Seminars archive

Catch me if you can -- the case for a 'runaway' black hole in the Orion Nebula Cluster

Ladislav Subr

Process of physical collisions of stars in star clusters is considered as one of possible mechanisms of the formation of massive black holes. I will present numerical simulations of this process and discuss a possibility that the Orion Nebula Cluster has undergone a period of violent relaxation which implies tight interactions of massive OB stars.

Integral-field spectroscopy of NLRs in Seyfert galaxies

Ivana Stoklasova

The Narrow-Line Regions (NLRs) represent emitting gas extended approximately over the central kiloparsec of active galaxies, ionized dominantly by non-thermal radiation from the active galactic nuclei (AGN). The spatial extent of NLRs makes them convenient targets for studying the interplay between the AGN and the host galaxy. The gas acts as a tracer of the galactic potential, perturbed by energy injection from the AGN in the inner parts. Fully two-dimensional mapping of the kinematics is necessary for a correct interpretation, and has been made possible in an efficient way only recently with the advent of the integral-field spectroscopy. I am going to present results of my PhD thesis, where NLRs of 16 nearby Seyfert galaxies have been studied with the use of the integral-field spectrograph OASIS. Strong departures from circular rotation have been found, in the form of outflows, tilted rings or non-circular motions resulting from non-axisymmetric galactic potential. Besides the kinematics, I will discuss the ionization and density structure of NLRs, the distribution of dust and the composition of stellar populations in the region.

Evidence for warped disks of young stars in the Galactic center

Hendrik Bartko

The central parsec around the super-massive black hole in the Galactic Center hosts more than 100 young and massive stars. Outside the central cusp (R~1'') the majority of these O and Wolf-Rayet (WR) stars reside in one or two disks. Here we present the results from new observations of the Galactic Center with the AO-assisted near-infrared imager NACO and the integral field spectrograph SINFONI on the ESO/VLT.

Stationary stagnation point flows in the vicinity of a 2D magnetic null point

Dieter Nickeler

The classical reconnection solutions propose an X-type inflow of the plasma into the X-point structure of the magnetic field. We analyse systematically the implications of the different magnetic field structures of a field which depends linearly on the spatial coordinates. The linear dependence is valid if the Jacobian matrix of the magnetic field does not vanish at the null point. We solve the complete set of the resistive MHD equations for the case that the Jacobian of the velocity field does not vanish. To get a resistive and reconnective flow, it seems, that it is necessary to have a spatially varying resistivity. This leads us to the question which topological and geometrical type of flow is required to guarantee a stationary reconnection solution.

Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2008

M. Dovciak, J. Horak, G. Torok, M. Urbanec, A. Kawka, P. Bakala, J. Kovar, I. Stoklasova, P. Jachym, D. Kofron, P. Kolorenc

Michal Dovciak Thermal disc emission from a rotating black hole: X-ray polarization signatures Thermal emission from the accretion disc around a black hole can be polarized, due to Thomson scattering in a disc atmosphere. General Relativity effects strongly modify the polarization properties of the thermal radiation as observed at infinity. Among these effects, the rotation of the polarization angle with energy is particularly useful as a diagnostic tool. We assume the two cases of a Schwarzschild and an extreme Kerr black hole with a standard thin disc and a scattering atmosphere. We compute the expected polarization degree and the angle as functions of the energy as they could be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. We assume the thermal emission dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105 in the thermal state, we calculate the observed polarization. Jiri Horak Relations between high-frequency QPOs in neutron stars We review properties of high-frequency quasi-periodic oscillations (QPOs) in neutron star sources. In particular, we report on the recent study of occurrences and mutual relation of twin-peak QPOs in the source 4U 1636. We have found several prominent points in the frequency-frequency correlation where the two QPO peaks exchange their dominance (in terms of rms amplitudes and quality factors). Both QPOs are detected mainly in the vicinity of these points. As the two QPO frequencies are nearly commensurable there, these transitions likely correspond to internal nonlinear resonances between the two QPO modes. We confirm this idea in a simple toy-model consisting of two harmonic oscillator coupled by a general nonlinear force. Gabriel Torok Relating QPOs and EOS Constraints on neutron star properties following from the modelling of quasiperiodic oscillations (QPOs) have been often discussed but not much systematically treated. We consider a specific, relativistic precession, QPO model and the data of atoll source 4U 1636-53. We show that, in difference to the commonly accepted view, the model implies a mass-angular-momentum relation rather than a single combination of neutron star mass and angular momentum. We discuss similar relations for two other models. We confront these relations with predictions of miscellaneous neutron star equations of state (EOS). Martin Urbanec Equation of state of dense matter and rotating neutron stars We present some properties of slowly rotating neutron stars and how these properties are affected by equation of state of dense matter. Adela Kawka Analysis of white dwarfs in post-common envelope binaries Analysis of FUSE spectra of white dwarfs in post-common envelope binaries will be presented. By using available models, the temperature, surface gravity and chemical composition have been determined. Also, the observed photospheric lines have been used to trace the orbital path of the white dwarf, and hence place constraints on the binary parameters. Pavel Bakala Modelování power spekter záření horkých skvrn na povrchu neutronových hvÄ›zd a v tenkých akrečních discích Toky rentgenového záření horkých skvrn na povrchu neutronových hvÄ›zd a v jejich akrečních discích jsou významnÄ› ovlivnÄ›ny gravitačním lensingem v silném relativistickém poli tÄ›chto kompaktních objektů. SvÄ›telné kÅ™ivky i power spektra tohoto záření nesou tedy výrazný podpis geometrie systému zdroj–pozorovatel i parametrů centrálního objektu. Modulace epicyklickými frekvencemi orbitalního pohybu v akrečních discích je také často uvažována jako jeden z možných mechanismů vzniku QPOs – kvaziperiodických oscilací pozorovaných v power spektrech binárních systémů s neutronovou hvÄ›zdou či černou dírou. Vyvinuli jsme softwarový plnÄ› relativistický simulátor modelující svÄ›telné kÅ™ivky a power spektra záření horkých skvrn jak na povrchu neutronových hvÄ›zd, tak i v akrečních discích včetnÄ› modulace epicyklickými frekvencemi. Jiri Kovar Off-equatorial orbits in strong gravitational fields Near a black hole or an ultracompact star, the motion of particles is governed by a strong gravitational field. Electrically charged particles also feel the electromagnetic force arising due to currents inside the star or plasma circling around. We study the possibility that the interplay between gravitational and electromagnetic actions may allow for the stable, energetically bound off-equatorial motion of charged particles. This would represent the well-known generalized Störmer's 'halo orbits', which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of the Kerr Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow the existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity. Ivana Stoklasova Integral-field spectroscopy of narrow-line regions in Seyfert galaxies Our spatially-resolved spectroscopic study of narrow-line emission regions in a sample of 16 nearby Seyfert galaxies reveals strong departures from circular rotation, due to non-axisymmetric gravitational potentials and/or outflows of matter. We also study the gas density and ionization structure, and the composition of underlying stellar populations. Pavel Jachym Gas stripping Ram pressure stripping of galaxies in clusters can yield gas deficient disks. Previous numerical simulations based on various approaches suggested that except for near edge-on disk orientations, the amount of stripping depends very little on the inclination angle. Following our previous numerical and analytical study we extend the set of parameters with the disk tilt angle and study the stripping of the interstellar content (ISM) of galaxies in various environments of clusters with compact or large distributions of the intra-cluster medium (ICM). A grid of numerical simulations with varying parameters is performed using tree/SPH code GADGET with a modified method for calculating the ISM-ICM interaction. The simulations confirm the general trend of less stripping at orientations close to edge-on case. Although various hydrodynamical effects are present in the ISM-ICM interaction, the main quantitative stripping results appear to be roughly consistent with a simple scenario of the momentum transfer from the encountered ICM. We propose a fitting formula to reproduce the numerical results. The dependence on the disk tilt angle and the ICM column density is superior to that on the peak ram pressure. Similar behaviour can also be found in previous simulations. David Kofron Obecná relativita a její Newtonovská limita – první ilustrace na prostoročasech s pohybujícími se zdroji Newtonovská limita obecnÄ› relativistických Å™eÅ¡ení je významná jak z obecného hlediska srovnání různých teorií tak zvláštÄ› pro pochopení fyzikálních vlastností často matematicky velmi komplikovaných Å™eÅ¡ení rovnic pro gravitační pole v obecné relativitÄ›. Nastíníme, jak Newtonovskou limitu provést pro gravitační pole rovnomÄ›rnÄ› urychlených částic či černých dÄ›r a poukážeme na nÄ›které podobnosti s Newtonovskou limitou "klasického" Schwarzschildova Å™eÅ¡ení. ZávÄ›rem zmíníme nÄ›které možnosti zobecnÄ›ní, například pÅ™idání elektrického náboje a odpovídajícího elektromagnetického pole. Premysl Kolorenc ICD – ultrafast decay pathway of ionized clusters Core ionization of atoms and molecules produces ions in their highly excited states, lying well above the second ionization threshold. Such states can decay by electron emission. This fundamental process is known as Auger effect. Contrary to the core ionization, ionization of electrons from the inner-valence subshell produces much less excited species making the slow photon emission the only possible decay mode as long as the ion is isolated. The situation differs radically in clusters. Due to the possibility to distribute the positive charges over two or more cluster subunits the double ionization thresholds of the clusters lie much lower than those of the isolated atoms or molecules. This brings about an interatomic (or intermolecular) decay process in which the excess energy of the ion with a hole in the inner-valence subshell is utilized to ionize a neutral neighbour. The two positive charges distributed over the cluster usually lead to the explosion (break-up) of the cluster into two singly-charged ions. The decay process occurs via the Coulomb interaction between the electrons of the two cluster subunits and is thus called an inter-atomic Coulombic decay (ICD).

New dynamical model and black hole spin determination for LMC X-1

Aleksander Sadowski

Recently a new dynamical model of the high mass X-ray binary LMC X-1 has been developed. Detailed analysis of the light curves and spectra made accurate determination of the system parameters possible. These results were used to estimate the dimensionless spin parameter of the central BH basing on X-ray spectra obtained by RXTE. (Talk based on arXiv:0810.3447 and arXiv:0901.0920.)

Entropy distribution in non-barytropic Polish Doughnuts

Marek Abramowicz

I review recent improvements in analytical models of perfect fluid tori rotating around black holes, called Polish Doughnuts. I concentrate on the problem of entropy distribution in non-barytropic Polish Doughnuts. I will derive step-by-step all relevant formulae on a blackboard (or whiteboard). The classical Polish Doughnuts are assumed to be made of barytropic fluids, with a one-parameter equation of state, P = P(\rho). In this case the relativistic Euler equation has a trivial first integral, allowing the analytic solution, but this case also leads to a fundamental difficulty known (in the theory of rotating stars) as "the von Zeipel paradox". One possible way out is a strong meridional circulation, another one is a non-barytropic fluid. Both possibilities call for a more subtle mathematical treatment than that known for the classic Polish Doughnuts.

Microphysics of accretion discs

Odele Straub

Accretion discs are made of rotating gas and are present in various astrophysical objects, such as young stellar systems, black hole or neutron star binaries, active galactic nuclei and gamma ray bursts. The main difficulty is to properly understand the nature of viscosity, i.e., the very process by which the gas looses angular momentum to accrete onto the central object. Recent studies of the microscopic viscosity and resistivity of plasma give information about the role of magneto-hydrodynamical turbulence. (This talk based on a few recent articles, e.g., Rossi et al. 2008.)

Molecular gas in the Magellanic Clouds; Physical conditions of star forming clouds probed by sub-mm spectra

Yasuo Fukui

The Magellanic Clouds are an ideal laboratory to study star formation in molecular clouds. We have been doing a systematic study of molecular clouds by suing mm- and sub-mm molecular spectra. The NANTEN2 telescope in Chile offers a powerful tool to probe density and temperature of the molecular gas and to relate the physical properties with star formation. I present the most recent progress obtained by the study and discuss how molecular gas is formed from HI gas and cause the active star formation.

Lecture Day: Discussion with students from the International Max Planck Research School (IMPRS), Heidelberg

J. Merten, J. Palous, V. Karas, J. Dale, P. Jachym et al.

An informal discussion about research topics in astronomy and astrophysics carried out in Prague and Heidelberg. All students and researchers are welcome to participate.

Approaching the dawn of gravitational wave astronomy: new inputs from magnetars and short gamma ray bursts

Luigi Stella

The first generation of gravitational wave interferometers has now reached design sensitivity and extensive coordinated scientific runs are about to start. Second generation interferometers, such as Advanced LIGO/Virgo will follow in 4-5 years, are effectively opening the new field of gravitational wave astronomy. Much is still to be learned about the gravitational wave sources from current astrophysical observations and theory. I will survey the recent progress achieved in this field through the study of magnetars, neutron stars whose electromagnetic emission is powered by the decay of their extremely high magnetic field, and short gamma ray bursts, which are believed to originate from coalescing neutron star/neutron star of neutron star/black hole binaries.

Newman-Penrose formalism and Walker-Penrose theorem

Michal Dovciak

In general relativity in the limit of geometrical optics the polarization vector is parallelly transported along the light ray. Therefore in general relativistic polarization calculations the parallel transfer of vectors along the null geodesic is important. According to the Walker-Penrose theorem there exists a complex constant that fully describes this transfer in the black hole solutions of Einstein's equations. In this talk a proof of the theorem will be given (or rather sketched). To this purpose firstly the basic concept of Newman-Penrose formalism will be shown.

Our Galactic neighbourhood - a melting pot of migration

Birgitta Nordstrom

Spiral galaxies are an important part of the visible Universe. In the prototype, our own Milky Way, we can observe the most important component of a spiral galaxy - the disk - in unprecedented detail. In the Solar neighbourhood we can determine the numbers, ages, detailed chemical compositions, and galactic orbits of stars from the entire history of the disk with a completeness and accuracy not available anywhere else in the Universe. Therefore, the solar neighbourhood is a fundamental benchmark for all models of the evolution of galaxy disks. The Geneva-Copenhagen Survey (Nordström et al. 2004 and Holmberg et al. 2007, 2008) has full spatial, kinematic, metallicity and age information for 14,000 long-lived stars and provides a rich source of data for tests of models of evolution and formation of the Galaxy. We find that classical evolution paradigm of gradual enrichment and dynamical heating of the Galactic disk seem to fail several of the standard tests related to the stellar metallicity distribution, age-metallicity relation, and age-velocity relation. Both dynamical and kinematic evolution need to be taken into account in sufficient detail by the models to match the best data. A search for signatures of past accretion events in the Milky Way (Helmi et al. 2006) has yielded evidence of ancient substructure in the Galactic Disk and a project to study possible chemical signatures is ongoing.

Between us and particle astrophysics: high-energy hadronic interactions

Jan Ebr

With the advent of large ground-based cosmic-ray observatories, from which the Pierre Auger Observatory is currently the largest, a new window into the universe has opened. Particles with laboratory energies as large as 10<sup>20</sup> eV are observed and the statistic of the data in the 10<sup>19</sup> eV range increases rapidly. Nevertheless, the interpretation of the data is not easy: not only do we observe only the extensive air showers of secondary particles in the atmosphere instead of "seeing" the incoming particle directly - the crucial difficulty lies in the fact that the sensitivty range of ground-based detectors does not overlap with the energies currently accessible in accelerator experiments. Thus some way of extrapolation is necessary - a genuinely tricky task considering the lack of fundamental understading even for the existing data in the accelerator energy range. This talk aims to clarify these difficulties to a non-particle physicist. A brief introduction to strong interaction phenomenology is given. The Quantum Chromodynamics (QCD) is introduced, showing its strengths and weaknesses alike. A small digression into the field-theoretical aspects is made, explaining the important differences between different coupling regimes of QCD, gently touching the crucial idea of a running coupling constant. Then the somehwat mystical pomeron is introduced and the general ideas of current highest-energy hadronic Monte Carlo event generators are presented. At the end, the listener shall understand why, even with the amazing amount of data and theories at hand, the utility of ground-based cosmic ray observatories as "particle telescopes" for astrophysics is still sohewhat limited.

On the multi-critical properties of axisymmetric matter flow in the Kerr metric

Tapas K. Das

I will demonstrate that the accretion of non self-gravitating classical fluid onto astrophysical black holes may have multiple critical points by identifying the equations describing the space gradient of the dynamical flow velocity of the accreting matter to be equivalent to first order autonomous dynamical systems. Fixed point analysis ensures that such a flow must be multi-transonic for certain astrophysically relevant initial boundary conditions, and such multi-transonic flow allows the formation of Rankine-Hugoniot shock in the accretion disc. I also plan to demonstrate how such multi-transonic shocked flow model can be applied to explain various astrophysical phenomena, like generation of the AGN outflow and the X-ray flares from our Galactic centre, or the origin of the quasi periodic oscillations of black hole candidates.

The black hole accretion

Marek Abramowicz

I will describe the main results of the theory of black hole accretion. A fairly complete description includes all classical results. Namely: the Balbus-Hawley theory of turbulent stresses; the Shakura-Sunyaev accretion disc; Polish doughnuts, ion tori, slim discs, and the `adafs'; disco-seismology and the Papaloizou-Pringle instability &mdash; as well as recent developments in calculating spectra by advanced ray-tracing. I do not assume any detailed knowledge of general relativity or radiative processes. I will introduce all the necessary physics step by step, in a way that (I hope) will be helpful for beginners and not boring for experts. This is the first lecture in the series of six, scheduled to be presented during autumn 2009.

Accretion discs around magnetized neutron stars

Ulf Torkelsson

Ghosh & Lamb constructed the standard model for an accretion disc around a magnetic star in 1979. The dipole field of the star produces a toroidal magnetic field in the disc because of the mismatch between the rotation of the star and the Keplerian rotation in the disc. The resulting magnetic tension is then responsible for an exchange of angular momentum between the star and the disc. Since then, through the work by Balbus & Hawley, we have become aware of that the accretion disc can produce a magnetic field on its own. This magnetic field can significantly enhance the torque between accretion disc and the neutron star. Furthermore the torque reversals that have been observed in disc-accreting X-ray pulsars by Bildsten and co-workers can then be interpreted as reversals of the magnetic field in the accretion disc. Belay Tessema and Torkelsson have recently produced self-consistent models of such accretion discs.

A few unsolved problems in accretion theory: stability, oscillations, limit cycles

Marek Abramowicz

I will review the present state of understanding of time behavior of black hole accretion disks, stressing the issues of: Papaloizou-Pringle instability, Balbus-Hawley instability, diskoseismology, and thermal limit cycles.