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Coherent dusty and gaseous structures near the Galactic center
Štofanová, Lýdia ; Karas, Vladimír (advisor) ; Kovář, Jiří (referee)
Sagittarius A*, a compact source in the centre of the Milky Way, is the nearest supermassive black hole (SMBH) in our cosmic neighbourhood, where various astrophysical processes take place. In consequence, variety of structures emerge near the Galactic centre and bow shocks that are closely studied in this work represent an example of them. The introductory part of this thesis is a brief review of the history of the Galactic centre research and its discovery in radio wavelengths. The main body of the thesis is focused on a simplified model of the bow- shock structures that are generated by stars moving supersonically with respect to the ambient medium. We discuss how these structures vary along the orbit. To this end, we consider four different models: (a) without the presence of any gaseous medium emerging from or accreting onto the SMBH, (b) taking an outflow from the SMBH into account, (c) the case of an inflow onto the SMBH, and finally (d) the combined model involving both an outflow and an inflow at the same time. We discuss symmetries of each model (or lack of them) and we find that the model considering the ambient medium at rest appears symmetrical with respect to the pericentre passage. The combined model manifests itself as the most asymmetrical one of them all. We show profiles for the...
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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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Coherent dusty and gaseous structures near the Galactic center
Štofanová, Lýdia ; Karas, Vladimír (advisor) ; Kovář, Jiří (referee)
Sagittarius A*, a compact source in the centre of the Milky Way, is the nearest supermassive black hole (SMBH) in our cosmic neighbourhood, where various astrophysical processes take place. In consequence, variety of structures emerge near the Galactic centre and bow shocks that are closely studied in this work represent an example of them. The introductory part of this thesis is a brief review of the history of the Galactic centre research and its discovery in radio wavelengths. The main body of the thesis is focused on a simplified model of the bow- shock structures that are generated by stars moving supersonically with respect to the ambient medium. We discuss how these structures vary along the orbit. To this end, we consider four different models: (a) without the presence of any gaseous medium emerging from or accreting onto the SMBH, (b) taking an outflow from the SMBH into account, (c) the case of an inflow onto the SMBH, and finally (d) the combined model involving both an outflow and an inflow at the same time. We discuss symmetries of each model (or lack of them) and we find that the model considering the ambient medium at rest appears symmetrical with respect to the pericentre passage. The combined model manifests itself as the most asymmetrical one of them all. We show profiles for the...
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