|
| |
|
| |
|
|
Astrofyzikální procesy v blízkosti kompaktních objektů
Sochora, Vjačeslav ; Karas, Vladimír (advisor) ; Schee, Jan (referee) ; Semerák, Oldřich (referee)
Title: Astrophysical processes near compact objects: studying extremal en- ergy shifts from accretion rings Author: Vjačeslav Sochora Department: Academy of Sciences of the Czech Republic, Astronomical In- stitute Supervisor: doc. RNDr. Vladimír Karas, DrSc.; Academy of Sciences of the Czech Republic, Astronomical Institute Abstract: The X-ray emission from inner regions of an accretion disk around black holes provides wealth of information about matter in extreme con- ditions. A spectral profile of radiation from a narrow circular ring has a characteristic double-horn profile. Red and blue peaks of the profile are close to the extremal values of the energy shift. We describe a useful approach to calculate the extremal energy shifts in the regime of strong gravity. We dis- cuss if the radial structure of the disk emission could be reconstructed using extremal energy shifts of the individual rings. For this purpose, we simulate artificial data from a bright active galactic nucleus and show that the re- quired sensitivity and energy resolution can be reached with the proposed LOFT mission. Keywords: black hole physics, accretion disks, galactic nuclei
|
|
|
Neutron stars near a galactic centre
Zajaček, Michal ; Karas, Vladimír (advisor) ; Šubr, Ladislav (referee)
In this work we study the processes near the Galactic centre, which serves as a paradigm for low- luminosity galactic nuclei. The introductory part of the thesis is a brief review on the radio source Sagittarius A* in the Galactic centre and on its immediate surroundings. The main part of the thesis focuses on the hypothetical population of neutron stars that should be present in large numbers in this region. We analyse the predictions concerning the encounters of this observationally unexplored population with the ambient interstellar medium and we discuss the distribution of their interaction modes with respect to the parameters of the system. We find out that this distribution is strongly dependent on the density of the ambient medium, whereas only weakly dependent on its temperature. The effect of the prolongation of rotational period is negligible on the time-scale of about ten thousand years. In the second part, we predict the evolution of the high-eccentricity passages of clouds and dust- enshrouded stars (with pericentre distances at about 1000 Schwarzschild radii from the black hole). In all studied cases a major part of the matter is diverted from the original path. Powered by TCPDF (www.tcpdf.org)
|
|
|
Accretion discs in the context of tidal disruption of stars in nuclei of galaxies
Štolc, Marcel ; Karas, Vladimír (advisor) ; Schee, Jan (referee)
Stars can be stretched and ripped apart by the super-massive black hole at the core of a galaxy. The remnant gaseous trail gradually circularizes in a ring of mass that spreads by the viscous forces into an accretion disc. In this thesis we have studied the spectral line profle time evolution of radiation refected by the accretion disc located around a super-massive black hole. We assume the central body to be a slowly rotating or non-rotating super-massive black hole with no charge, in the frst approximation represented by the Schwarzschild solution. In a sense of Shakura-Sunyaev standard accretion disc model with the kinematic viscosity parameter α ≈ 1 we allow the accretion disc evolution to be guided by the angular momentum transfer equation with the initial mass ring located at the tidal radius being the product of tidal disruption of a star passing by a super-massive black hole. During the simulations we keep varying the mass of the central body while we keep the mass and the radius of the star constant (M = 1M⊙ and R = 1R⊙), i.e. taking into account the solar-type stars only. We defer the prospects of the full analysis involving spin (and charge) of the central body for the future study as it will be necessary to use the equations for the redshift factor and the accretion disc evolution...
|
|
| |
|
|
Magnetické pole v jádru Galaxie
Hamerský, Jaroslav ; Karas, Vladimír (advisor) ; Kovář, Jiří (referee)
In the present work we study the properties of accretion tori orbiting black hole. Our approach to this problem comes from the solving of general relativistic magnetohydrodynamic equations, which follow from conservation of the energy-momentum tensor, the particle number and from Maxwell's equations. We solve these equations by numerical methods which are described in Chapter 1. The formalism of tori which we consider here is described in Chapter 2. We are interested in tori with constant density of angular momentum and Fishbone-Moncrief tori mainly. We study accretion rates in these tori when the mass of black hole is increased suddenly and so the equilibrium in the torus is corrupted. For tori with constant density of angular momentum we study the influence of the presence of toroidal magnetic field on accretion rates.
|
|
|
Accretion discs in the context of tidal disruption of stars in nuclei of galaxies
Štolc, Marcel ; Karas, Vladimír (advisor)
Stars can be stretched and ripped apart by the super-massive black hole at the core of a galaxy. The remnant gaseous trail gradually circularizes in a ring of mass that spreads by the viscous forces into an accretion disc. In this thesis we have studied the spectral line profle time evolution of radiation refected by the accretion disc located around a super-massive black hole. We assume the central body to be a slowly rotating or non-rotating super-massive black hole with no charge, in the frst approximation represented by the Schwarzschild solution. In a sense of Shakura-Sunyaev standard accretion disc model with the kinematic viscosity parameter α ≈ 1 we allow the accretion disc evolution to be guided by the angular momentum transfer equation with the initial mass ring located at the tidal radius being the product of tidal disruption of a star passing by a super-massive black hole. During the simulations we keep varying the mass of the central body while we keep the mass and the radius of the star constant (M = 1M⊙ and R = 1R⊙), i.e. taking into account the solar-type stars only. We defer the prospects of the full analysis involving spin (and charge) of the central body for the future study as it will be necessary to use the equations for the redshift factor and the accretion disc evolution...
|
|
| |
|
|
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...
|
guest ::
RSS feed.