Seminars 2012
Gabriel Torok, Pavel Bakala
Institute of Physics, Silesian University, Opava
Modelling time variability of relativistic X-ray lightcurves
Abstract
Gabriel Torok, Pavel Bakala
Very informal discussion of the current status and future prospects of the project.
Gunther Witzel
I. Institute of Physics, University of Cologne, Germany
Intrinsic variability properties of the near-infrared source SgrA*
Abstract
Gunther Witzel
The variable near-infrared source Sagittarius A* is associated with the supermassive black hole at the center of our Milky Way. The polarized near-infrared radiation is dominated by Synchrotron processes in the immediate surrounding of the black hole, originating either from the accretion process itself or from a short jet. Polarized outbursts on timescales of hours and with flux densities 20 times brighter than the faintest states close to the detection limit are typical for the variability of Sgr A*. These flares often exhibit smaller flux density variations on a 10 minute timescale and could be interpreted in the frame work of hot spots in the accretion disk of the black hole. I present a comprehensive phenomenological description of the statistics of the variability of Sgr A* using Fourier transform based surrogate data for comparison and discuss the instrumental prerequisites for a study of the intrinsic polarimetric states of the source.
Jiri Svoboda
European Space Astronomy Centre, Madrid, Spain
Relativistic iron lines as the probe of black hole innermost accretion discs
Abstract
Jiri Svoboda
X-ray spectroscopy of active galaxies and black hole binaries provides an opportunity to explore the innermost regions of black hole accretion discs. I will make a brief introduction to relativistic distortion of the iron fluorescent lines and its application for measurement of black hole angular momentum. Very steep radial decrease of the disc reflection emissivity has been detected in several X-ray sources suggesting the disc-irradiating corona to be compact and very centrally localised. I will discuss whether the special conditions on the corona properties are indeed required, and/or whether the steep radial emissivity could be an artifact of model assumptions. I will present two different effects which might account for the steep radial emissivities, the angular directionality of the reflected radiation properly calculated in the fully relativistic regime and the radial dependence of the accretion disc ionisation. I will show that these effects may also influence the measurements of the black hole angular momentum.
Richard Wunsch
Astronomical Institute, Academy of Sciences, Prague
Tree-based gravity solver for grid-based hydrodynamic codes
Abstract
Richard Wunsch
I will present our MPI parallel tree gravity solver for the hydrodynamic code Flash. Finite difference hydrodynamic codes (like Flash) traditionally use either spectral methods (FFT, multipole expansion) or multi-grid methods to solve for the gravitational potential. On the other hand, particle based codes (N-body, SPH) typically compute the gravitational potential either by direct integration or by tree based algorithms.
We develop an octal tree-based gravity solver for grid based hydrodynamic codes that is efficient on massively parallel architectures. There are two main reasons for this efficiency: (i) the simplicity of the algorithm allows us to make the communication between processes - often the main bottleneck of parallel algorithms - minimalistic; (ii) the regular structure of the grid enables very efficient implementation of interaction lists that make the tree-walk faster. Another advantage of the tree-code is that it can be relatively easily implemented on graphics card based architectures. The accuracy and scaling tests show that the algorithm is competitive with multi-grid methods and scales very well at least up to 512 processors.
Jaroslav Hamersky
Astronomical Institute, Academy of Sciences, Prague
Magnetized accretion tori
Abstract
Jaroslav Hamersky
We are carrying out a student project concerning the study of accretion of two-dimensional magnetized tori with a radially increasing angular momentum density, l(R). We discuss differences between these tori and those with l=const. In an informal talk we will present the progress and the current status of the project, as well as open questions.
S. Ehlerova, P. Hadrava, L. Subr, M. Bursa, F. Hueyotl-Zahuantitla, D. Kofron, J. Kovar, M. Urbanec
Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2011
Abstract
S. Ehlerova, P. Hadrava, L. Subr, M. Bursa, F. Hueyotl-Zahuantitla, D. Kofron, J. Kovar, M. Urbanec
Soňa Ehlerová: Fragmented walls of the Carina Flare Supershell
CO observations of the molecular cloud in the wall of the Carina Flare Supershell made with the APEX telescope are presented. Clumps in the cloud are identified and interpreted as results of the gravitational fragmentation process in the shell, which is also affected by the pressure of the ambient medium. The mass spectrum of identified clumps is compared to theoretical predictions.
Petr Hadrava: Evolution of First Stars with Mass-loss
The first stars or the so called Population III stars were formed from the matter with primordial chemical composition determined by the conditions in the early universe, i. e. with extremely low content of heavy elements. It is supposed that the first stars had mostly very high masses and hence large luminosities and short evolution time-scale. Their radiation caused re-ionization of the universe and their remnants enriched the universe by heavy elements. However, this process depends on the form and rate of mass-loss by these stars which can take place already during their evolution before the final collapse. To investigate the role of this process, we have computed models of first-stars' evolution with parameterized mass-loss.
Ladislav Šubr: Dynamical evolution of the young stellar disc in the Galactic center
Origin of several hundreds of young stars in within the distance $\lesssim 1\,\pc$ from the Galactic supermassive black hole still represents an open problem of contemporary astrophysics. In this contribution we further investigate the model which assumes their formation in-situ via fragmentation of a self-gravitating gaseous disc. We show that currently observed configuration of the system of young stars can be obtained an outcome of a dynamical evolution of a single, initially very thin stellar disc. Our model assumes the long-term evolution of the stellar disc to be determined by gravitational influence of a distant molecular torus (CND) and mutual resonances of stellar orbits within the disc.
Michal Bursa: Modeling thermal spectra of accretion disks
Accurate theoretical modeling of accretion disk spectra and their comparison with observation data provides an important technique for getting a better understanding of processes playing role in accretion disks driven by strong gravity. At the same time we can not only test how good are our theoretical models, but we can also probe properties of the disk's central engines - black holes.
Filiberto Hueyotl-Zahuantitla: Formation of the AGN torus in a nuclear starburst environment
I will present a model for the formation of the torus in active galactic nuclei from the matter reinserted by a rotating young nuclear star cluster (NSC). A necessary condition to form the torus is that the NSC must be in a catastrophic cooling regime. By using typical parameters of NSCs (mass, size, etc,... ) and black holes (BHs) in Seyfert galaxies, the model predicts dimensions of the torus as those inferred from observations. A simple analytic formula to estimate the radius of the torus in terms of the parameters of the NSC and BH will be presented.
David Kofroň: The radiation properties of the C-metric
By casting the charged C-metric in the global boost-rotation symmetric form we are able to investigate its radiation properties in more details. This is a complicated task, and, unfortunately not yet completely done, as we are dealing with the gravitational radiation of an extended body, not only of a point-like particle. Also the gravitational-electromagnetic interaction is involved.
Jiří Kovář: Charged perfect fluid structures near compact objects
Following our previous studies of charged test particles and uncharged perfect fluid tori encircling compact objects, we introduce here a simple test model of a charged perfect fluid torus in strong gravitational and electromagnetic fields. In contrast to ideal magnetohydrodynamic models, we consider here the opposite limit of negligible conductivity, where the charges are tied completely to the moving matter. To clearly demonstrate features of our model, we present three-dimensional axisymmetric charged toroidal configurations around Reissner-Nordstroem black holes and compare them with equivalent configurations of electrically neutral tori. The details of the study can be found in Kovar et al. (2011, Physical Review D, 84, 084002). Next, we describe an ongoing work on the Newtonian version of our model, which is, due to its simplicity, very suitable to illustrate the basic properties of the system. The Newtonian approach allows us to find the off-equatorial structures. These represent a generalization of our recent studies of the off-equatorial halo motion of charged test particles.
Martin Urbanec: Rotating compact stars
We will present results of compact star modeling based on Hartle-Thorne approach assuming matter being described by standard equations of state for neutron star matter and also equation of state describing the so called strange matter. Typical differences between neutron and strange star properties will be demonstrated. We will show implications on the astrophysical phenomena in the vicinity of these objects and how models of these phenomena could be tested using our models.
Vladimir Karas
Astronomical Institute, Academy of Sciences, Prague
Influence of frame-dragging on magnetic null points near rotating black hole
Abstract
Vladimir Karas
In an informal talk I will report on work done recently in collaboration with Ondrej Kopacek and Devaky Kunneriath. Based on a paper in Classical and Quantum Gravity (to appear).
Understanding the mechanisms of particle acceleration from the vicinity of black holes poses a challenge. Electromagnetic effects are thought to be a prime suspect, but details still need an explanation. To this end, we study a three-dimensional structure of oblique magnetic fields near a rotating black hole in vacuum. It has been proposed that such a set-up can lead to efficient acceleration when plasma is injected near a magnetic null point. We focus our attention especially on the magnetic field in the immediate neighborhood of the magnetic null point, which was previously shown to occur in the equatorial plane. By employing the Line-Integral-Convolution (LIC) method, we visualize the magnetic field lines and explore the electric lines rising out of the equatorial plane.