We develop a general algorithm that enables the consistent embedding of any four-dimensional static and spherically symmetric geometry into any five-dimensional single-brane braneworld model, characterized by an injective and nonsingular warp factor. Furthermore, we supplement the algorithm by introducing a method that allows one to, in principle, reconstruct 5D field theories that support the aforementioned geometries. This approach is based on a conformal transformation of the metric with the conformal factor being identified with the warp factor of the bulk geometry. The reconstructed theories depend solely on the induced brane geometry, since the warp factor is model-independently represented by a scalar field in the Lagrangian density. As a first application of our reconstruction method, we present for the first time a complete theory that supports the five-dimensional brane-localized extension of the Schwarzschild black hole, for any warp factor. The same method is subsequently utilized to illustrate the process of coherently embedding a de Sitter brane in braneworld models.
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We study the accretion flows towards a central Kerr super-spinning attractor, discussing the formation of the flow inversion points, defined by condition $u^{phi}=0$ on the particles flow axial velocity. We locate two closed surfaces, defining emph{inversion coronas} (spherical shells), surrounding the central attractor. The coronas analysis highlights observational aspects distinguishing the central attractors and providing indications on their spin and the orbiting fluids. The inversion corona is a closed region, generally of small extension and thickness, which is for the counter-rotating flows of the order of $lesssim 1.4 M$ (central attractor mass) on the vertical rotational axis. There are no co-rotating inversion points (from co-rotating flows). The results point to strong signatures of the Kerr super-spinars, provided in both accretion and jet flows. With very narrow thickness, and varying little with the fluid initial conditions and the emission process details, inversion coronas can have remarkable observational significance for primordial Kerr super-spinars predicted by string theory. The corona region closest to the central attractor is the most observably recognizable and active part, distinguishing black holes solutions from super-spinars. Our analysis expounds the Lense--Thirring effects and repulsive gravity effects in the super-spinning ergoregions.
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Our analysis focus on the dragging effects on the accretion flows and jet emission in Kerr super-spinars. These attractors are characterized by peculiar accretion structures as double tori, or special dragged tori in the ergoregion, produced by the balance of the hydrodynamic and centrifugal forces and also effects of super-spinars repulsive gravity. We investigate the accretion flows, constituted by particles and photons, from toroids orbiting a central Kerr super-spinar. As results of our analysis, in both accretion and jet flows, properties characterizing these geometries, that constitute possible strong observational signatures or these attractors, are highlighted. We found that the flow is characterized by closed surfaces, defining inversion coronas (spherical shell), with null the particles flow toroidal velocity ($u^phi=0$) embedding the central singularity. We proved that this region distinguishes proto-jets and accretion driven flows, co-rotating and counter-rotating flows. Therefore in both cases the flow carries information about the accretion structures around the central attractor, demonstrating that inversion points can constitute an observational aspect capable of distinguishing the super-spinars.
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String loop vibrations in a central plane of a Schwarzschild black hole are investigated for various string equations of state. We discuss string loop stability and derive frequencies of vibrational modes. Using the vibrating string loop model we fit the quasi-periodic oscillation (QPO) observed in X-ray signal coming from some compact sources. We demonstrate how the string-loop parameters are related to the radial and vertical fundamental vibration modes, and how the vibrational instability can be related to the Q-factor characterizing the observed QPOs.
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The detailed and comprehensive analysis of radiation processes in accretion disks consisting of electrically charged particles around black holes may provide powerful information about the spacetime geometry of the central black hole. We investigate the circular orbits of electrically charged particles around an electrically charged black-bounce Reissner–Nordström (RN) black hole, known as an RN Simpson–Visser (SV) black hole. We also study the profiles of the innermost stable circular orbits (ISCOs), energy, and angular momentum of the particles in their ISCOs, as well as the efficiency of energy release processes in the accretion disk in the Novikov–Thorne model. Finally, we calculate and study the effects of the black-bounce parameter as well as the black-hole charge on the intensity of the radiation of ultrarelativistic charged particles orbiting the charged RN SV black hole along circular orbits and falling into the black hole. It is observed that the black-bounce parameter essentially decreases the ISCO radius, and consequently the energy extraction and intensity of electromagnetic radiation.
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We study the optical properties of spacetime around a novel regular black hole (BH) in general relativity (GR) coupled to nonlinear electrodynamics (NED), which is asymptotically flat. First, we study the angular velocity and Lyapunov exponent in unstable photon circular orbits in the novel spherically symmetric BH spacetime. Later, the rotating regular BH solution is obtained using the Newmann-Janis algorithm, and the event horizon properties of the BH are determined. We analyze the effective potential for the circular motion of photons in the spacetime of the novel rotating BH. Also, we analyze the photon sphere around the novel BH and its shadow using celestial coordinates. We obtain that an increase of the BH spin and charge as well as NED field nonlinearity parameters causes an increase in the distortion parameter of the BH shadow, while, the area of the shadow and its oblateness decrease. Moreover, we also obtain the constraint values for the BH charge and the nonlinearity parameters using Event Horizon Telescope data from shadow sizes of supermassive BHs Sgr A* and M87*. Finally, the emission rate of BH evaporation through Hawking radiation is also studied.
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One of the most important issues in relativistic astrophysics is to explain the origin mechanisms of (ultra)high energy charged particle components of cosmic rays. Black holes (BHs) being huge reservoirs of (gravitational) energy can be candidates for such particle sources. The main idea of this work is to study the effects of scalar-tensor-vector gravity (STVG) on particle acceleration by examining charged particle dynamics and their acceleration through the magnetic Penrose process (MPP) near magnetized Kerr-MOG BHs. First, we study the horizon structure of the BH. Also, we study the effective potential to gain insight into the stability of circular orbits. Our results show that the magnetic field can extend the region of stable circular orbits, whereas the STVG parameter reduces the {instability} of the circular orbit. The motion of charged particles around the magnetized BH reveals various feasible regimes of the ionized Keplerian disk behavior. Thus, from the examination of particle trajectories we observe that at fixed values of other parameters, the Schwarzschild BH captures the test particle; in the case of Kerr BH, the test particle escapes to infinity or is captured by the BH, while in Kerr-MOG BH, the test particle is trapped in some region around BH and starts orbiting it. On investigating the MPP, we found that with increasing magnetic field, the behavior of orbits becomes more chaotic. As a result, the particle escapes to infinity more quickly.
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We study motion of a phonon, a particle representing the quanta of the sound wave in the (2+1) spacetime of the acoustic analogous axially symmetric black hole, so-called acoustic (sonic) black hole. Similar to the real objects known as black holes in relativity theories, the phenomenon called acoustic black hole possesses the ergoregion whose area is increasing with increasing rotation of the black hole, leading to more phonons being affected by the supersonic flow. It is found that phonons in the ergoregion of an acoustic black hole behave differently than those outside of it. Specifically, we found that the phonons in the ergoregion are affected by the supersonic flow of the fluid, causing them to move in different directions than those outside the ergoregion. Moreover, we presented calculations of the deflection angle and time delay of the phonon in the field of the acoustic black hole in the weak field regime that can be useful to test the geometry of the acoustic black hole in the laboratory.
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The present work is devoted to the study of the dynamics of charged particles around Simpson-Visser black holes (with the length parameter l ≤ 2) and wormholes (l >2 ) immersed in an external asymptotically uniform magnetic field. To do this, first, we solve the Maxwell equation for 4-potentials of the electromagnetic field and show that the difference between the numerical solution and Wald's solution is small enough to neglect it, which may allow us to use the solution obtained by Wald. We also study fundamental frequencies of in the vertical and radial oscillations of charged particles around circular stable orbits around the magnetized black hole. The effects of the magnetic interaction and length parameters on the fundamental frequencies. We investigate the quasiperiodic oscillations (QPOs) around the black hole in relativistic precession and epicyclic resonance models. It is also shown that the combined effects of magnetic interaction for negatively charged particles and length parameters can mimic the spacetime effects of the Schwarzschild black hole compensating for their effects, as well as the spin of rotating Kerr black holes. The distance between an orbit where a QPO is generated with the ratio of upper and lower frequencies 3: 2 and innermost stable circular orbits is also studied. It is found that the QPO orbits are very close to ISCO in the RP model at l <2 . This implies that the obtained result helps to determine the ISCO around black holes. We also study the applications of observed QPOs around stellar-mass black holes in microquasars and supermassive black holes.
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In this article, we try to capture the influence of deviation from standard Kerr black hole spacetime on observed high-frequency quasi-periodic oscillations signal. We explore the dynamics of test particles in the field of rotating compact objects governed by the various modifications of the standard Kerr black hole spacetime and apply the model of epicyclic oscillations of Keplerian discs to the observed microquasars and active galactic nuclei high-frequency quasi-periodic oscillations data. We presented a generalized formalism for the fitting of the high-frequency quasi-periodic oscillations models so-called epicyclic resonance and relativistic precession models, under the assumption of stationary, axisymmetric, and asymptotically flat spacetimes. Recently, we have used the same set of stationary, axisymmetric, and asymptotically flat spacetimes, and estimated the restrictions of spacetime parameters with the help of hot-spot data of three flares observed at Sgr~A* by GRAVITY instrument citep{Shahzadi-et-al:2022:EPJC:}. The aim of this work is not to test a particular theoretical model or to determine and constrain its parameters, but to map a set of well-astrophysically motivated deviations from classical Kerr black hole spacetime and demonstrate which ones provide the best fit for high-frequency quasi-periodic oscillations data and could be fruitful for future exploration.
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