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Accreting black holes via X-ray polarimetry
Mikušincová, Romana ; Svoboda, Jiří (advisor) ; Matt, Giorgio (referee)
Black hole spin is an essential parameter, for it provides us with the infor- mation on the black hole formation and growth. In this Thesis, we simulated observations for an upcoming X-ray polarimetric mission IXPE (Imaging X- ray Polarimetry Explorer) with the aim to study the robustness of black hole spin and inclination measured via X-ray polarimetry. To simulate polarization spectra, we used a multicolor blackbody emission model accounting for thermal radiation from the accretion disk. For the case of maximally rotating black hole (spin a = 0.998), we were able to reconstruct both spin and inclination of the system with a high precision (∆a ≤ 0.2 for spin and ∆θ ≤ 15 deg for inclination). For less rotating black holes, the spin was correctly recovered, but with a large uncertainty. The inclination is well constrained for any spin value. We conclude that polarimetric measurements will be useful to make independent black hole spin measurements, that would be compared with the spectral-fitting and timing methods. Therefore, X-ray polarimetric missions will be highly desirable. 1
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Relativistic corrections in hard X-ray spectra of accreting black holes
Štofanová, Lýdia ; Svoboda, Jiří (advisor) ; Róžaňska, Agata (referee)
Hard X-ray spectra of accreting black holes in active galactic nuclei and X- ray binaries are characterized by a power-law shape with an exponential cut-off energy at several tens up to few hundreds of keV. The value of the cut-off energy is related to the temperature of a hot corona that reprocesses and inversely Comptonizes thermal emission from the accretion disc. The exact geometry of the corona is still unknown. Several observations suggest it to be very compact and in a close proximity to the black hole. This implies strong relativistic effects such as gravitational redshift, Doppler shift, light bending and beaming to shape the resulting spectra. However, the relativistic effects on primary X-ray emission are often neglected in the data spectral fitting. In this work, we investigate how large uncertainty is introduced by neglecting these relativistic effects. To this purpose, we performed simulations of X-ray spectra for different coronal geometries, and compared the intrinsic and observed values of the cutoff energy. We re-analyzed NuSTAR observations of an active galactic nucleus 1H0419-577 and X-ray binary GRS 1915+105. We found that the extremely low coronal temperatures observed in these sources may be explained by the gravitational redshift due to the proximity of the compact corona to the black hole....
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Kinematics of particle collisions in the ergosphere of Kerr black hole
Skoupý, Viktor ; Ledvinka, Tomáš (advisor) ; Žofka, Martin (referee)
In the thesis we deal with an effect which can be used to extract energy from a rotating black hole, so-called collisional Penrose process. First we investigate the ways to find the equations of motion in the general relativity using Hamilto- nian. Then we examine the equations of motion and their consequences in several coordinate systems for the space-time in the vicinity of a rotating black hole. Fi- nally we look into ways to create a particle capable to escape to infinity with as big energy as possible using Compton scattering and annihilation. The biggest energy found is approximately 14 times the energy of the incoming particles. The efficiency decreases with the distance from the horizon and with the decreasing specific angular momentum of the black hole. 1
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Rotating thin disc around a Schwarzschild black hole: properties of perturbative solution
Kotlařík, Petr ; Semerák, Oldřich (advisor) ; Ledvinka, Tomáš (referee)
In 1974, Will presented a solution for the perturbation of a Schwarzschild black hole due to a slowly rotating and light thin disc given in terms of a multipole expansion of the perturbation series. In a recently submitted paper, P. Čížek and O. Semerák generalized this procedure to the perturbation by a slowly rotating finite thin disc, using closed forms of Green functions rather than the multipole expansion. The method is illustrated there, in the first perturbation order, on the constant-density disc. In this thesis, we summarize, check and plot some of the obtained properties, and show how the presence of the disc changes the geometry of a horizon and the position of significant circular orbits. 1
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Fields of current loops around black holes
Vlasáková, Zuzana ; Semerák, Oldřich (advisor) ; Svítek, Otakar (referee)
The magnetic field of a test circular current loop placed symmetrically around a Schwarzschild black hole has been determined several times in the literature and solutions has been expressed by different formulas. We compare these formulas analytically as well as numerically, and show, in particular, how they behave on the symmetry axis, in the equatorial plane and on the horizon. The problem is relevant for modelling accretion discs around black holes.
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Tidal disruption in the vicinity of black holes in active galaxies
Štolc, Marcel ; Svoboda, Jiří (advisor) ; Haas, Jaroslav (referee)
In this thesis we studied tidal disruptions of stars in the vicinity of com- pact objects and performed spectral re-analysis of XMMSL1 J074008.2-853927, one of the most prominent candidates of tidal disruption events (TDE) around super-massive black holes. This particular TDE shows signs of thermal and non- thermal radiation component and is also indicating the energy excess correspond- ing to the iron line emission of Fe Kα line. We analysed X-ray spectra from two XMM-Newton observations - from the 30-th of April 2014 and the 12-th of Jan- uary 2015 that were follow-up observations after the discovery of this TDE. We confirmed the possible thermal and non-thermal profile of radiation and the pres- ence of Fe Kα line in the data of the first XMM-Newton follow-up observation. We studied in detail the parameters of the iron line, which cannot be to tightly constrained due to insufficient quality of data. However, we also showed that in case of modelling the excess in lower energy band with relativistic reflection the parameters can be constrained. 1
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Chaotic motion in Johannsen-Psaltis spacetime
Zelenka, Ondřej ; Loukes Gerakopoulos, Georgios (advisor) ; Kopáček, Ondřej (referee)
The Johannsen-Psaltis spacetime is a perturbation of the Kerr spacetime de- signed to avoid pathologies like naked singularities and closed timelike curves. This spacetime depends not only on the mass and the spin of the central object, but also on extra parameters, making the spacetime deviate from Kerr; in this work we consider only the lowest order physically meaningful extra parameter. In this thesis we summarize the basics of the theory of regular and chaotic dynamics and we use numerical examples to show that geodesic motion in this spacetime can exhibit chaotic behavior. We study the corresponding phase space by using Poincaré sections and rotation numbers to show chaotic behavior both directly and indirectly (e.g. Birkhoff chains), and we use Lyapunov exponents to directly estimate the sensitivity to initial conditions for chaotic orbits. 1
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Space-times with toroidal horizons
Pešta, Milan ; Semerák, Oldřich (advisor) ; Ledvinka, Tomáš (referee)
Basic results of the uniqueness theorems and the topological-censorship theorem are presented. Properties of the well-known solution of Einstein's equations with a toroidal event horizon are examined and one of possible visualizations of the coordinate system that helps to better understand the behaviour of various invariants in the vicinity of the singularity is suggested. Apart from this solution, two solutions with a ring singularity are introduced as potential candidates for space-times with toroidal horizons whose properties are interpreted intuitively using the toroidal or Weyl coordinate systems. Last part is devoted to apparent horizons of the considered solutions and the differential equation for the apparent horizon of an arbitrary solution of the Weyl class is derived. The numerical solution of this equation is not presented.
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Chaos in deformed black-hole fields
Witzany, Vojtěch ; Semerák, Oldřich (advisor) ; Kopáček, Ondřej (referee)
The consequences of two key approximations of accretion-disc physics near black holes are studied in this thesis. First, the question of effective ``pseudo-Newtonian" potentials mimicking a black hole is investigated both through numerical simulations and analytical means, and second, the neglect of additional gravitating matter near accreted-upon black holes and its consequences are put to test. After some broader discussion of integrability, resonance and chaos, a general "pseudo-Newtonian" limit for geodesic motion is derived, and applied for the case of null geodesics near a glowing toroid and for time-like geodesics in the Kerr metric. Afterwards, a new Newtonian gravitational potential for non- singular toroids is proposed and its usefulness for the so-called Weyl space-times is discussed. Finally, a new pseudo-Newtonian potential is introduced and applied alongside already known potentials in models of free test particle motion in the field of a black hole with a disc or ring, in complete analogy with previous exact-relativistic studies, and the previous conclusion of chaos in disc/ring-hole models is confirmed. Overall, the pseudo-Newtonian framework is able to reproduce a number of key features of the original systems with notable differences arising only as a consequence of extremely strong or...
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Magnetic fields of current loops around black holes
Pejcha, Jakub ; Semerák, Oldřich (advisor) ; Ledvinka, Tomáš (referee)
Magnetic field of equatorial current loop around Schwarzschild (or Kerr) black hole has been studied in many papers and solutions expressed in different forms. In this work we summarize derivations of some of these solutions, illustrate them on specific examples and compare these examples. We also indicate analytic com- parison of some of the formulas. Published formulas lead, as expected, to same results, but some of them are more convenient for numerical evaluation. 1
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