We study the accretion flow from a counter-rotating torus orbiting a central Kerr black hole (BH). We characterize the flow properties at the turning point of the accreting matter flow from the orbiting torus, defined by the condition uϕ = 0 on the flow toroidal velocity. The counter-rotating accretion flow and jet-like flow turning point location along BH rotational axis is given. Some properties of the counter-rotating flow thickness and counter-rotating tori energetics are studied. The maximum amount of matter swallowed by the BH from the counter-rotating tori is determined by the background properties. The fast spinning BH energetics depends mostly on BH spin rather than on the properties of the counter-rotating fluids or the tori masses. The turning point is located in a narrow orbital corona (spherical shell), for photons and matter flow constituents, surrounding the BH stationary limit (outer ergosurface), depending on the BH spin-mass ratio and the fluid initial momentum only. The turning corona for jet-like flow has larger thickness, it is separated from the torus flow turning corona and it is closer to the BH stationary limit. Turning points of matter accreting from torus and from jets are independent explicitly of the details of the accretion and tori model. The turning corona could be observable due to an increase of flow luminosity and temperature. The corona is larger on the BH equatorial plane, where it is the farthest from the central attractor, and narrower on the BH poles.
Read More
The Tolman VII solution is considered by some as one of the few analytical solutions to Einstein's equations, which describes approximately well the interior of neutron stars (NSs). This solution is characterized by the mass M , radius R , and an energy density that varies quadratically with the radial coordinate r . Recently, Jiang and Yagi proposed a modification of this solution, the so-called modified Tolman VII (MTVII) solution, by introducing an additional quartic term to the energy density radial profile. The MTVII solution is an approximate solution to Einstein's equation, which includes a new parameter α that allows the solution to have a better agreement with the energy density profiles for realistic NSs. Here we consider the MTVII solution, showing that for certain values of the parameter α and compactness C this solution manifests a region of negative pressure near the surface which leads to negative values of the tidal Love number. To alleviate these drawbacks, we introduce an exact version of the MTVII solution obtained by solving numerically Einstein's equations for the MTVII energy density profile. As an application of our new exact MTVII (EMTVII) solution, we calculate the tidal Love number and tidal deformability, as a function of C , for different values of the parameter α . We find that the EMTVII solution predicts a positive tidal Love number for the whole range of allowed values of parameters (C ,α ), in agreement with previous results for realistic NSs.
Read More
We study the existence of charged fluid nonconducting structures orbiting in the background given by a Schwarzschild black hole immersed in a monopolelike magnetic field introduced in the context of the Blandford-Znajek process. Due to fact that the split-monopole magnetic field is not defined in the equatorial plane, where typical accretion disks are located, we focus on searching for off-equatorial charged toroidal structures. We demonstrate that charged nonconducting tori can arise very close to the symmetry axis of the magnetized black hole; thus representing a possible obstacle for jets created due to the Blanford-Znajek process.
Read More
We are fitting dynamics of electrically neutral hot-spot orbiting around Sagittarius A* (Sgr A*) source in Galactic center, represented by various modifications of the standard Kerr black hole (BH), to the three flares observed by the GRAVITY instrument on May 27, July 22, July 28, 2018. We consider stationary, axisymmetric, and asymptotically flat spacetimes describing charged BHs in general relativity (GR) combined with nonlinear electrodynamics, or reflecting the influence of dark matter (DM), or in so called parameterized dirty Kerr spacetimes, and test them using the hot-spot data. We show that the orbital frequencies as well as positions of the hot-spots orbiting the considered BHs fit the observed positions and periods of the flare orbits, and give relevant constraints on the parameters of the considered BH spacetimes and the gravity or other theories behind such modified spacetimes.
Read More
We study evolution of the braneworld Kerr-Newman (K-N) naked singularities, namely their mass M , spin a , and tidal charge b characterizing the role of the bulk space, due to matter in-falling from Keplerian accretion disk. We construct the evolution in two limiting cases applied to the tidal charge. In the first case we assume b =const during the evolution, in the second one we assume that the dimensionless tidal charge β ≡b /M2=const . For positive values of the tidal charge the evolution is equivalent to the case of the standard K-N naked singularity under accretion of electrically neutral matter. We demonstrate that counterrotating accretion always converts a K-N naked singularity into an extreme K-N black hole and that the corotating accretion leads to a variety of outcomes. The conversion to an extreme K-N black hole is possible for naked singularity with dimensionless tidal charge β <0.25 , and β ∈(0.25 ,1 ) with sufficiently low spin. In other cases the accretion ends in a transcendental state. For 0.25 <β <1 this is a mining unstable K-N naked singularity enabling formally unlimited energy extraction from the naked singularity. In the case of β >1 , the corotating accretion creates unlimited torodial structure of mater orbiting the naked singularity. Both nonstandard outcomes of the corotating accretion imply a transcendence of such naked singularity due to nonlinear gravitational effects.
Read More
Recently, the Gravitational Decoupling through the Minimal Geometric Deformation was applied to study a mixture of a spherically symmetric internal solution of the Einstein gravitational equations with a polytropic fluid, giving interesting results of the energetic interchanges in the special case of the Tolman IV solution. In this work, we extend these newly introduced methods to the case of Tolman VII space-times that are currently considered as a convenient exact solution of Einstein equations representing relatively precisely realistic neutron stars.
Read More
The motion of photons, neutral and charged particles around wormhole described by exponential metric which is the solution of the Einstein-scalar field equations has been studied. It is also proven that the wormhole solution is a special case of the well-known JNW spacetime and that the scalar field corresponds to the phantom field which has the anti-gravitating repulsive feature. It is also shown that the characteristic radii, namely, innermost stable circular orbit (ISCO), marginally bound orbit (MBO) and photon sphere in the spacetime described by exponential metric are less than that obtained in the Schwarzschild spacetime. However, the size of the shadow of the wormhole is greater than the shadow of the Schwarzschild black hole. The energy efficiency in the spacetime described by exponential metric is relatively large than that in the Schwarzschild spacetime. Later it is assumed that the wormhole is embedded in the asymptotically uniform magnetic field which behaves similar to the dipole-like field near the origin. The dynamical motion of charged particles around the wormhole described by exponential metric implanted in the asymptotically uniform magnetic field has been studied assuming that charged particles are acted by gravitational and Lorentz forces, simultaneously. It is shown that the ISCO position for charged particle will be always less than that of neutral particle.
Read More
The X-ray emission from blazars has been widely investigated using several space telescopes. In this work, we explored statistical properties of the X-ray variability in the blazars S5 0716+714, OJ 287, Mkn 501, and RBS 2070 using the archival observations from the XMM-Newton telescope between the period 2002-2020. Several methods of timing and spectral analyses, including fractional variability, minimum variability time-scale, power spectral density analyses, and countrate distribution, were performed. In addition, we fitted various spectral models to the observations, as well as estimated hardness ratio. The results show that the sources are moderately variable within the intraday time-scale. Three of the four sources exhibited a clear bi-modal pattern in their countrate distribution, revealing possible indication of two distinct countrate states, that is, hard and soft countrate states. The slope indices of the power spectral density were found to be centred around 0.5. Furthermore, the spectra of the sources were fitted with single power law, broken power law, log-parabolic, and blackbody + log-parabolic models (the latter only for OJ 287). We conclude that for most of the observations log-parabolic model was the best fit. The power-spectral-density analysis revealed the variable nature of PSD slopes in the source light curves. The results of this analysis could indicate the non-stationary nature of the blazar processes on intraday time-scales. The observed features can be explained within the context of current blazar models, in which the non-thermal emission mostly arises from kilo-pc scale relativistic jets.
Read More
We present a simple, analytic and straightforward method to elucidate the effects produced by polytropic fluids on any other gravitational source, no matter its nature, for static and spherically symmetric spacetimes. As a direct application, we study the interaction between polytropes and perfect fluids coexisting inside a self-gravitating stellar object.
Read More
The light curve of an isolated bright spot in a Keplarian orbit is studied to investigate the signature of the firewall around the event horizon of the black hole. An increase in total observed flux is found. In addition to that, for firewall case comparatively a longer time radiation is observed.
Read More
