Seminars archive

What have we learned from the observations of the NIR-excess source DSO/G2 in the Galactic centre?

Michal Zajaček

The Dusty S-cluster Object (DSO/G2) orbiting the supermassive black hole (Sgr A*) in the Galactic centre has been monitored in both near-infrared continuum and line emission. There has been a continued discussion about the character of the object: interpreting it as either a core-less gas cloud or a dust-enshrouded star. A recent analysis of polarimetry data in K_s-band (2.2 μm) by Shahzamanian et al. (2016) allows us to put further constraints on the geometry of the DSO. I will present our recent results of the radiative transfer modelling, including NIR polarized continuum, and discuss possible models of the DSO/G2 in the context of Galactic centre stellar populations.

Revealing the structure of the outer disks of classical Be stars - are Be disks circumbinary?

Robert Klement

Classical Be stars possess self-ejected gaseous circumstellar disks governed by viscous forces. The structure of the inner parts (<20 stellar radii) of these disks is well explained by the viscous decretion disk model (VDD), which is able to reproduce multi-technique observable properties of most of the so-far studied objects. Due to the nature of the emission mechanism responsible for the IR and radio continuum excess (free-free emission), the outer parts of the disks are observable at radio wavelengths only. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for the handful of Be stars that were observed in radio, but the physical reason for this feature remained mostly unknown. I will present results from the multi-technique modeling of beta CMi, for which we obtained new sub-mm data from the APEX telescope. The SED turndown observed in beta CMi could be reproduced only when assuming a truncated disk. The most plausible explanation for the truncation is the presence of a faint companion, which has been just independently confirmed as predicted via RV analysis of the H-alpha line. Results from the SED modeling of 5 additional stars, 4 of which have new multiband VLA data, will be presented. All the studied disks are found to be truncated, while only one of the objects is a previously known binary. The detailed structure of the radio SED revealed by the VLA observations allow for studying the exact nature of the disk truncation. The truncation is clearly not as sharp as expected, and certain features indicate that the disks may extend beyond the orbits of the companions, thus offering a possibility that Be disks are actually circumbinary disks. Impacts of the suspected high binarity fraction of Be stars on their possible common evolutionary scenario will be discussed.

Observing with ESO telescopes

Bruno Leibundgut

ESO offers a wide range of telescopes and instruments. Selecting the right combination for astronomical observations can be tricky and decisive for the project. I will present the ESO process to provide a better understanding of what is required by the proposer. The talk covers proposal preparation, selection, scheduling, observations and data products as returned from ESO. It will also will touch on related topics, like available instrumentation and their science capabilities, science archive, student- and fellowship programmes. Open discussion on all topics is encouraged.

Generalized conservation law near spinning black holes

Vojtěch Witzany

Conservation of energy and angular-momentum fluxes is a well known property of the evolution of any matter field in any axisymmetric stationary metric. However, there is another flux corresponding to the „hidden symmetry“ of the Kerr space-time undergoing well-defined evolution which can be understood as a generalized conservation law. In this talk I derive the flux and its properties and discuss applications to simulations of accretion onto black holes.

Comparing origins of low-frequency quasi-periodic oscillations with spectral-timing

Abigail Stevens

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:

White dwarfs and their astrophysical implications

Kuantay Boshkayev

In my talk I will speak about classical and relativistic, cold and hot, static and rotating white dwarfs. The main attention will be drawn to the white-dwarf model of magnetars and the finite temperature effects in white dwarfs.

A study of star formation sites in tightly interacting systems

Anna Saburova

On the example of three strongly interacting systems (Arp 270, Arp 194, NGC4656) we study the star forming sites in the main bodies of the galaxies and in their vicinities: gas and stellar kinematics, metal abundance and stellar age. For this purpose we used the long-slit spectral observations carried out at the Russian 6m telescope. All three systems are very different in their star forming history. In Arp 270 the emission gas is well chemically mixed, whereas in Arp 194 there are strong velocity and abundance gradients between the galaxies. In Arp 194 and NGC4656 we observe the evidences of continuing gas accretion onto the galaxies. In Arp194 we found a tidal dwarf candidate falling back into the parent galaxy. The close low surface brightness satellite interacting with NGC 4656 in contrast appears to be a metal-poor discy dwarf galaxy resembling ultra-diffuse galaxies where the mass of stellar population is low in comparison with the mass of dark matter and HI.

Galaxies with prolate rotation in the Illustris simulation

Ivana Ebrová

Tens of early type galaxies have been recently reported to possess prolate rotation, i.e. significant amount of rotation around the major axis, including two cases in the Local Group. Although expected theoretically, this phenomenon is rarely observed and remains elusive. In order to explore its origin we study the population of well-resolved galaxies in the Illustris cosmological simulation. We follow their evolution back in time and find that the emergence of prolate rotation is strongly correlated with the time of the last significant merger the galaxy experienced, although other evolutionary paths leading to prolate rotation are also possible. (

Magnetism in Interacting Binary Stellar Systems

Ivan L. Andronov

The highlights of the theoretical study and own photometrical (since 1978) and polarimetrical (1989) monitoring of magnetic cataclysmic variables are reviewed in a comparison to other magnetic binaries. The characteristic timescale of variability of these objects is from seconds to decades and (extrapolating) even more. The monitoring of the first star of our sample AM Her was initiated by Prof. V.P. Tsesevich (1907-1983). Since more than 364 ADS papers were published on these and other variable stars in a frame of the international "Inter-Longitude Astronomy" (ILA) project. In this short review, we present some highlights of observations and mathematical modeling of interacting binary stars of different types: classical (AM Her, QQ Vul, V808 Aur = CSS 081231:071126+440405, FL Cet), asynchronous (BY Cam, V1432 Aql), intermediate (V405 Aql, BG CMi, MU Cam, V1343 Her, FO Aqr, AO Psc, RXJ 2123, 2133, 0636, 0704) polars and magnetic dwarf novae (DO Dra) with 25 timescales corresponding to different physical mechanisms and their combinations.

Probing the close environment of supermassive black holes through X-ray spectral and temporal approaches

Elias Kammoun

The spin is one of the fundamental observable properties of black holes (BHs). The spin measurement, particularly in active galactic nuclei (AGN), is of great interest for understanding the physical processes on scales ranging from the circumnuclear region out to the host galaxy. Among the various methods used to measure spins of accreting BHs, the detection of a strong relativistic reflection component in the X-ray spectra is potentially the most powerful, and it has been shown to provide robust results mainly in the least obscured sources. It would be then timely to test how reliable the reflection-based BH spin measurements that can be currently achieved are. I will present in the first part of my talk an attempt to answer this question through blind-fitting a set of simulated high-quality XMM-Newton and NuSTAR spectra, considering the most generic configuration of AGN spectra. Our main result shows that, in spite of their quality, single-epoch observations mostly return poor constraints on the actual BH spin value, except for the case of extreme relativistic effects. I will discuss also the possible impact of ATHENA in this field and how these limitations can be overcome. In the second part of my talk, I will present the "flux-flux plots" method that can provide model-independent clues regarding the X-ray variability of AGN. We applied this method to simultaneous XMM-Newton and NuSTAR observations of MCG-6-30-15 and we were able to identify both a variable and a constant (over >4.5 days) component of the X-ray emission in this source. The variable emission is consistent with a primary plus relativistic reflection that are modified by warm absorption. While the constant emission consists of a blackbody (kT ~ 0.1 keV) responsible of the constant soft excess and a neutral reflection, from distant neutral material, dominating at energies above ~1.6 keV.

"Per aspera ad astra simul": an ERAMUS+ collaboration

David Jones (IAC), and Petr Kabath (ASU)

The "Per aspera ad astra simul" ("Through difficulties to the stars together") ERASMUS+ programme represents a strategic collaboration between the Instituto de Astrofisica de Canarias (IAC, Spain) and various institutions across the Czech Republic and Slovakia, with unique opportunities for students, postdocs and more senior researchers alike. In this talk, David Jones (via Skype) will introduce the IAC and its observatories, including the Roque de Los Muchachos Observatory - home to the World's largest optical telescope, the Gran Telescopio Canarias or GranTeCan. With Petr Kabath we will also highlight some of the planned activities of the ERASMUS+ programme, including student placements at the IAC and GranTeCan, Summer schools on astronomical observing techniques, and opportunities for Czech and Slovak institutions to host IAC researchers.

Reading patterns in the sky

Sarah Jaffa

Star formation is a chaotic process, operating over a range of scales. This makes direct comparison between individual objects, whether observed or simulated, at best very difficult, and in many situations not very useful. We need unbiased statistical measures both to identify and classify the important structures; to enable a quantitative comparison of observed and simulated data; and thereby to reveal the underlying physics. I will describe a range of algorithms for detecting and quantising structure in star forming clouds and star clusters, using statistical and morphological metrics, and illustrate their application to a few representative sources.

The Hubble Deep Field and its Legacy

Robert Williams

The history of how and why the original Hubble Deep Field was undertaken will be described, including the features that made it unique. The characteristics of subsequent HST deep fields will be explained, including how they helped lead to the discoveries of dark energy, the distribution of dark matter, and the rate of star formation since the Big Bang. The results of all the deep fields combined with recent detailed numerical simulations from supercomputers have now produced a realistic model for galaxy formation and evolution over cosmic time.

Accretion flows in the collapsing massive stars

Agnieszka Janiuk

In 2015, LIGO discovered gravitational waves from a binary black hole merger, marking the beginning of a new era in astrophysics. This revolutionary discovery challenged our understanding of compact objects, including the formation of very heavy black holes from the direct collapse of massive stars. The process of collapse inevitably involves feeding the forming black hole with mass and angular momentum. For the first time, we perform self-consistently a numerical simulation of such process. We account for the change of black hole mass and spin during the accretion of the mass-energy through the black hole horizon, and we evolve the Kerr spacetime metric accordingly. Another important issue is feedback during the collapse of a star into a black hole, which may unbind the outer layers of the star and halt accretion, resulting in a less massive black hole. Our preliminary simulations using HARM code (High Accuracy Relativistic Magnetohydrodynamics) are aimed to estimate constraints on the angular momentum of the quasi-spherical flow needed to form the massive black holes that LIGO observed, as well as the possible resultant spin.

Exploring the impact of XRBs during reionization of the Universe

Vanesa Douna

Reionization is one of the main cosmological phase transitions of the Universe. Observational evidence shows that ~1Gyr after the Big Bang, the intergalactic medium was ionized, although it had become neutral much before, when the Cosmic Microwave Background was released ~0.3 Myr after the Big Bang. Despite the increasing knowledge of the Epoch of Reionization in light of the new observational skills, the sources of reionization are still being debated. Although UV photons from massive stars seem to be the main reionization agents, X-rays from accreting black-hole populations and cosmic rays have also been considered. In this talk, I will summarize two of the main results of my PhD research, that intend to explore the effect of stellar accreting sources at cosmic dawn. The importance of the effect of high mass X-ray binaries (HMXBs) on the primordial Universe depends on the hypothesized increase in the number and luminosity of HMXBs in low-metallicity galaxies. In the first place, to test this hypothesis we compile from the literature a set of observational data and, by means of Bayesian inference, we fit simple Monte Carlo models for the dependence on metallicity of the size and luminosity of the HMXB populations. We find that HMXBs are typically ten times more numerous per unit star formation rate in low-metallicity galaxies (namely <20% solar) than in solar-metallicity galaxies. A subgroup of X ray binaries, known as microquasars (MQs), exhibit powerful relativistic jets that interact with the surrounding medium. We explore the contribution of electrons accelerated in the jets of high-redshift microquasars to heating and ionizing the intergalactic medium (IGM). We develop Monte Carlo simulations of the propagation and energy deposition of these electrons as they travel away from the source to the IGM. We find that MQs contribute significantly to heating the intergalactic medium and are effective ionizers only near the galaxies.

CPK Workshop in Prague and Ondrejov

Lecture Days

Takes place over 11&ndash;15 December 2017 Talks: o Ondrej Pejcha, Explosive deaths of stars: core-collapse supernovae and stellar mergers o Tayebeh Tahamtan, Nonlinear electrodynamic in GR o Anabella Araudo, On the maximum energy of particles accelerated in the termination shocks of AGN jets o Hirofumi Noda, Structure of accreting matter in AGNs revealed by X-ray and optical monitoring and X-ray microcalorimeter spectroscopy o Georgios-Loukes Gerakopoulos, Chaos, entropy and black holes o Marzieh Parsa, Parameter estimation via MCMC o Petr Kabath, 2m telescope and exoplanets o Vladimir Karas, Mach, Meissner and rotating black holes o Daniel Seifried, Hydrodynamical simulations in Flash Social and sightseeing options: o Ondrejov Observatory, excursion (Fricova str.) o Czech Impressionism, exhibition (Prague Castle Riding School) o National Technical Museum, exhibition (Kostelni str.) o Kepler Museum, exhibition (Karlova str.) o Christmas concert Dixit Maria, Petra (Klimentska str.) o Youthful Science &ndash; Useful Science, seminar (Narodni str.)

Properties of faint meteors studied by video observations

Vlastimil Vojáček

PhD Thesis Discussion Parallel double-station video observations paired with spectroscopic video observations are a good way to study millimetre-sized meteoroids. Almost two decades of video observations of meteors at the Ondrejov observatory give us broad database to study large quantities of meteoroids and their properties. In this work we combined 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 origin, internal structure etc. of these millimetre-sized interplanetary bodies. Meteoroids that contained small grains tend to release the sodium early. Since there is a smaller amount of sodium for Na depleted meteoroids, the sodium was released earlier than it was released for meteoroids with same grain sizes and without the sodium depletion. Overall, meteoroids with sodium depletion showed different composition: they were composed of stronger material without very small grains and they did not showed very bright wakes. Two iron meteoroids on Halley type orbits were observed. They are probably remnants of complicated early years of our solar system. The distribution of grain sizes of Jupiter-family members was in a good agreements with results from the COSIMA instrument from the ROSETTA mission.

High-resolution radio interferometry: from jets to the supermassive black hole

Silke Britzen

Very Long Baseline Interferometry (VLBI) observations provide the highest resolution information in Astronomy. They allow us to study the jets and the immediate vicinities of the active supermassive black holes in galaxies. In my talk, I will present our most recent results on two prototypical active black holes. OJ287 is the best candidate galaxy for hosting a supermassive binary black hole at very close separation. To our knowledge, this is the first time we can explain the historic light-curve variability and long-term jet kinematics in geometrical terms - by jet precession and jet rotation. The jet might be launched from the ergosphere of the active black hole. For the nearby, giant radio galaxy M87 with a supermassive black hole of 10^9 solar masses, we find strong indication for turbulent processes connecting the accretion disk and the jet. For both active galaxies we can provide insights into the long-standing problem of the origin of astrophysical jets.