the proper limit admits the known solution in Einstein gravity coupled with self-interacting scalar field. Finally, we establish the equations of a general scalar–tensor field coupled to nonlinear electrodynamics (NED) field in 2+1 dimensions without searching for exact solutions.