Seminars
Our seminars take place in the library of the Main Building (Praha–Sporilov, see Contacts).
Scheduled:
Jaroslava Hlavacova
Institute of Formal and Applied Linguistics, Faculty of Mathematics and Physics, Charles University in Prague
How computers understand natural languages
Abstract
Jaroslava Hlavacova
Talk about natural language processing
I will discuss language corpora, several levels of linguistic tagging, electronic dictionaries, parsing of sentences, briefly about main applications, for instance machine translation, question answering, speech analysis and synthesis. Examples will be taken mainly from the Czech language.
Previous:
P. Jachym, O. Svitek, M. Urbanec, D. Kofron, J. Svoboda, J. Kovar, O. Kopacek
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Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2010
Abstract
P. Jachym, O. Svitek, M. Urbanec, D. Kofron, J. Svoboda, J. Kovar, O. Kopacek
Pavel Jáchym: Gas stripping in cluster galaxies: NGC 4254
Deep HI observations have revealed a 250 kpc tail extending from the Virgo cluster galaxy NGC 4254. Within the tail an interesting elongated gaseous structure occurs, VIRGOHI 21, that exhibits a velocity gradient as large as 200 km/s. Several mechanisms were suggested to be able to produce the tail, including gas infall or high-speed tidal interaction with another galaxy. We attempt to model this system with ram pressure stripping (RPS) by surrounding intra-cluster material and discuss under what conditions RPS can form such long, thin stripping structures.
David Kofroň: Charged rotating black hole in electric field
The Ernst method of removing nodal singularities from the charged C-metric representing uniformly accelerated black holes with mass m, charge q and acceleration A by ``adding'' an electric field E is generalized. Utilizing the new form of the C-metric found recently, Ernst's simple ``equilibrium'' condition mA=qE valid for small accelerations is generalized for arbitrary A. The nodal singularity is removed also in the case of accelerating and rotating charged black holes, and the corresponding equilibrium condition is determined.
Ondřej Kopáček: Stable and chaotic motion of charged particles in the vicinity of compact objects
We investigate charged test particle motion in off-equatorial potential lobes appearing above the horizon of the Kerr black hole immersed into the large scale asymptotically homogeneous magnetic test field. Employing various techniques we study the dynamics of test particles in this setup being especially curious how do initial conditions affect the dynamic regime of motion and whether regular or chaotic motion dominates in given setup.
Jiří Kovář et al: Off-equatorial structures orbiting compact objects
We have discussed properties of the off-equatorial orbits of electrically charged particles within the general relativistic approach near two samples of compact objects: rotating magnetic compact star approximated by the Schwarzschild metric and a test magnetic field of a rotating dipole, and Kerr black hole immersed in an asymptotically uniform magnetic field of external origin. Especially, we have studied the orientation of motion along stable circular off-equatorial orbits of constant radius (halo orbits) and shapes of the related effective potential halo lobes, where the general off-equatorial motion can be bound. In both the considered cases of compact objects, two different regimes exist. Namely, the one when completely disjoint lobes occur symmetrically above and below the equatorial plane, and the one when the lobes are joined across the equatorial plane. A possible application of such model concerns the structure of putative circum-pulsar discs consisting of dust particles. It is possible that particles can acquire a small (but non-zero) value of the net electric charge, which drives them to form a halo lobes. In other words, we can speak of a model of very diluted charged off-equatorial structures or discs. These results were published in the journal Classical and Quantum Gravity (volume 27) in 2010.
Otakar Svítek: Dynamical horizon in Robinson-Trautman spacetimes
We study past horizons in the class of
type II Robinson-Trautman vacuum spacetimes with a cosmological constant. These exact radiative solutions of Einstein’s equations exist in the future of any sufficiently smooth initial data, and they approach the corresponding spherically symmetric Schwarzschild–(anti-)de Sitter metric. By analytic methods we investigate the existence, uniqueness, location, and character of the past horizons in these spacetimes. In particular, we generalize the Penrose-Tod equation for marginally trapped surfaces, which form such white-hole horizons, to the case of a nonvanishing cosmological constant, and we analyze the behavior of its solutions and visualize their evolutions. We also prove that these horizons are explicit examples of an outer trapping horizon and a dynamical horizon, so that they are spacelike past outer horizons.
Jiří Svoboda: Looking into the inner black hole accretion discs
My research deals with astrophysical black holes, including stellar-mass black holes in Galactic binaries and super-massive black hole in nuclei of active galaxies. I will present our recent results in studying various effects on the spin measurements via relativistically smeared X-ray emission lines. For instance, precision of the spectra fitting procedure could be compromised by inappropriate accounting for the angular distribution of the disc emission. Particularly, a limb-darkening law has been frequently assumed in reflection models although some radiation transfer computations exhibit an emission excess towards grazing angles (limb brightening). I will show that an improper usage of limb darkening can lead to an over-estimation of the spin value and can partly mimic a steeper profile of the radial emissivity. I will demonstrate these results on the case of the XMM-Newton observation of the Seyfert galaxy MCG-6-30-15, and on the simulated data for the next-generation X-ray missions. An unknown geometry of the disc corona is another issue which will be discussed. Fully relativistic self-consistent models with lamp-post scheme (in addition to an isotropically distributed corona) have been currently developing in our group. Application of these models on high-quality X-ray data is among my future research interests.
Martin Urbanec: Rotating compact stars and QPO observations
Properties of rotating neutron and strange star models will be compared. We will show that external Hartle-Thorne spacetime of both neutron and strange stars could be very accurately approximated by Kerr spacetime if mass of the star is close to its maximal value for given equation of state. This limit will be used to show that mass estimate from QPO observations will be increased if rotation is taken into account. We will also present how combination of parameters obtained by neutron and strange stars modeling could be used to put limits on QPO models.
We try hard to meet regularly to discuss ongoing research projects and recent astro-ph papers. If you would like to give a seminar in our group, please contact Vladimir Karas. Everybody is welcome.