
Chaotic motion in the JohannsenPsaltis spacetime
Zelenka, O. ; LukesGerakopoulos, Georgios
The JohannsenPsaltis spacetime is a perturbation of the Kerr spacetime designed to avoid pathologies like naked singularities and closed timelike curves. This spacetime depends not only on the mass and the spin of the compact object, but also on extra parameters, making the spacetime deviate from Kerr. In this work we consider only the lowest order physically meaningful extra parameter. We use numerical examples to show that geodesic motion in this spacetime can exhibit chaotic behavior. We study the corresponding phase space by using Poincaré sections and rotation numbers to show chaotic behavior, and we use Lyapunov exponents to directly estimate the sensitivity to initial conditions for chaotic orbits.


Polarization properties of bow shock sources close to the Galactic centre
Zajaček, M. ; Karas, Vladimír ; Hosseini, E. ; Eckart, A. ; Shahzamanian, B. ; ValenciaS, M. ; Peissker, F. ; Busch, G. ; Britzen, S. ; Zensus, J. A.
Several bow shock sources were detected and resolved in the innermost parsec from the supermassive black hole in the Galactic centre. They show several distinct characteristics, including an excess towards midinfrared wavelengths and a significant linear polarization as well as a characteristic prolonged bowshock shape. These features give hints about the presence of a nonspherical dusty envelope generated\nby the bow shock. The Dusty Scluster Object (also denoted as G2) shows similar characteristics and it is a candidate for the closest bow shock with a detected proper motion in the vicinity of Sgr A*, with the pericentre distance of only approx. 2000 Schwarzschild radii. However, in the continuum emission it is a pointlike source and hence we use Monte Carlo radiative transfer modeling to reveal its possible threedimensional structure. Alongside the spectral energy distribution, the detection of polarized continuum emission in the nearinfrared Ksband (2.2 micrometers) puts additional constraints on the geometry of the source.

 

Hysteresis behavior of shocks in low angular momentum flows
Suková, Petra
In this work we present the GRMHD 1D simulations of accreting matter with variable angular momentum. We focus on the existence and behaviour of the shock in the flow. We show that the location of the shock front responds to the change of the angular momentum, which also causes the accretion rate onto the black hole to vary on different time scales. We study the possible hysteresis behaviour of the shock\nfront during the time evolution of the flow. We discuss the potential observational effects of this phenomenon.


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 nontrivial, 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.

 

Epicyclic oscillations of thick relativistic disks
Horák, Jiří ; Straub, O. ; Šrámková, E. ; Goluchová, K. ; Török, G.
We study epicyclic oscillations of thick relativistic tori with constant specific angular momentum distribution using the finite element numerical method. We have compared frequencies of the axisymmetric and nonaxisymmetric modes with the analytic formulae obtained by Straub and Šrámková (2009) and Fragile et al. (2016). We have found excellent agreement in the case of axisymmetric radial epicyclic modes. In the case of the axisymmetric vertical epicyclic modes and nonaxisymmetric modes in general, the analytic approximation agrees with numerical results only for tori of moderate thicknesses. Our analysis also revealed an instability of the thick constant angular momentum tori with respect to the radial epicyclic oscillation.


Wave excitation at Lindblad resonances using the method of multiple scales
Horák, Jiří
In this note, the method of multiple scales is adopted to the problem of excitation of non–axisymmetric acoustic waves in vertically integrated disk by tidal gravitational fields. We derive a formula describing a waveform of exited wave that is uniformly valid in a whole disk as long as only a single Lindblad resonance is present. Our formalism is subsequently applied to two classical problems: trapped p–mode oscillations in relativistic accretion disks and the excitation of waves in infinite disk.

 
 