In this letter a new Lagrangian variational principle is proved to hold for the Einstein field equations, in which the independent variational tensor field is identified with the Ricci curvature tensor Rμ ν rather than the metric tensor gμ ν. The corresponding Lagrangian function, denoted as LR, is realized by a polynomial expression of the Ricci 4-scalar R ≡gμ νRμ ν and of the quadratic curvature 4-scalar ρ ≡Rμ νRμ ν . The Lagrangian variational principle applies both to vacuum and non-vacuum cases and for its validity it demands a non-vanishing, and actually also positive, cosmological constant Λ >0 . Then, by implementing the deDonder-Weyl formalism, the physical conditions for the existence of a manifestly-covariant Hamiltonian structure associated with such a Lagrangian formulation are investigated. As a consequence, it is proved that the Ricci tensor can obey a Hamiltonian dynamics which is consistent with the solutions predicted by the Einstein field equations.
Read More
We study the epicyclic oscillatory motion around circular orbits of the traversable asymptotically flat and reflection-symmetric wormholes obtained in the Einstein-Dirac-Maxwell theory without applying exotic matter in their construction. We determine frequencies of the orbital and epicyclic motion in the Keplerian disks having inner edge at the marginally stable circular geodesic of the spacetime. The obtained frequencies are applied in the so-called geodesic models of high-frequency quasiperiodic oscillations (HF QPOs) observed in microquasars and active galactic nuclei containing a supermassive central object. We show that even the simplest epicyclic resonance variant of the geodesic models can explain the HF QPOs observed in many active galactic nuclei for realistic choices of the wormhole parameters, but there are some of the sources where only wormholes with unrealistically large values of the parameters can be sufficient for the explanation. On the other hand, in the case of microquasars, the observed HF QPOs strongly restrict the acceptable values of the wormhole parameters.
Read More
Recently introduced exact solution of the Einstein gravity coupled minimally to an anisotropic fluid representing dark matter can well represent supermassive black holes in galactic nuclei with realistic distribution of dark matter around the black hole, given by the Hernquist-like density distribution. For these fluid-hairy black hole spacetimes, properties of the gravitational radiation, quasinormal ringing, and optical phenomena were studied, giving interesting results. Here, using the range of physical parameters of these spacetimes allowing for their relevance in astrophysics, we study the epicyclic oscillatory motion of test particles in these spacetimes. The frequencies of the orbital and epicyclic motion are applied in the epicyclic resonance variant of the geodesic model of quasiperiodic oscillations (QPOs) observed in active galactic nuclei to demonstrate the possibility to solve the cases where the standard vacuum black hole spacetimes are not allowing for explanation of the observed data. We demonstrate that the geodesic model can explain the QPOs observed in most of the active galactic nuclei for the fluid-hairy black holes with reasonable halo parameters.
Read More
An analytical solution representing traversable asymptotically flat and symmetric wormholes was obtained without adding exotic matter in two different theories independently: in the Einstein-Maxwell-Dirac theory and in the second Randall-Sundrum brane-world model. Further, a smooth normalizable asymmetric wormhole solution has been recently obtained numerically in the Einstein-Maxwell-Dirac theory. Using the time-domain integration method we study quasinormal ringing of all these wormholes with emphasis to the regime of mimicking the near extremal Reissner-Nordström black holes, which is characterised by echoes. In addition, we calculate radius of shadows cast by these wormholes.
Read More
We explore the possibility of jet collisions with accreting tori orbiting around super-massive black holes. The analysis provides constraints on the formation and the observational evidence of the host configurations. We use a General Relativistic Hydrodynamic model, investigating the light surface contraints in aggregates of misaligned tori orbiting a central static Schwarzschild black hole. Each (toroidal) configuration of the agglomeration is a geometrically thick, pressure-supported, perfect fluid torus. Aggregates include proto-jets, the open cusped solutions associated with the geometrically thick tori. Collision emergence and the stability properties of the aggregates are considered at different inclination angles relative to a fixed distant observer. We relate the constraints to the relevant frequencies of the configurations and fluid specific angular momentum, separating the constraints related to the fluid hydrodynamics and those related to the geometric backgrounds. We analyze the existence of accreting tori supporting jet-emission. We discuss the existence of orbit-replicas that could host shadowing effects in replicas of the emissions in two regions; close to and far from the BH (horizon replicas in jet shells). Our investigation clarifies the role of the pressure gradients of the orbiting matter and the essential role of the radial gradient of the pressure in the determination of the disk verticality. Finally, we analyze the possibility that a toroidal magnetic field could be related to the collimation of proto-jets.
Read More
In many astrophysical scenarios the charge of the black hole is often neglected due to unrealistically large values of the charge required for the Reissner-Nordström spacetime metric. However, black holes may possess small electric charge due to various selective accretion mechanisms. In this paper we investigate the effect of a small hypothetical electric charge of a Schwarzschild black hole on the ionization of a freely falling neutral particle and subsequent escape of the ionized particle from the black hole. We show that the energy of ionized particle can grow ultrahigh and discuss distinguishing signatures of particle acceleration by weakly charged black holes. We also discuss a possible application of the proposed mechanism as an alternative cosmic ray acceleration scenario. In particular we show that the Galactic center supermassive black hole is capable to act as a PeVatron of protons. The presented mechanism can serve as a simple toy model of a nonrotating compact object acting as a particle accelerator with a potential astrophysical implementations related to the cosmic ray physics and beyond.
Read More
We present a review of the Penrose process and its modifications in relation to the Kerr black holes and naked singularities (superspinars). We introduce the standard variant of this process, its magnetic version connected with magnetized Kerr black holes or naked singularities, the electric variant related to electrically charged Schwarzschild black holes, and the radiative Penrose process connected with charged particles radiating in the ergosphere of magnetized Kerr black holes or naked singularities. We discuss the astrophysical implications of the variants of the Penrose process, concentrating attention to the extreme regime of the magnetic Penrose process leading to extremely large acceleration of charged particles up to ultra-high energy E∼1022 eV around magnetized supermassive black holes with mass M∼1010M⊙ and magnetic intensity B∼104 G. Similarly high energies can be obtained by the electric Penrose process. The extraordinary case is represented by the radiative Penrose process that can occur only around magnetized Kerr spacetimes but just inside their ergosphere, in contrast to the magnetic Penrose process that can occur in a more extended effective ergosphere determined by the intensity of the electromagnetic interaction. The explanation is simple, as the radiative Penrose process is closely related to radiated photons with negative energy whose existence is limited just to the ergosphere.
Read More
The combined effect of a strong gravitational field and electromagnetic field in the vicinity of a generic regular black hole related to non-linear electrodynamics with Maxwellian weak-field limit on the radiation flux of a hot spot orbiting the regular black hole on the Keplerian disc has been studied. The frequency shift due to the strong gravitational field and magnification of the radiation related to gravitational lensing have been calculated. We compare the flux related to the Maxwellian regular black hole to the flux of the hot spots moving with the same orbital period around standard Schwarzschild and Raissner-Nordstrom black holes to illustrate the role of the effective geometry governing photon motion around the regular black hole.
Read More
Region of trapped null geodesics hidden inside of extremely compact objects is of astrophysical importance because of trapping of gravitational waves, or neutrinos. The trapping effect of null geodesics was extensively studied for spherically symmetric extremely compact objects. Recently, influence of rotation of the extremely compact objects on the trapping of null geodesics was treated in the simplest possible model of internal linearized Hartle-Thorne spacetime with uniform energy density distribution and uniform emissivity distribution of null geodesics. Here, we extend the study of the rotation influence on the trapping effect in the case of linearized Hartle-Thorne spacetimes based on the Tolman VII spherically symmetric solutions, where we assume the emissivity of the null geodesics proportional to the energy density of the Tolman VII object having quadratic radial profile. We demonstrate enhancement (suppression) of the trapping effect in the case of counter-rotating (co-rotating) null geodesics due to the behavior of the effective potentials and escape cones of the null geodesics in the linearized Hartle-Thorne-Tolman VII spacetimes. In dependence on the parameters of these spacetimes, we determine the "local" and "global" coefficients of efficiency of the trapping and compare the results to those related to the rotating spacetimes based on the internal Schwarzschild spacetimes. We demonstrate that in the Tolman VII spacetimes the trapping is more efficient, being allowed in objects with radii larger than those of the trapping internal Schwarzschild spacetimes, occurring even for R >3.3 M.
Read More
The exact analytical solution of Einstein-Maxwell-scalar (EMS) field equations has been explored which covers several well-known solutions such as the Reissner-Nordström, Schwarzschild-MOG (Modified Gravity), Janis-Newman-Winicour, and Schwarzschild solutions. It has been assumed that the interactions between the tensor, vector, and scalar fields are negligible. The newly obtained solution is characterized by three free parameters as the total mass M, electric (magnetic) charge Qe (Qm), and scalar charge C (or n parameter) of the gravitational compact object. It is also shown that dual solution for the vector potential Aϕ =Qm cos θ is satisfied by the EMS field equations and the electric charge can be safely replaced by magnetic charge Qm. Finally, we have studied curvature invariants and test particle motion around the gravitational source described the obtained new spacetime metric. We have also provided analysis of degeneracy values of spin parameter of the rotating Kerr black hole and charge parameter of compact object described by the new spacetime metric through comparison of radii of ISCO, photonsphere and energy efficiency. It is shown that new black hole solution in Einstein-Maxwell-scalar field theory can mimic spin parameter of the Kerr black hole up to a∗ ≲ 0 . 6 while the astrophysical black hole' observations that it reaches up to a∗ ≲ 0 . 99 . Consequently, one may conclude that the obtained new black hole solution can be considered as realistic candidate for the astrophysical black holes with a∗ ≲ 0 . 6 .
Read More
