Seminars archive

How computers understand natural languages

Jaroslava Hlavacova

<i>Talk about natural language processing</i> 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.

Charged particles in black hole magnetospheres

Vladimir Karas

In an informal talk I will report on work done recently in collaboration with Jiri Kovar et al. Based on a paper published in Physical Review D, vol. 84, id. 084002 (arXiv:1110.4843).

Analysis of non-gaussian data using Karhunen-Loeve expansion

Peter Greskovic

Most of currently used methods for statistical analysis of astronomical measurements is based on assumption of gaussian distribution of noise. There is no universal framework to analyse data that violates this assumption. However, such data is not uncommon in astrophysics. For example lightcurves of X-ray sources appear to have strongly non-gaussian nature. We will discuss some problems with analysis of this data. We will also present a universal method to deal with this problem that is based on Karhunen-Loeve expansion of random processes and Bayesian statistics. Advantages of this method will be shown in some numerical experiments and it's sensitivity and usability will be discussed.

The turbulent evolution of the multi-phase ISM

Stefanie Walch

In this talk I will summarise our recent paper on the turbulent evolution of the two-phase interstellar medium (warm and cold atomic phases), where we study the influence of gas metallicity, turbulence, and non-equilibrium chemistry, in order to constrain the initial conditions for star formation prevailing in turbulent gas in different environments. We perform high-resolution simulations in 3D, including a realistic non-equilibrium treatment of the ionization state of the gas, and examine both driven and decaying turbulence.

Using numerical relativity to explore fundamental physics and astrophysics

Luciano Rezzolla

Recent years have seen a major progress in numerical relativity and the solution of the simplest and yet among the most challenging problems in classical general relativity: that of the evolution of two objects interacting only gravitationally. I will review the results obtained so far when modelling binaries of black holes or of neutron stars and also discuss the impact these studies have in detection of gravitational-waves, in astrophysics, and in our understanding of general relativity.

Hydrodynamics in X-ray binaries - A new hydrocode for wind simulation

Jan Cechura

The Roche lobe overflow and the isotropic radial stellar wind were traditionally considered as two competitive scenarios to explain the regime of mass loss from a companion star which can feed an accretion disc around a compact object in interacting binaries and X-ray binaries in particular. We have developed a radiation-hydrodynamic model of circumstellar matter and used it to numerically simulate the properties of the evaporative stellar wind from a supergiant component of a binary. Those simulations include radial approximation and also full 3D radiation-hydrodynamics taking into account the Roche potential, Coriolis force and radiative pressure in the continuum and lines. The Coriolis force influences substantially the mass-loss and thus also the accretion rate. Our results show that non-radial models are necessary to reach quantitative information about the angular modulation of the wind and the mass-loss and accretion rates with an accuracy of about tens of percents. Furthermore, the focusing of the stellar wind by the gravitational field of the compact companion leads to the formation of a gaseous tail behind the companion which can be understood as a modified version of the Bondi-Hoyle-Lyttleton accretion. Nowadays, the existence of such gaseous tails in interacting binaries is confirmed observationally.

Recoil velocity induced by electromagnetic radiation

Yasufumi Kojima

We examine the recoil velocity induced by the superposition of the magnetic dipole and quadrupole radiation from a pulsar/magnetar born with rapid rotation. The resultant velocity depends not on the magnitude, but rather the ratio of the two moments and their geometrical configuration. The model does not necessarily lead to high spatial velocity for a magnetar with a strong magnetic field, which is consistent with the recent observational upper bound. The maximum velocity predicted with this model is slightly smaller than that of observed fast-moving pulsars.

Relativistic magnetic reconnection at X-type neutral points

Yasufumi Kojima

The relativistic effects in the oscillatory damping of magnetic disturbances near two-dimensional X-points are investigated. By taking displacement current into account, we study new features of extremely magnetized systems, in which the Alfven velocity is almost the speed of light. The frequencies of the least-damped mode are calculated using linearized relativistic MHD equations for wide ranges of the Lundquist number $S$ and the magnetization parameter $\sigma$. The oscillation and decay times depend logarithmically on $S$ in the low resistive limit. This logarithmic scaling is the same as that for nonrelativistic dynamics, but the coefficient becomes small as $\sim \sigma^{-1/2}$ with increasing $\sigma$. These timescales approach constant values in the high resistive limit: the oscillation time becomes a few times the light crossing time, irrespective of $\sigma$, and the decay time is proportional to $\sigma$ so is longer for a highly magnetized system.

Preparation of ALMA observing proposals using the Observing Tool

Bartosz Dabrowski

The Atacama Large Millimeter/submillimeter Array (ALMA) is an array of high-precision antennas operating at millimeter/submillimeter wavelengths, currently being assembled at a 5000 m high site in northern Chile. The Joint ALMA Observatory (JAO) invites members of the astronomical community to submit proposals for Early Science observations with ALMA. Successful projects are expected to be executed between September 30, 2011 and June 30, 2012. The Cycle 0 capabilities comprise sixteen 12-m antennas, receiver bands 3, 6, 7 and 9 (wavelengths of about 3, 1.3, 0.8 and 0.45 mm) and two antenna configurations with maximum baselines of 125 m and 400 m. The purpose of Early Science is to deliver scientifically useful data to the astronomical community and to facilitate the ongoing characterization of ALMA systems and instrumentation as the capability of the array continues to grow. Proposals for ALMA are prepared and submitted using the ALMA Observing Tool (OT). The OT is a Java application used for the preparation and submission of ALMA Phase I (observing proposal) and Phase II (telescope runfiles for accepted proposals) materials.

Portrait of a Dark Face

Tapas K. Das

The massive black hole at our Galactic centre possesses largest apparent angular size of the event horizon. This allows the astronomers to introduce a novel VLBI technique using which one can observationally resolve the length scale comparable to the size of the event horizon of our Galactic centre black hole, and to directly image the event horizon in radio wave. On the theoretical front, a consistent modeling of the general relativistic black hole accretion in Kerr metric is necessary to predict how that image would look like in reality, and how the spin of the black hole can observationally be estimated. With an accurate measurement of the black hole mass already made, such observational signature of black hole spin as well as the direct imaging of the event horizon can have profound impact on our attempt to understand the strong gravity black hole space time at the close vicinity of the event horizon.

Meteorologické družice Meteosat druhé generace

Martin Setvák

Meteosat Second Generation (MSG) a organizace EUMETSAT. Stručná historie a status organizace, členství ČR, licenční a datová politika EUMETSATu. Ukázky snímků a odvozených produktů generovaných z dat MSG (operativní, experimentální), dostupnost aktuálních snímků MSG.

Radiative transfer approach for X-ray astronomy

Agata Rozanska

The Galactic center region

Devaky Kunneriath

High angular resolution radio interferometry at high frequencies enables us to study in great detail the emission mechanisms of the central region of our Galaxy, which harbours the supermassive black hole source Sagittarius A*. This talk will focus on recent results from our observations with two radio interferometer arrays, the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and Australia Telescope Compact Array (ATCA), and the infrared Very Large Telescope (VLT). I will also present results from the flare modelling of Sgr A* based on multi-wavelength observing campaigns, along with high-resolution maps and a spectral index analysis of the extended emission of the central region (the "mini-spiral") at 3 and 1.3 mm, and compare them to maps in near-infrared Br-gamma (2.16 micron) and mid-indfrared (8.6 micron) wavelengths. I will discuss physical properties of gas and dust that can be deduced from these measurements, such as spectral index, emission measure, density, mass, and extinction.