National Repository of Grey Literature 158 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
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
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
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....
Variational Concepts with Applications to Microstructural Evolution
Fischer, F D. ; Svoboda, Jiří ; Hackl, K.
In systems at elevated temperature the development of the microstructure of a material is controlled by diffusional and interface migration processes. As first step the description of the microstructure is reduced to a finite number of time-dependent characteristic parameters (CPs). Then the Thermodynamic Extremal Principle (TEP) is engaged to develop the evolution equations for these characteristic parameters. This treatment is demonstrated on a bamboo-structured material system predicting the spatial and time distribution of chemical composition as well as the deformation state.
SEMI-SOLID PROCESSING OF METAL-OXIDE COMPOSITE
Jirková, H. ; Aišman, D. ; Mašek, B. ; Svoboda, Jiří
One of the available routes to developing a new material resistant to high-temperature creep is to create a microstructure consisting of a metal matrix and dispersed stable particles. For making intricately shaped components from such materials, new processes must be found to allow near net shape products to be manufactured in a simple and rapid manner. A semi-solid processing chain relying on mini-thixoforming could become one such process. For this purpose, an unconventional technology chain was designed in the present experiment. The chain comprises mechanical alloying, powder metallurgy techniques and thermomechanical treatment with transition through the semi-solid state. In this chain, thanks to the intensive deformation at the thixo-forming stage, the desired shape is achieved effectively. The second requirement was the good creep resistance of the material. To this purpose, two different powder materials consisting of metals and oxides were proposed. In both cases, the metal constituent contained iron and aluminium. The primary difference between the materials denoted as A and B was the nature of the oxides acting as strengthening particles. The powder mixture was prepared by mechanical alloying and compacted using various techniques. The powder mixture for the A material was compacted using high-pressure torsion (HPT). The B material was compacted by heating the mechanically alloyed powder enclosed in a steel container in a furnace and by subsequent intensive compressive deformation in a press. From the resulting products, cylindrical specimens for semi-solid processing were cut. The rate of heating to the semi-solid processing region was high. In the first stage, appropriate temperatures and heating rates were sought and optimized. These are crucial in obtaining the desired fine and adequately uniform dispersion of particles providing the strengthening effect.
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
X-ray spectroscopy of polar-scattered Seyfert 1 galaxies
Mikušincová, Romana ; Svoboda, Jiří (advisor) ; Horák, Jiří (referee)
In this thesis we present a spectral re-analysis of the Fairall 51 galaxy. It is optically a Seyfert 1 galaxy. It does, however, show a very strong polarization in the optical spectrum and as this feature is not characteristic for Seyfert 1 objects, this galaxy may represent a borderline object between Seyfert 1 and Seyfert 2 galaxies and provide with valuable information about the structure of Active Galactic Nuclei. We studied two archival sets of observations performed in September 2005 and March 2006 by the European satellite XMM-Newton and four observations performed in September 2013 by the Japanese observatory Suzaku. It follows from the spectral analysis that there are at least two or three ionized absorbers in the studied AGN. Based on the spectral variability, we were also able to estimate the location of the variable absorber which appears to be 3 to 60 light days from the central engine, which indicates its origin in the Broad Line Region. 1
Hosts and transmission of the crayfish plague pathogen Aphanomyces astaci
Svoboda, Jiří
The crayfish plague pathogen, the oomycete Aphanomyces astaci, has been decimating populations of European crayfish species for more than 150 years, and is therefore considered one of the 100 worst world's invasive species. A. astaci is highly specialised for a parasitic life, but it can be isolated from moribund crayfish and grown on synthetic media, as it is the case also for several other oomycetes (chapter 7). The life of A. astaci includes three basic forms: mycelium in host's tissues, and the infective units occurring in water, zoospores and cysts. All North American crayfish species tested so far have shown some resistance to A. astaci, i.e., they could carry the infection for long, serving as vectors of the pathogen. Massive sporulation from infected North American crayfish starts when the host is moulting, stressed, or dying (chapter 4). However, I could show in my experiments that some sporulation occurs even from apparently healthy and non-moulting American crayfish hosting A. astaci, so infected North American crayfish must be considered a permanent source of the infection (chapter 4). Five genotype groups of A. astaci have already been distinguished. Strains from a particular genotype group probably share the same original host crayfish species of North American origin. Nevertheless, they can...
Information System Design
Steklý, Tomáš ; Svoboda, Jiri (referee) ; Koch, Miloš (advisor)
Diploma thesis deals with the design of the information system for the financial department of the company of Kiwi.com. Target of the work is to evaluate the current status of the information system of the financial department and to analyze its lacks and weaknesses. Based on these analysis and requirements will be determined the design of the new information system. Final part of the diploma thesis will be dedicated to the benefits and economical evaluation of the designed measures.

National Repository of Grey Literature : 158 records found   1 - 10nextend  jump to record:
See also: similar author names
28 SVOBODA, Jakub
53 SVOBODA, Jiří
6 Svoboda, J.
28 Svoboda, Jakub
63 Svoboda, Jan
2 Svoboda, Jan,
38 Svoboda, Jaroslav
1 Svoboda, Jindřich
2 Svoboda, Jonáš
11 Svoboda, Josef
1 Svoboda, Jáchym
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