
Chaos v porušených polích černých děr
Witzany, Vojtěch ; Semerák, Oldřich (advisor) ; Heyrovský, David (referee)
The loss of complete geodesic integrability is one of the important consequences (and thus indicators) of deviation from the Kerrtype spacetime. Indeed, it has been confirmed many times in the literature that even a highly symmetric perturbation of the Kerr or Schwarzschild metric can make the free testparticle motion chaotic. In this thesis, we study the testparticle dynamics in the field of a Schwarzschild black hole surrounded by a thin disc or ring, using, however, Newton's gravity with a simple "pseudo Newtonian" potential to mimic the black hole. The Poincaré sections show that the (pseudo)Newtonian system is slightly more chaotic than the general relativistic one. The difference seems to be correlated with the phasespace allowed region being more open towards the center in the pseudoNewtonian case. Powered by TCPDF (www.tcpdf.org)

 

Missing bright red giants in the Galactic center: A fingerprint of its once active state?
Zajaček, Michal ; Araudo, Anabella ; Karas, Vladimír ; Czerny, B. ; Eckart, A. ; Suková, Petra ; Štolc, Marcel ; Witzany, V.
We propose a novel scenario for the bright redgiant depletion based onthe collisions between red giants and the nuclear jet, which was likely active in the Galactic center a few million years ago and could have led to the formation of the largescaleγray Fermi bubbles. The process of the jetinduced ablation of red giants appears to be most efficient within∼0.04 pc(Scluster), while at larger distances it was complemented by star–accretion disc collisions and at smaller scales, tidal stripping operated. These three mechanisms likely operated simultaneously and createdan apparent core of latetype stars within∼0.5 pc.


Perturbing the accretion flow onto a supermassive black hole by a passing star
Suková, Petra ; Zajaček, M. ; Witzany, V. ; Karas, Vladimír
The close neighbourhood of a supermassive black hole contains not only accreting gas and dust, but also stellarsized objects like stars, stellarmass black holes, neutron stars, and dustenshrouded objects that altogether form a dense nuclear starcluster.These objects interact with the accreting medium and they perturb the otherwise quasistationary configuration of the accretion flow. We investigate how the passages of a star can influence the black hole gaseous environment with GRMHD 2D and3D simulations. We focus on the changes in the accretion rate and the associated emergence of outflowing blobs of plasma.


Dynamics of spinning test particles in curved spacetimes
Zelenka, Ondřej ; Loukes Gerakopoulos, Georgios (advisor) ; Witzany, Vojtěch (referee)
The motion of a test particle in the Schwarzschild background models the merger of a compact object binary with extremely different masses known in the literature as Extreme Mass Ratio Inspiral. In the simplest geodesic approxima tion, this motion is integrable and there is no chaos. When one takes the spin of the smaller body into account, integrability is broken and prolonged resonances along with chaotic orbits appear. By employing the methods of Poincaré surface of section, rotation number and recurrence analysis we show for the first time that there is chaos for astrophysically relevant spin values. We propose a uni versal method of measuring widths of resonances in perturbations of geodesic motion in the Schwarzschild spacetime using actionanglelike variables. We ap ply this novel method to demonstrate that one of the most prominent resonances is driven by second order in spin terms by studying its growth, supporting the expectation that chaos will not play a dominant role in Extreme Mass Ratio Inspirals. Last but not least, we compute gravitational waveforms in the time domain and establish that they carry information on the motion's dynamics. In particular, we show that the time series of the gravitational wave strain can be used to discern regular from chaotic motion of the source. 1


Chaos in deformed blackhole fields
Witzany, Vojtěch ; Semerák, Oldřich (advisor) ; Kopáček, Ondřej (referee)
The consequences of two key approximations of accretiondisc physics near black holes are studied in this thesis. First, the question of effective ``pseudoNewtonian" potentials mimicking a black hole is investigated both through numerical simulations and analytical means, and second, the neglect of additional gravitating matter near accretedupon black holes and its consequences are put to test. After some broader discussion of integrability, resonance and chaos, a general "pseudoNewtonian" limit for geodesic motion is derived, and applied for the case of null geodesics near a glowing toroid and for timelike geodesics in the Kerr metric. Afterwards, a new Newtonian gravitational potential for non singular toroids is proposed and its usefulness for the socalled Weyl spacetimes is discussed. Finally, a new pseudoNewtonian 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 exactrelativistic studies, and the previous conclusion of chaos in disc/ringhole models is confirmed. Overall, the pseudoNewtonian 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...


Chaos v porušených polích černých děr
Witzany, Vojtěch ; Semerák, Oldřich (advisor) ; Heyrovský, David (referee)
The loss of complete geodesic integrability is one of the important consequences (and thus indicators) of deviation from the Kerrtype spacetime. Indeed, it has been confirmed many times in the literature that even a highly symmetric perturbation of the Kerr or Schwarzschild metric can make the free testparticle motion chaotic. In this thesis, we study the testparticle dynamics in the field of a Schwarzschild black hole surrounded by a thin disc or ring, using, however, Newton's gravity with a simple "pseudo Newtonian" potential to mimic the black hole. The Poincaré sections show that the (pseudo)Newtonian system is slightly more chaotic than the general relativistic one. The difference seems to be correlated with the phasespace allowed region being more open towards the center in the pseudoNewtonian case. Powered by TCPDF (www.tcpdf.org)
