Seminars archive

Influence of frame-dragging on magnetic null points near rotating black hole

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.

Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2011

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.

Magnetized accretion tori

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.

Tree-based gravity solver for grid-based hydrodynamic codes

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.

Relativistic iron lines as the probe of black hole innermost accretion discs

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.

Intrinsic variability properties of the near-infrared source SgrA*

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.

Modelling time variability of relativistic X-ray lightcurves

Gabriel Torok, Pavel Bakala

Very informal discussion of the current status and future prospects of the project.

Influence of cosmological constant and other effects on the movement of Magellanic Clouds

Jan Schee

This seminar will summarize our study of the influence of the cosmological constant (Lambda) on the movement of Magellanic Clouds in the gravitational field of Milky Way. It turns out that Lambda affects the binding mass of both clouds. In some cases it may produce the cosmic repulsion effect that is comparable to the influence of dynamical friction.

Cosmology and cluster astrophysics with the South Pole Telescope

Robert Suhada

The South Pole Telescope (SPT) is 10-meter telescope operating at millimeter wavelengths whose primary goal is to search for clusters of galaxies via the Sunyaev-Zel'dovich (SZ) effect - the distortion of the cosmic microwave background imprinted by the hot intercluster plasma. The mass and redshift distribution of the cluster population is sensitive to the geometry of the Universe and to the rate of structure growth. Galaxy clusters thus provide important insights into cosmological questions such as the nature of cosmic acceleration and the Gaussian character of underlying density perturbations. They can also serve as a consistency test of the general theory of relativity on cosmological scales. I will explain the principles of cluster cosmology and give an overview of the SPT survey. I will discuss the current cosmological constraints from the SPT cluster data, highlighting the constraints on the dark energy equation of state, and the sum of the neutrino masses. I will also demonstrate the use of SPT clusters to answer astrophysical questions such as the cosmic evolution of gas cooling in the cluster cores and give examples of interesting individual systems discovered by SPT, raging from spectacular merging clusters to the most massive relaxed objects in the Universe.

Environmental Effects Which Drive the Evolution of Virgo Cluster Spiral Galaxies

Jeff Kenney

Clusters and their outskirts encompass a wide range of environmental conditions and their galaxies experience a variety of interactions. Virgo is the nearest cluster, so processes in its member galaxies can be observed at high resolution with a wealth of available data. Most of the spiral galaxies with the largest star formation rates in Virgo are HI-rich galaxies with very extended HI distributions, located in the cluster outskirts. We propose that the disks of these galaxies are actively accreting gas, which enhances their star formation rates. Most cluster galaxies (located closer to the cluster center) are gas-deficient and have reduced star formation rates. Many galaxies show clear evidence for ongoing ram pressure stripping, including "radio deficit regions" and ridges of polarized radio continuum emission on the leading sides, and HI tails on the trailing sides. From stellar population studies and comparisons of observations with simulations, we can identify galaxies in different stages of stripping. Many galaxies which were stripped in the past have truncated and symmetric gas disks but normal stellar disks, and have properties intermediate between spiral and S0 galaxies. While ram pressure stripping is undoubtedly responsible for producing the many cluster galaxies with truncated gas disks, the origin of anemic galaxies, with extended but low surface brightness disks of gas and star formation, has been less clear. We propose that these disks are starved, with their gas reservoirs at large radii (perhaps like those seen in the gas-rich galaxies, although on average less extreme) removed by either tidal forces or ram pressure stripping, which stops accretion onto the disks.

Practical Radioastronomy for Students using small Telescopes

Joachim Köppen

Practical work is an invaluable and essential tool in the education of students in astrophysics and in the physical sciences. By performing direct observations and analysing the data students experience the reality and limitations of measurements, and learn how scientific results are extracted from data. Small radio telescopes offer the unique possibility for students to observe the sky at wavelengths invisible to direct human perception. I like to give an overview of experiments and instruments I have been using during the past few years for education at various levels and for outreach, and to report on results and experiences gained. Instruments include not-overly-expensive telescopes on 10 GHz and 1.42 GHz which permit determination of surface temperatures of the Sun and Moon, the spiral structure and rotation curve of hydrogen gas in the Milky Way, hence a hint for the enigmatic Dark Matter. Standard radio equipment allows to listen to solar eruptions and Jovian magnetic storms. At practically zero cost, receivers in the audio range at several kHz permit us to observe, record, and analyse natural radio emissions. Present day electronics and computers gives students the chance to operate or even to build equipment which easily get results that were the cutting edge of research 50 years ago!

Seminář opavské fyziky (kompaktní rentgenové zdroje, akreční disky, spektra, variabilita)

Kolektiv autorů

Seminář opavské fyziky na téma kompaktní rentgenové zdroje, akreční disky, spektra, variabilita. Proběhne série tématických přednášek určená především studentům fyziky na Opavské univerzitě.

Tidal disruption by black holes

Andrej Cadez

Tidal disruption by a black hole is a violent event occuring when an object - a star, planet, asteroid or solid debris - wanders in the vicinity of a black hole and finds itself on a no return orbit. Energetics, characteristic features of light curves, and scenarios leading to tidal disruption will be discussed.

Kinetic theory for astrophysical plasmas

Claudio Cremaschini

In this talk I will present an overview of the work done during my PhD, which concerns the theoretical foundations of kinetic theory for astrophysical plasmas that can be found in accretion discs around compact objects and in relativistic jets. First, I will discuss the formulation of kinetic theory for non-relativistic accretion disc plasmas, with particular reference to collisionless axisymmetric magnetized and gravitationally-bound plasmas. I will show how exact quasi-stationary kinetic solutions (equilibria) of the Vlasov equation can be constructed which permit the treatment of quasi-stationary plasmas characterized by non-uniform fluid fields, including temperature and pressure anisotropies, azimuthal and poloidal flows and differential rotation. The significance of these solutions and the implications concerning the self-generation of stationary magnetic fields (kinetic dynamo) in accretion discs will be pointed out. Second, I will present a solution of the so-called electromagnetic (EM) radiation-reaction (RR) problem characterizing the dynamics of finite-size classical relativistic charged particles. The derivation is based on a variational formulation of the EM RR effect. I will show that, from the Hamiltonian representation of the non-local EM RR effect, a canonical formulation of the kinetic theory for relativistic collisionless plasmas subject to RR can be obtained. This also permits the derivation of the corresponding relativistic fluid equations in which the non-local EM RR contribution can be proved to act as a non-conservative collisional operator. The physical properties of the solution and its astrophysical applications will be discussed.

Galactic centers near and far: First interferometric observations of 3 galaxies of the Borderline Type 1 QSO Sample and of the Galactic Center (Sgr A*) with the SMA and ALMA

Lydia Moser

In my talk I will give a short insight into my recent work which can be divided into two research topics related to black holes, i.e. active galactic nuclei (AGN) and the Galactic Center. The strong correlation between the black hole mass and the central velocity dispersion of galaxies suggests a coeval growth of supermassive black holes (SMBH) and the surrounding stellar bulges. The growth is believed to be regulated by an interplay of a nuclear fueling, i.e. inflow of gas which is then consumed by star formation and accretion onto the SMBH, and a feedback from these regions such as winds, outflows and radiation. The diverse mechanisms that might be involved in these processes (e.g. galactic interactions or secular processes) are topics of current research. Going down to smaller scales little is known about the interaction of the SMBH with its immediate environment. Our own Galactic Center offers us a unique laboratory to study the physics around the SMBH, Sagittarius A*, on sub-parsec scales. In the first part of my talk I present interferometric observations of three barred galaxies from our Borderline Type 1 QSO (B1Q) sample with the Submillimeter Array (SMA) in the CO(2-1) and (3-2) line transition. The goal of the sample is to investigate the nuclear fueling in type-1 active galactic nuclei (AGN) with magnitudes around the classical Seyfert/Quasi-Stellar Object (QSO) demarcation, all located in the volume 0.01 < z < 0.06. We intend to study the evolutionary link between the local population and the more active galaxies at higher redshifts. As preliminary result we find that all three galaxies are rich in molecular gas with masses ranging from 1.3&mdash;11 x 10^9 M_sun, and two of them can be classified as luminous infrared galaxies (LIRGs) indicating high star formation activity. The region of CO emission is very compact, i.e. FWHM < 1.7 kpc, and only in one case the molecular gas also extends along the galactic bar. From the two galaxies with a compact CO emission region, one shows signatures of an inflow. The continuum flux is not detected which is consistent with an expected dust contribution of less than 50 muJy. The second part is about radio continuum and line emission maps at 230 GHz and 345 GHz obtained from a single night's observation of the Galactic Center with the SMA and - for the first time - with the Atacama Large Millimeter Array (ALMA) at 230 GHz. While the ALMA mosaic covers only the mini-spiral region, the SMA data includes also most parts of Sagittarius A West and the surrounding circumnuclear disk. Sgr A West is partially detected in continuum emission and the H30alpha (~231 GHz) recombination line emission map outlines well the distribution of the ionized gas as well as its radial motion. I present the first high resolution (5") map of the Galactic Center region in CO(3-2) emission - it traces the molecular gas in the circumnuclear disk. For future analyses of the variability of Sgr A*, I extracted the lightcurves of the Sgr A*. They show a significant difference in the average flux level between the two observations with ALMA and SMA, that are separated by only 2.5 hours.

The Large Observatory for X-ray Timing (LOFT)

Luigi Stella

High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m2-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m2 (more than an order of magnitude larger than any spaceborne predecessor) in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of <260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.