Seminars
Our seminars take place in the library of the Main Building,
Praha–Sporilov.
Next:
20.06.2011 14:00
Yasufumi Kojima
Department of Physics, Hiroshima University, Japan
Recoil velocity induced by electromagnetic radiation
Abstract
Yasufumi Kojima
Recoil velocity induced by electromagnetic radiation
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.
22.06.2011 14:00
Yasufumi Kojima
Department of Physics, Hiroshima University, Japan
Relativistic magnetic reconnection at X-type neutral points
Abstract
Yasufumi Kojima
Relativistic magnetic reconnection at X-type neutral points
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.
24.06.2011 14:00
Bartosz Dabrowski
Astronomical Institute, Academy of Sciences, Ondrejov
Preparation of ALMA observing proposals using the Observing Tool
Abstract
Bartosz Dabrowski
Preparation of ALMA observing proposals using the Observing Tool
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.
Previous:
03.05.2011 14:00
Jan Cechura
Astronomical Institute, Academy of Sciences, Prague
Hydrodynamics in X-ray binaries - A new hydrocode for wind simulation
Abstract
Jan Cechura
Hydrodynamics in X-ray binaries - A new hydrocode for wind simulation
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.
If you would like to give a seminar in our group, please contact Vladimir Karas.
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