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Thin accretion disk structure equation in a general metric
Hrubeš, Jan ; Bursa, Michal (advisor) ; Horák, Jiří (referee)
In the physics of accretion disks around extremely massive objects, the effect of spa- cetime curvature is manifested on the behaviour of gas and radiation. Black holes and the accretion disks around them are commonly described by the Kerr metric. However, other metrics are being tested to see if their predictions correspond better to real astronomical objects. The aim of this thesis is to generalize the description of the accretion disk develo- ped by I. D. Novikov and K. S. Thorne for a general stationary axially symmetric metric. The results are then verified by substitution of the Kerr solution. In the last section the equations are solved for the stationary axially symmetric metric with charge parameter. 1
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Thin accretion disks with magnetic advective term
Vavřička, Radek ; Bursa, Michal (advisor) ; Horák, Jiří (referee)
Accretion disks around black holes with gas radiating out parts of its gravitational potential energy have long served as objects of both theoretical and observational studies. By solving the structure equations of the disk it is possible to predict the outgoing radia- tive flux and the observed spectrum of the disk and test the validity of the theory against direct observations. The standard thin disk model (Shakura-Sunyaev, Novikov-Thorne) shows, however, a still unexplained non-negligible deviance in the observed spectrum at higher mass accretion rates. To amend to the set of proposed explanations, in this thesis we examine the effect of the magnetic pressure on the trapping of some of the internal energy generated by viscous dissipation processes in the disk and advecting this energy to the black hole. A phenomenological description of heat advection mediated by a highly heterogenous magnetic field will be given, as well as its effect on the spectrum and observed effective temperature. 1
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Understanding the iron K alpha line emissivity profile with GR radiative transfer code
Zhang, Wenda ; Dovčiak, Michal ; Bursa, Michal ; Svoboda, Jiří ; Karas, Vladimír
In this work, we present calculations of the illumination and the iron K alpha emissivity profiles performed with the GR radiative transfer code Monk that employs the Monte Carlo method. In most previous studies the distinction between the illumination and emissivity profiles was not clearly made. For AGN discs, the emissivity profile has a similar shape with the illumination profile, but in the inner most region the former is steeper than the latter, where as for accretion discs in black hole X-ray binaries, the distinction between the two profiles is more dramatic. We find out that the different behavior between AGN and black hole X-ray binary discs is due to the different energy spectra of the illuminating radiation. This suggests that the emissivity profile of the iron Kαline cannot be determined by black hole spin and corona geometry alone and the energy spectrum of the illuminating radiation has to be taken into account. We also study the dependence of the emissivity profile on the geometry of the corona.
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Energy balance of corona in black-hole accretion disks
Tynianskaia, Valeriia ; Bursa, Michal (advisor) ; Svoboda, Jiří (referee)
The presence of X-ray radiation that comes from the innermost regions of Active Galactic Nuclei indicates a presence of a hot gas component located close to the central black hole. The exact location and geometry of this so called corona is not known and various configurations are being considered in the literature. One of the suggestions for the geometry is a rather compact region located on the symmetry axis of the black hole (lamp-post model). Another variant is a layer that surrounds the accretion disk on both sides (slab model). Other models consider anything in between also in combination with a truncated disk. One of the key questions connected with the corona that is not often addressed is how is the corona energized, i.e. where does it take energy from. Assuming the disk is the only source of energy in an accreting system and that its internal energy is partly radiated and partly used to support magnetic fields, we evaluate geometrical constraints on the corona from the energy conservation condition. Lastly, we try to investigate the total emitted spectrum of a system consisting of a central black hole, a thin accretion disk and a slab corona. 1
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Numerical implementation of equations for photon motion in Kerr spacetime
Bursa, Michal
Raytracing is one of the essential tools for accurate modeling of spectra and variability of various astrophysical objects. It has a major importance in relativisticenvironments, where light endures to a number of relativistic effects. Because the trajectories of light rays in curved spacetimes, and in Kerr spacetime in particular, are highly non-trivial, we summarize the equations governing the motion of photon\n(or any other zero rest mass particle) and give analytic solution of the equations thatcan be further used in practical computer implementations.
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