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Astrophysical black holes embedded in organized magnetic fields
Karas, Vladimír
Large-scale magnetic fields pervade the cosmic environment where the astrophysical black holes are often embedded and influenced by mutual interaction. In thiscontribution, we outline the appropriate mathematical framework to describe magnetized black holes within General Relativity, and we show several examples of how these can be employed in the astrophysical context. In particular, we examine the magnetized black hole metric in terms of an exact solution of electro-vacuum Einstein-Maxwell equations under the influence of a non-vanishing electric charge. New effects emerge: the expulsion of the magnetic flux out of the black-hole horizon depends on the intensity of the imposed magnetic field.
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MAD UFOs: Magnetically Arrested Discs with persistent Ultra-Fast Outflows
Suková, Petra ; Zajaček, M. ; Karas, Vladimír
We study an outflow that develops in the MAD state in 3D GRMHD simulations. We show that the outflow can be accelerated to\nrelativistic velocities and persist over the course of our simulation. We compare the properties of the outflow from MAD discs with those launched by orbiting secondary at close orbit. The main difference is that the orbiting body launches a more coherent, quasiperiodic ultrafast outflow at lower velocities (v < 0.5c) while the outflow launched in the MAD state (without the body) has a stochastic behaviour and has anapproximately flat velocity distribution between lower anf higher outflow velocities, 0.2c < v < 0.3c and v > 0.5c.
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Magnetic fields of current loops around black holes
Vrba, Šimon ; Semerák, Oldřich (advisor) ; Karas, Vladimír (referee)
We summarize and explain the mathematical procedure that allows us to find the closed form of the magnetic field generated by a test current loop in Kerr spacetime. We consider axisymmetric placement of the loop for all three cases of the Kerr background: below-extreme black hole, extreme black hole, and naked singularity. The field is obtained by differentiating the effective Green function of the Debye potential, which is expressed in terms of elliptic integrals. 1
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Structure of a boundary layer between a star and an accretion disk: analytical models
Trachta, Tomáš ; Horák, Jiří (advisor) ; Karas, Vladimír (referee)
In this work, we attempt to find a three-dimensional solution to the Navier-Stokes equations describing an α-disk accreting onto a neutron star. The solution for great radial distances is well-known. However, this solution leads to a singularity at the zero-torque radius. A boundary layer arises in the neighbourhood of this radius since the solution for great radial distances cannot fulfil the inner boundary condition, therefore it stops being valid. Mathematically we are dealing with singular perturbations. In order to eliminate this singularity, we will use the method of matched asymptotic expansions, which leads to new equations of motion. First, we review known results, then we provide arguments for rescaling the equations of motion, and we attempt to solve the rescaled equations. 1
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Transition from regular to chaotic motion in black hole magnetospheres
Kopáček, Ondřej ; Karas, Vladimír (advisor) ; Kulhánek, Petr (referee) ; Rezzolla, Luciano (referee)
Cosmic black holes can act as agents of particle acceleration. We study properties of a system consisting of a rotating black hole immersed in a large-scale organized magnetic field. Electrically charged particles in the immediate neighborhood of the horizon are influenced by strong gravity acting together with magnetic and induced electric components. We relax several constraints which were often imposed in previous works: the magnetic field does not have to share a common symmetry axis with the spin of the black hole but they can be inclined with respect to each other, thus violating the axial symmetry. Also, the black hole does not have to remain at rest but it can instead perform fast translational motion together with rotation. We demonstrate that the generalization brings new effects. Starting from uniform electro-vacuum fields in the curved spacetime, we find separatrices and identify magnetic neutral points forming in certain circumstances. We suggest that these structures can represent signatures of magnetic reconnection triggered by frame-dragging effects in the ergosphere. We further investigate the motion of charged particles in these black hole magnetospheres. We concentrate on the transition from the regular motion to chaos, and in this context we explore the characteristics of chaos in...
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Evolution of the stellar disc in the Galactic centre
Kobrle, Stanislav ; Šubr, Ladislav (advisor) ; Karas, Vladimír (referee)
In this thesis we will study dynamical models of Galactic centre analyse them and compare with available observational data. Our main target is to investigate origin of young stars which are being observed here. We examine if they could have been born in a single accretion disc. Next target is to confirm or disprove the possibility of existence one or more disc-like structures which are frequently discussed by scientists. We introduce methods used to detect stellar structures and apply them to artificial and observational data.
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