Seminars in 2016


Ondrej Kopacek

Deterministic chaos in a black hole magnetosphere

Dynamics of electrically charged particles in the vicinity of a rotating black hole embedded in the external large-scale magnetic field is numerically investigated. In particular, we consider a non-axisymmetric model in which the asymptotically uniform magnetic field is inclined with respect to the axis of rotation. We study the effect of inclination onto the prevailing dynamic regime of particle motion, i.e. we ask whether the inclined field allows regular trajectories or if instead, the deterministic chaos dominates the motion. In this contribution we further discuss the role of initial condition, particularly, the initial azimuthal angle. To characterize the measure of chaoticness we compute maximal Lyapunov exponents and employ the method of Recurrence Quantification Analysis.


Joachim Wambsganss

Gravitational lensing: giant luminous arcs, Einstein rings and exoplanets

Public lecture of the Learned Society of the Czech Republic. Two decades ago we had no idea whether planets orbiting stars other than the Sun existed at all. Today more than 500 exoplanets have been discovered, and the field of exoplanet research has advanced to become one of the most captivating branches of astronomy. Observational techniques now aim to address questions such as what the atmosphere and weather are like on some of these planets, and to determine their global statistical properties.


Attila Meszaros

Gamma-ray bursts: an overview

The gamma-ray bursts (GRBs) were discovered already half a century ago, but until today their astrophysical meaning remains unclear. There are short and long GRBs. The long GRBs can be connected to supernovae, but for the short ones no supernova-GRB relation has been proven yet. On the other hand, a gravitational wave--short GRB relation cannot be excluded. In addition, other types of GRBs can also exist. Simply, the situation cannot be declared as satisfactory. Because GRBs are probably the farthest objects of the Universe, and are seen also in the plane of Milky Way, they can serve as an observational test of the cosmological principle. Remarkably, these tests are against this principle. The lecture is an attempt to survey the present day situation of the topic.


Ondřej Pejcha

Death throes of binary stars: cool and luminous transients

A fraction of binary stars evolve through a phase of dramatic angular momentum loss accompanied by envelope mass ejection, which might result in the merger of the two stars. The discovery of a contact binary with rapidly decreasing orbital period followed by an outburst in V1309 Sco established a connection between such catastrophic phases of binary star evolution and a group of transients characterized by their red color and the luminosity lying in the gap separating classical novae and supernovae. To utilize the wealth of information gathered on these transients, we explore the dynamics of outflows from mass-losing binary stars using smoothed particle radiation-hydrodynamic simulations with realistic equation of state and opacities. We identify several distinct outflow regimes as a function of the parameters of the perishing binary star and make the connection with the properties of the observed transients.


Michal Zajaček

An update about Dusty S-cluster object (DSO/G2) near the supermassive black hole

I will briefly summarize our recent findings about the DSO/G2 infrared-excess object that is now in its post-peribothron phase. In particular, I will show my models that can explain the basic observational characteristics of the object. In the end, I will present a generalized model of the star-ambient medium interaction near Sgr A* and how it can be used to constrain the properties of the interstellar medium in the immediate vicinity of the supermassive black hole.


Lukáš Ledvina

Microlensing of distant quasars

In quasar microlensing, flux from a distant quasar is modulated by the gravitational lensing effect of individual stars in a galaxy lying in the foreground. The stars form a network of caustics that scan the surface of the quasar accretion disk passing in the background. Here we illustrate the effects of microlensing on the neutral iron Kα line emitted from the innermost disk. Changes in the line profile were most clearly observed by Chartas et al. (2012) in the lensed quasar RX J1131–1231. We use a fully relativistic Kerr-metric thin-disk model (Dovčiak et al. 2004) to demonstrate the connection between features of the line profile and the caustic position on the emission map of the quasar. In the future this method may prove to be a very powerful technique for spatially resolving the X-ray emission from the innermost accretion disk. We introduce a new fast numerical approach for calculating the specific flux from a (microlensed) accretion disk using GPU. In the future microlensing may prove to be a very powerful technique for spatially resolving the X-ray emitting region of the quasar.


Johannes Andersen, Birgitta Nordström

Chemical enrichment processing in the early galaxy

The most extreme metal-poor stars (EMP; [Fe/H] <= -2.5) are faint and their metal lines very weak. UVES@VLT revealed that they are chemically surprisingly homogeneous, but a few subgroups exhibit dramatic overabundances of notably carbon and neutron-capture elements. The standard, local, explanation is transfer and dilution of processed matter from a former, more evolved binary companion to the surface of the lower-mass EMP star we see today, and the binary frequency of stars enriched in C and s-process elements is indeed high. But accurate long-term radial-velocity monitoring has now proved that stars enriched in only C or r-process are generally single, so their anomalous abundances must have been implanted already in their natal ISM clouds by external sources at interstellar distances. This is not what current models of galaxy formation would predict.


Joachim Koeppen

Multifrequency Radioastronomy with a large Amateur Ground Station

(Why I could not come to Praha in 2015 ...) Since my retirement in 2013 I have been a member of a group of radioamateurs in Kiel who operate several parabolic radio antennas (4 to 9m diameter) to communicate by reflection of radio signals via the Moon, to run a ground station for NASA spacecraft, and to do some radio astronomy. Our institute also uses the facilities to allow our students to perform real observations. After describing the instrumentation, I shall give an overview of the some of labours that held me back in Kiel last year: measurement of the pointing errors and their compensation, the solar eclipse in March, the total lunar eclipse in Sept, and observations of Jupiter earlier this year. If time permits, I might say a few words on measurements of pulsars and atmospheric absorption.


Simone Recchi

New ideas on the formation of globular clusters

Several ideas have been put forward in the recent past about the formation of globular clusters (GCs), but none of them seem to be fully satisfactory. In particular it is very challenging to explain the presence of multiple populations of stars and the anticorrelations between some light elements (in particular the Na/O and Mg/Al anticorrelations) within GC. For most of the proposed scenarios, in order to fit these anticorrelation the GC must lose a very large fraction of first generation stars, but it is unclear how to disperse such a huge fraction of stars. We propose here that first generation stars are formed in a supershell, swept up after the explosion of a very massive primordial star (the so-called popIII star). Since these supershell stars are born at some distance from the center of the GC, and since they inherit the outward-directed radial velocity of the supershell, it is likely that most of them will be unbound. On the other hand, a significant fraction of the ejecta of these supershell stars can propagate, almost unimpeded, towards the center of the GC. They pile up there, cool (because of the large densities reached in the center) and can form a new generation of stars. We will present in this seminar the details of this scenario and 1-D chemo-dynamical simulations corroborating it.


Andreas Eckart

New insights into the nature of the center of the Milky Way

The compact and very massive object located at the center of the Milky Way is currently the best candidate for a supermassive black hole (SMBH) in our immediate vicinity. The strongest evidence for this is provided by measurements of stellar orbits, variable X-ray emission, and strongly variable polarized near-infrared emission originating from the radio source Sagittarius A* (SgrA*) in the middle of the central stellar cluster. Simultaneous near-infrared and X-ray observations of SgrA* have revealed insights into the emission mechanisms responsible for the powerful flares from within a few tens to one hundred Schwarzschild radii of such a putative SMBH. If Sgr A* is indeed a SMBH it will, in projection onto the sky, subtend the largest event horizon and will certainly be the first and the most important target of the Event Horizon Telescope (EHT) Very Long Baseline Interferometry (VLBI) observations currently under intense preparation. The large body of evidence continues to discriminate the identification of Sgr A* as a SMBH from alternative possibilities. We will review observational facts, theoretical grounds, and conceptual aspects for the case of Sgr A* being the black hole.


Gerold Busch, Marzieh Parsa, Nastaran Fazeli, Grischa Karssen, Matthias Subroweit, Michal Zajacek

Lecture Day: Week of Doctoral Students

Contributed talks at the Symposium on Theoretical Physics, Astronomy and Astrophysics 2016: Busch G., Eckart A., Valencia-S. M., Fazeli N., Moser L., Zajacek M.: Correlations Between Supermassive Black Holes and Their Hosts in Active Galaxies Parsa M., Eckart A., Shahzamanian B., Peissker F., Valencia-S. M.: The Relativistic Motion of Stars near Supermassive Black Hole in the Galactic Center Fazeli N., Eckart A., Valencia-S. M., Busch G., Moser L., Zajacek M.: Near Infrared Integral Field Spectroscopy of Near by Active Galaxies Karssen G. D., Bursa M., Eckart A., Valencia-S. M., Dovciak M., Horak J.: Analysis of the Four Brightest X-ray Flares from Sgr A* Subroweit M., Garcia-Marin M., Eckart A., Borkar A., Witzel G., Shahzamanian B., Straubmeier C.: Submillimeter and Radio Variability of Sgr A* — A Statistical Analysis Zajacek M., Eckart A., Shahzamanian B., Karas V., Valencia-S. M.: Stability of Circumstellar Gaseous–Dusty Material Close to the Galactic Center Conference website:


Lýdia Štofanová, Kateřina Jarkovská

Bachelor Theses 2016

Lydia Stofanova: Coherent dusty and gaseous structures near the Galactic centre Abstract. Sagittarius A*, a compact source in the centre of the Milky Way, is the nearest supermassive black hole (SMBH) in our cosmic neighbourhood, where various astrophysical processes take place. In consequence, variety of structures emerge near the Galactic centre and bow shocks that are closely studied in this work represent an example of them. The introductory part of this thesis is a brief review of the history of the Galactic centre research and its discovery in radio wavelengths. The main body of the thesis is focused on a simplified model of the bow-shock structures that are generated by stars moving supersonically with respect to the ambient medium. We discuss how these structures vary along the orbit. To this end, we consider four different models: (a) without the presence of any gaseous medium emerging from or accreting onto the SMBH, (b) taking an outflow from the SMBH into account, (c) the case of an inflow onto the SMBH, and finally (d) the combined model involving both an outflow and an inflow at the same time. We discuss symmetries of each model (or lack of them) and we find that the model considering the ambient medium at rest appears symmetrical with respect to the pericentre passage. The combined model manifests itself as the most asymmetrical one of them all. We show profiles for the tangential velocity in the shell and the mass surface density of the bow-shock shell along the stellar orbit for all considered models. Katerina Jarkovska: Annihilation and creation operators in Lie algebra theory and physics Abstract. We show the use of the theory of Lie algebras, especially their oscillator realizations, in the context of quantum mechanics. One can construct oscillator realizations from matrix realizations. In the case of symplectic and special orthogonal algebra, we demonstrate an alternative method of obtaining oscillator realizations from symmetric or exterior power of a vector space of annihilation and creation bosonic or fermionic operators. We find Lie algebra of polynomials of degree at most two in phase space of a mechanical system, which form the semidirect product of the Heisenberg algebra and symplectic algebra. It is shown that a classical system with Hamiltonian function in this algebra can be quantized by two equivalent representations - Schrödinger or Bargmann-Fock representation. The second mentioned representation generates the same operators of symplectic algebra as we got from their previous formal construction from symmetric power of a vector space of bosonic operators. Quantization is demonstrated on the bosonic harmonic oscillator. We use the similarities between bosonic and fermionic oscillator realizations to define the fermionic harmonic oscillator. Some properties of spinor representations of special orthogonal algebra are illustrated on its state space.


Anne Verhamme

Escape of Lyman photons from galaxies

One of the key questions in observational cosmology is the identification of the sources responsible for cosmic reionization. The general consensus is that a population of faint low-mass galaxies must be responsible for the bulk of the ionizing photons. However, until recently, attempts at identifying individual galaxies showing Lyman continuum (LyC) leakage have only found very few such galaxies, both at high and low redshifts. We recently proposed a new indirect diagnostic for LyC leakage based on the shape of the Lyman-alpha line, that I will first review (Verhamme+15). A breakthrough was recently achieved by our team (Izotov et al. 2016a,b), detecting LyC emission in low-redshift (z~0.3) luminous compact star-forming galaxies (LCGs) with high LyC escape fractions using HST/COS observations. I will present these new results, and the Lyman-alpha emission from these galaxies, confirming our theoretical predictions. Finally, I will propose another complementary Lyman-alpha signature for LyC leakage, discussing theoretical predictions on the Lyman-alpha spatial extent of LyC emitters: LyC leakers should have no/faint Lyman-alpha halos.


Zeinab Khorrami

A sharpened close-up of R136 and NGC3603: unshrouding the nature of their stellar population

I will present the photometric analysis of the core of two young massive clusters, R136 and NGC3603, on the basis of HST data in the visible and the VLT high dynamic imaging that we obtained in the infrared thanks to the SPHERE focal instrument operated since 2015 and its extreme Adaptive Optics. In an extensive photometric study of these data we discovered a significantly larger number of faint low-mass stars in the core of both these clusters compared to previous works. These stars are often detected in the vicinity of known massive bright objects. By comparing HST and SPHERE measures, NGC3603 does not show any signature of mass segregation in its core since the MF slope of the very core and the next radial bin are similarly flat and agree well with the MF found in previous works of the outer regions. On the other hand R136 is partially resolved using the SPHERE/IRDIS mode with most of the massive stars having visual companions. Considering the spectroscopic and photometric errors on the extinction and the age of cluster members, I estimate a mass range for each detected star. The MF is plotted at different ages with given errors on stellar masses. Finally I demonstrate that we need more resolution to go further on studying R136 which is 7-8 times further than NGC3603.


Robert Williams

The scientific legacy of Hubble Telescope’s Deep Fields

The history of how and why the original Hubble Deep Field was undertaken will be described, including the features that made it unique. The improved characteristics of the 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 has now produced a realistic model for galaxy formation and evolution over cosmic time.


Mikolaj Grzedzielski

Theory of thermal instabilities in accretion disks and its applications to the X-ray sources in all scales

The radiation emitted in the accretion disks comes from the turbulent viscous processes. Thermal instability of the accretion disk leads to the limit-cycle oscillations. An example of the thermal instability is radiation pressure instability, existing in inner regions of the disk. Since the universality of the optically thick and geometrically thin accretion disk models, we can enable our prescription to the objects with central mass ranging many orders of magnitude. We performed a large grid of generalized alpha-disk models, with another magnetization parameter mu. We proposed several formulas connecting model parameters and observables (period and parametrized shape). We applied this method to the determination of mass of a possible intermediate-mass black hole in the Ultraluminous X-ray source HLX-1. In further work we plan to include in our model GR e ffects from black hole spin and influence of heavy elements on scattering function. We plan also to analyze the magnetization of the Active Galactic Nuclei from the sample of sources.


Jason Dexter

Resolving the event horizon of the black hole in the Galactic Center

Interferometric sub-mm and infrared observations of the Galactic center black hole, Sgr A*, are now resolving angular scales comparable to the expected size of its event horizon. I will discuss how these observations could probe strong field general relativity, focusing especially on the expected appearance of the black hole shadow in emission models of Sgr A*.


Jean-Marck Hure

Methods for gravity: from soft to hyper-potentials

Gravitation plays a key-role in structuring most structures in the Universe (stars, planets, galaxies, etc). Unfortunately, the computation of the Newtonian potential and forces, needed in many simulations, remains tricky inside matter and challenging (both numerically and analytically). We will first remind a few basic properties of the gravitational potential, and discuss current point mass singularity-avoidance methods, including gravity ``softening''. We will then discuss the possibility of deriving optimal (i.e. better Newtonian-preserving) prescriptions for the softening parameter. A possible alternative to common techniques (multipole expansions, softening, etc.) is the use of hyper-potentials, which will be outlined in brief.

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