Seminars archive

Leftovers from the model of spotted accretion discs

Vladimír Karas

A `spotted' accretion disc near a black hole has been studied for a long time as a suitable phenomenological scheme to explain a wide range of variability phenomena. Variety of open issues still persist with this scenario. Based on a recent paper (Wellons et al, <a href=""></a>), we discuss a possibility to further explore effects of strong gravity in the X-ray signal from the inner regions of accreting compact objects.

Study of black holes in active galactic nuclei

Jiri Svoboda

We cordially invite all staff and guests of the Astronomical Institute ASCR to the ceremony of Jan Fric Premium for the year 2013. The ceremony will take place in the library of the Astronomical Institute ASCR in Ondrejov on Monday, January 20, from 13:00. Since 2009, Astronomical Institute grants the Premium of Jan Fric to young researchers of the Institute for their extraordinary results, which contribute to the international prestige of the Institute. The laureate for 2013 is the employee of the Department of Galaxies and Planetary Systems, Dr. Jiří Svoboda. He received the Premium for the study black holes in active galactic nuclei.

Propagating star formation in swept-up shells

Frantisek Dinnbier

Shells of swept-up gas are commonly observed at borders of HII regions and interstellar bubbles. As they expand and accrete new mass, they can fragment and form new stars. If massive stars are formed, they create new bubbles and this scenario repeats leading to propagation of star formation. Despite the importance of shell fragmentation, there is no consonance in literature how long the unstable wavelengths are and how fast they grow, especially in the case of higher values of pressure that confines the shell. In my work I study fragmentation of such shells by hydrodynamic simulations including a self gravity. A shell which is a subject to my study is thick and stratified in a direction perpendicular to its surface. At larger distances, it is truncated by the external pressure from both sides. I will present first results where I study the fragment growth rate as a function of the external pressure and compare my results with analytical estimates made by another authors in the past.

The Astro-H X-ray observatory

Norbert Werner

I will talk about the upcoming Astro-H satellite, the next large international X-ray observatory. Astro-H will carry the first high-resolution X-ray imaging spectrometers and will provide an unprecedented wide band coverage that will revolutionize the study of the physics of X-ray plasmas. I will summarize the technical capabilities of the satellite and will present some of the most exciting science goals that we expect to reach with this mission.

Particle-in-Cell (PIC) simulations of instabilities in weakly ionized plasmas

Miroslav Horky

I will talk about the research done during my stay at the University of Oslo. We simulated plasma instabilities in weakly ionized plasmas under the presence of E x B fields. Both, electrons and ions were magnetised in our simulations and we changed only the type of ion-neutral collisions. For these different ion-neutral collision types we found different electron heating rates and other instability characteristics, such as electrostatic potential fluctuations and electrostatic wave spectra. At first I will present theoretical background of our research, then our results and in the end I would like to introduce current analytical study of the simulated instabilities.

Galactic center Mini-spiral: Interaction modes of neutron stars

Michal Zajacek

Streams of gas and dust in the inner parsec of the Galactic Center form a distinct feature known as Minispiral, which has been studied in radio and infrared wavebands (Kunneriath et al., 2012). Large fraction of the Minispiral gas is ionized by radiation of stars present in the Nuclear Star Cluster (NSC). Based on the inferred mass in the inner parsec about 10^6 solar masses, over ~10^4 neutron stars should move in the sphere of gravitational influence of the SMBH. We estimate that a fraction of them propagate through denser (ionized) medium concentrated along three arms. Based on the density and the temperature of the gaseous medium, we discuss interaction regimes of magnetised neutron stars passing through the Minispiral region. The simulation results may be applied to other galactic nuclei hosting NSC where the expected distribution of interaction regimes may be quite different.

X-ray timing and spectral studies from medium-weight black holes

Maria Caballero-Garcia

A unified characterization of the spectral evolution of black hole binaries has been done in the last 30 yrs. The study of the spectral and aperiodic variability characterizing the X-ray emission from stellar-mass black-hole binaries has revealed to be a very useful tool to understand the mass of the black hole and the physics of accretion on these sources. Ultra-Luminous X-ray sources (ULXs) are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. Their X-ray properties have been seen to be different from the case of stellar-mass black hole binaries. I will present the results that we have obtained from two outstanding ULXs (NGC5408 X-1 and the ULXs in M82) and discuss on the properties that can be derived from the study of their X-ray emission.

X-ray echos from Active Galactic Nuclei

William Alston, Erin Kara, Michal Dovčiak

Recently, time lags between hard and soft X-ray band emission in AGN have been observed by X-ray observatory XMM-Newton. These may be naturally interpreted as the delays between the intrinsic fluctuations in corona that emits primary power-law radiation and their reflection from the accretion disc below. In this talk we will summarize the basic observed properties of these X-ray lags. We will also show some theoretical results from our numerical modelling of these systems in a toy model with a lamp-post geometry.

Einstein's Relativity for everyone

Naresh Dadhich

Prof Dadhich will give a talk on “Einstein For Everyone”, accessible to people of all backgrounds including non-scientists. His lecture has been designed to convey the basic message of Einstein’s Relativity in non-technical terms using purely common sense arguments. Přednášku uvede Prof. J. Bičák, v případě zájmu ji bude překládat do češtiny.

Monitoring the Galactic center with ATCA

Abhijeet Borkar

The center of the Milky Way hosts a highly variable radio, NIR & X-ray source, Sagittarius A* (Sgr A*) which is associated with a 4 million solar mass super-massive black hole (SMBH). Sgr A* is known to have a flaring activity occurring from a few hours to few days which has been observed at X-ray and NIR wavelengths with radio/sub-mm flare following approximately 100 min later. This suggests that Sgr A* undergoes accretion events sporadically. To study the underlying mechanism of the flaring of Sgr A*, we have observed the Galactic Center with ATCA. In this talk, I will present the first results of these observations of the GC.

Environmental effects on dwarf galaxies - ram pressure and harassment

Rory Smith

Using numerical simulations, we study the effects of ram pressure stripping on dwarf galaxies. It is commonly assumed that ram pressure only effects the gas component of a galaxy. We find that actually it can effect the dynamics of the stars, and even the dark matter surrounding the disk - an effect dubbed 'ram pressure drag'. We study the effects of ram pressure drag on tidal dwarf galaxies, and find the response is very strong. Tidal dwarfs maybe entirely destroyed by gas removal, and their stellar dynamics may appear heavily dark matter dominated where no dark matter exists. In a separate study we conduct a comprehensive study (>150 different orbits) of how the effects of harassment on early type dwarfs depend on their orbit within the cluster. We find that harassment is only effective for orbits that pass deep within the cluster core, and so strongly harassed galaxies beyond the core must climb out of the cluster potential well. This causes strongly harassed galaxies to have systematically lower orbital velocity compared to normal galaxies at any chosen radius, and we show how phase-diagrams (plots of velocity vs radius) may be a useful tool for identifying strongly harassed galaxies.

Black hole growth and star formation through merger evolution

Paula Calderon

Various galaxy evolution scenarios suggest that galaxy merger processes affect star formation rate, black hole growth, morphology, among other properties of a galaxy. These changes vary dependent on the merging stage (pre-collision, interacting, etc). Hence, for my PhD thesis we will investigate a significant sample of mergers (3,373 systems) in different stages of merging. I will show some of my preliminary results. I hope this may generate some interest, and I would be very appreciative of your thoughts, comments and input. This sample is taken from the Galaxy Zoo project, which allows to citizen scientists to contribute with the selection of these objects.

Emergence of chaos in black hole magnetosphere

Ondrej Kopacek

While the motion of particles near a rotating, electrically neutral (Kerr), and charged (Kerr-Newman) black hole is always strictly regular, a perturbation in the gravitational or the electromagnetic field generally leads to chaos. The transition from regular to chaotic dynamics is relatively gradual if the system preserves axial symmetry, whereas non-axisymmetry induces chaos more efficiently. Here we study the development of chaos in an oblique (electro-vacuum) magnetosphere of a magnetized black hole. Besides the strong gravity of the massive source represented by the Kerr metric, we consider the presence of a weak, ordered, large-scale magnetic field. An axially symmetric model consisting of a rotating black hole embedded in an aligned magnetic field is generalized by allowing an oblique direction of the field having a general inclination with respect to the rotation axis of the system. The inclination of the field acts as an additional perturbation to the motion of charged particles as it breaks the axial symmetry of the system and cancels the related integral of motion. The axial component of angular momentum is no longer conserved and the resulting system thus has three degrees of freedom. Our primary concern within this contribution is to find out how sensitive the system of bound particles is to the inclination of the field. We employ the method of the maximal Lyapunov exponent to distinguish between regular and chaotic orbits and to quantify their chaoticity. We find that even a small misalignment induces chaotic motion. Talk based on a recent paper published in The Astrophysical Journal, Volume 787, article id. 117.

Triggered oscillations and destruction of magnetized relativistic tori in 2D

Jaroslav Hamersky

Runaway instability operates in accretion tori around black holes, where it affects systems close to the critical (cusp overflowing) configuration. The runaway effect depends on the radial profile l(R) of the angular momentum distribution of the fluid, on the dimension-less spin a of the central black hole (|a| ≤ 1), and other factors, such as self-gravity. Here we discuss the role of runaway instability within a framework of an axially symmetric model of perfect fluid endowed with a purely toroidal magnetic field. Talked based on a recent contribution in Proceedings of the International Astronomical Union, Volume 303, pp. 424-426.

Signals of X-ray reverberation in the AGN "iron-line/continuum" delays

Anastasios Epitropakis

X-ray reverberation has been an important tool in constraining the X-ray source geometry and emission mechanism in AGN for more than a decade. With the discovery of the so-called "soft lags" in various AGN starting with 1H 0707-495 in 2009, their interpretation as "light echoes" from a hot disc of illuminated material orbiting close to a super-massive black hole has opened a new realm of possibilities for putting tight constraints on the X-ray continuum/reflector geometry. Contemporary works have shifted their attention to the study of delays between the X-ray continuum and the broad iron-line (~5-7 keV), observed in the X-ray spectra of several AGN, whose strength and shape makes it the best available candidate for having a clear reflection origin. In this talk I will discuss and present the latest results of the work that I have been doing as part of my PhD thesis regarding the study of these so-called "iron-line/continuum" delays.

Magnetic Penrose process: the astrophysical revival of Penrose process

Naresh Dadhich

Penrose process was a novel way of extracting energy from a rotating black hole simply by using spacetime geometry. However it was soon realized that the process cannot be efficient enough for powering the high energy sources like quasars because it required threshold velocity between the fragments to be greater than c/2 which was not astrophysically sustainable. It was shown in mid-1980s by the author and his colleagues that presence of magnetic field due to accretion disk can beautifully overcome this inconvenient velocity limit and thus PP as MPP got revived as a mechanism for powering the central engine. It is gratifying that the present detailed work on rotating black hole energetics bears out the predicted by MPP.

Outburst properties evolution of ESO 243-49 HLX-1

Zhen Yan

Hyperluminous X-ray source ESO 243−49 HLX-1 shows quasi-periodic outbursts in the past few years. We have performed a study of the outburst properties of HLX-1 with Swift/XRT monitoring observations. We found that the waiting time was increasing with the outburst sequence. We also found that the outburst duration, total radiated energy, and e-folding decay timescale were decreasing with the outburst sequence, which was also noticed by previous study. So during each cycle, HLX-1 spent less and less time during the outburst phase and more and more time during the quiescence phase in the sequence of last five outbursts. We compared the outburst properties of HLX-1 with bright Galactic low-mass X-ray binary transients (LMXBTs), and found that HLX-1 does not follow most correlations we found in bright Galactic LMXBTs if its distance is 95 Mpc, which indicate that they do not share the same outburst mechanism.

Lecture Day: Galactic center projects

V. Karas, D. Kunneriath, G. Karssen, F. Peissker. E. Mossoux

Vladimír Karas (AsU): Electrically charged matter near a supermassive black hole (15+5) Devaky Kunneriath (AsU): Mechanism of accretion from the Galactic centre minispiral onto Sgr A* supermassive black hole (15+5) Grischa Karssen (UCO): Relativistic disc models for SgrA* (15+5) Florian Peissker (UCO): Monitoring the DSO on its orbit around SgrA* (15+5) Enmanuelle Mossoux (CNRS): Results of our XMM-Newton 2014 campaign on Sgr A* (15+5)

Physics of X-ray binaries

Jan Čechura

We present a novel observation interpreting method for the high-mass X-ray binaries (HMXBs) based on a combination of spectroscopic data and numerical results of a radiation hydrodynamic model of stellar wind in HMXBs. By using an indirect imaging method of Doppler tomography, we calculate synthetic tomograms of a predicted emission in Low/Hard and High/Soft X-ray states and compare them with tomograms produced using phase-resolved optical spectra of Cygnus X-1, a prototype of HMXBs. The emissions of HMXBs are determined by the local conditions within the circumstellar medium - namely by local density, temperature, and ionization parameter. These quantities are computed by the radiation hydrodynamic code and strongly depend on the X-ray state of such systems. By increasing intensity of an X-ray emission produced by the compact companion in the HMXB-model, we achieved a complete redistribution of the circumstellar medium in the vicinity of the modelled system. These changes (which simulate the transitions between two major spectral states) are also apparent in the synthetic Doppler tomograms which are in a good agreement with the observational data.

The disc-spin-jet connection in stellar and supermassive black holes

Chris Done

Black hole spin is difficult to measure as it leaves an imprint only close to the horizon, but it may be required to produce most dramatic relativistic jets seen in the flat spectrum radio quasars (FRII) and BL Lacs (FRI). I will review what we know about the connection between the accretion flow, jet and spin in the stellar mass black hole binary systems and then scale this up to the supermassive black holes in AGN and quasars, including spin constraints from the new techniques which use fast variability to reverberation map the inner disc.