Přednášky a semináře
Semináře pořádáme nově s čtvrtletní periodou a s bohatším obsahem.
Příští seminářové odpoledne (od 13h)
03. 04. 2017
Modelling of solar flare processes: comparison of two RHD codes Flarix and RADYN
Abstract: The talk will introduce the topic of radiation hydrodynamic modelling (RHD) of solar flares and then focus on comparison of two autonomous, methodologically different RHD codes, Flarix and RADYN. RADYN code was developed by M. Carlsson at the University of Oslo for chromospheric modelling and has been extended and extensively used for the flare modelling. Flarix code is being developed at the Astronomical Institute with the primary purpose of the flare processes modelling.
Both codes can compute the time evolution a 1D flare loop heated by the beam electrons propagating from the injection point in the corona down to the lower atmosphere. The codes are based on different numerical schemes and can treat various processes relevant for solar flares.
I will present the results of testing the codes for a simplified case of the electron beam heating and compare the time evolution of the atmospheric structure (e.g. temperature, density) and the optically thick emission.
03. 04. 2017
Breakfast of the beast - Flaring activity of Sagittarius A* as seen with ATCA
Abstract: The supermassive black hole at the center of the Milky Way, associated with the compact radio source Sagittarius A*, is the closest galactic center to Earth, and a representative of low luminosity active galactic nuclei. Its proximity has allowed us to study the physics of the galactic nuclei with great detail. Sagittarius A* is currently classified as inactive, but it undergoes regular flaring events which arise from the innermost region of the accretion flow. These flares have been observed across different wavelengths, from NIR & X-ray to radio and submillimeter regimes. Multiwavelength observations have shown that the NIR & X-ray flares occur simultaneously, and are followed by the submm & radio flares. The study of the flaring activity gives us indirect constraints on the details of the emission mechanism and allows us to investigate the accretion around a low-luminosity Super-Massive Black Hole. Here, I will present the results of our observations of the Galactic Center at 3 mm wavelength using the Australia Telescope Compact Array, taken between 2010 and 2014. The flaring events in the lightcurves of Sagittarius A* were detected using the structure function analysis and Bayesian blocks representation methods. From this, six instances of flaring activity were detected, lasting 1.5 - 3 hours. I will present the results of the modelling of the flares using the adiabatically expanding plasmon model, and the subsequent physical properties of the source derived from it.
03. 04. 2017
When Be stars talking
Abstract: We are living in the era of the Big Data. Sky surveys like OGLE, MACHO, ASAS produce huge amount of the photometric data of the more the hundred million of the stars. It also means, that we need to change the way how to analyse such big amount of data and gather the most information.
A global space astrometry mission, Gaia will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars. Moreover, The Gaia Data Processing and Analysis Consortium (DPAC) process the raw data to be published in the largest stellar catalogue ever made. As the members of the DPAC consortium we are participate on this amazing space mission. In the CU7 unit our goal was to develop methods for detection and classification of the photometric variability of the Be stars. This is the very challenging task, due to the complexity of the Be stars variability (various type of variability ranging from short-term to long-term type variability) and amounts of the analyzed data (at the end of the mission we will have 1.5 billion stars observed by the GAIA).
We developed an automatic, robust and very fast method for classification and statistical analysis of the light curves of Be stars suitable for the GAIA data processing using novel approach. Namely for classification of the light curves and detection of important irregular events like outburst we use symbolic aggregate approximation alias words representation of light curve. Result of the classification can be helpful for finding connections between dynamics of the Be star discs and their long term photometric variability, detect new type of the variability and their physical mechanism and for finding new candidates of the Be stars which exhibits photometric outbursts. Our developed code is the part of the whole package VariAlgol for photometric analysis of the GAIA data.
03. 04. 2017
Properties of faint meteors studied by video observations
Abstract: Parallel double-station video observations combined with spectroscopic video observations can be a good way to study millimeter-sized meteoroids. Almost two decades of video observations of meteors at the Ondřejov observatory give us broad database to study large quantities of meteoroids and their properties.
We tried to combine spectral video observations and results of the modelling of the fragmentation of meteoroids. Along with complex information about meteoroid's trajectories and orbits, this can give us better understanding about internal structure of these millimeter-sized interplanetary bodies.
Archiv minulých seminářů
Dále se konají menší tématické semináře jednotlivých oddělení:
Semináře slunečního oddělení
Vždy v úterý ve 13:00 (září–červen, s výjimkou úterků po celoústavním semináři) v zasedací místnosti slunečního oddělení pracoviště Ondřejov.
Semináře oddělení GPS
Obvykle v pondělí od 14 hod v přednáškové místnosti pracoviště Praha–Spořilov.
22. 06. 2017, 14:00
Comparing origins of low-frequency quasi-periodic oscillations with spectral-timing
Abstract: X-ray spectral-timing is a new field that seeks to investigate how matter behaves in strong gravitational fields. Observations suggest that different types of low-frequency quasi-periodic oscillations (QPOs) are associated with different emitting-region geometries (e.g., disk-like or jet-like) in the innermost part of the X-ray binary, close to the neutron star or black hole. We developed a technique for phase-resolved spectroscopy of QPOs, and are applying it to Type B and Type C low-frequency QPOs from the black hole X-ray binary GX 339-4. On the QPO time-scale, we find that the energy spectrum changes not only in normalization, but also in spectral shape. We can quantify how the spectral shape changes as a function of QPO phase, and the two different QPOs show markedly different spectral changes. In our previous work, we inferred that the Type B QPO could be caused by a large-scale-height (i.e., jet-like) precessing region illuminating and heating overlapping azimuthal regions of the inner accretion disk. Preliminary results of the Type C QPO indicate that a small-scale-height (disk-like) precessing region may be responsible for the observed spectral changes. This talk will feature results from this paper: https://ui.adsabs.harvard.edu/#abs/2016MNRAS.460.2796S/abstract