Investigation of a fluid circulation in strong gravitational fields represents a fundamental method for theoretical exploration of accretion discs and related processes. The fluid is commonly considered as a neutral gas, or as a quasi-neutral and highly conductive plasma, which can form a toroidal-like structure centered and circling along equatorial plane of a central object. Such a scenario stands for a basic model of thick accretion disc around a black hole or compact star, where the accretion can occur close to an equatorial cusp. Here, we show that if the circling fluid is electrically charged so that it possesses a net non-zero charge transported only by convection, it can form unique structures supposed that a proper ambient large-scale electromagnetic field is present around; along with the pure equatorial toroidal structures with equatorial cusps, characteristic for a neutral fluid circulation, we also find unique off-equatorial toroidal structures with off-equatorial cusps, the so-called `levitating tori', or circling structures with polar cusps hovering above a central object, referred to as `polar clouds', etc. These structures, constructed within the presented general relativistic magneto-hydrodynamic model, demonstrate the importance of consideration of a fluid charge for investigation of accretion processes. Even small net charge of the circling fluid together with a sufficiently strong ambient electromagnetic field can distinctively affect the commonly studied equatorial accretion scenario -- shifting it up to the polar region.
Read More
Fluid circling in gravitational and electromagnetic fields around a central compact object can form an equilibrium toroidal structure - a scenario representing a basic model for studying accretion discs orbiting around a black holes or compact stars. For mapping of possible typical shapes and physical properties of such structures, we commonly use a general relativistic magneto-hydrodynamic model based on the energy-momentum conservation written in a standard representation, which works for neutral as well as for electrically charged fluids. Moreover, we introduce this model in terms of two covariant force representations, both based on a proper hypersurface projection of the energy-momentum conservation. Then, space-like forces following from a decomposition of the fluid four-acceleration can be defined in the related hypersurface. These representations provide us with an insight into a fluid flow. Moreover, they are also well reflected in the related conformal hypersurface geometries; especially, behavior of the centrifugal forces is directly related to geodesics of the conformal hypersurfaces and their embedding diagrams. In this respect, we present a correspondence between the charged fluid flow world-lines from an ordinary spacetime and the world-lines determined by the charged test particles equation of motion in a conformal spacetime. The introduced force formalism is very general, i.e. it is not restricted only to the circling fluids. Since it is based on the 3+1 splitting of the fundamental equations, it is very convenient for a general fluid flow investigation where an application of numerical procedures is necessary. We illustrate the most important results by considering the circling fluid taking shape of a torus settled in the equatorial plane of the Schwarzschild spacetime accompanied by an asymptotically uniform magnetic field.
Read More
We demonstrate an extraordinary effect of energy gain by a single radiating charged particle inside the ergosphere of a Kerr black hole in presence of magnetic field. We solve numerically the covariant form of the Lorentz-Dirac equation reduced from the DeWitt-Brehme equation and analyze energy evolution of the radiating charged particle inside the ergosphere, where the energy of emitted radiation can be negative with respect to a distant observer in dependence on the relative orientation of the magnetic field, black hole spin and the direction of the charged particle motion. Consequently, the charged particle can leave the ergosphere with energy greater than initial in expense of black hole's rotational energy. In contrast to the original Penrose process and its various modification, the new process does not require the interactions (collisions or decay) with other particles and consequent restrictions on the relative velocities between fragments. We show that such a radiative Penrose effect is potentially observable and discuss its possible relevance in formation of relativistic jets and in similar high-energy astrophysical settings.
Read More
A strong quasi-periodic modulation has recently been revealed in the X-ray flux of the X-ray source XMMUJ134736.6+173403. The two observed twin-peak quasiperiodic oscillations (QPOs) exhibit a 3:1 frequency ratio and strongly support the evidence for the presence of an active galactic nucleus black hole (AGN BH). It has been suggested that detections of twin-peak QPOs with commensurable frequency ratios and scaling of their periods with BH mass could provide the basis for a method intended to determine the mass of BH sources, such as AGNs. Assuming the orbital origin of QPOs, we calculate the upper and lower limit on the AGN BH mass M, reaching M ≈ 107-109 M⊙. Compared to mass estimates of other sources, XMMUJ134736.6+173403 appears to be the most massive source with commensurable QPO frequencies, and its mass represents the current observational upper limit on the AGN BH mass obtained from the QPO observations.
Read More
Deterministic chaos is phenomenon from nonlinear dynamics and it belongs to greatest advances of twentieth-century science. Chaotic behavior appears apart of mathematical equations also in wide range in observable nature, so as in there originating time series. Chaos in time series resembles stochastic behavior, but apart of randomness it is totally deterministic and therefore chaotic data can provide us useful information. Therefore it is essential to have methods, which are able to detect chaos in time series, moreover to distinguish chaotic data from stochastic one. Here we present and discuss the performance of standard and machine learning methods for chaos detection and its implementation on two well known simple chaotic discrete dynamical systems - Logistic map and Tent map, which fit to the most of the definitions of chaos.
Read More
