• 2019-19-09: Call for the Proceedings of RAGtime contribution is open
  • 2019-19-09: Look to the Photogallery!
  • 2019-13-09: The program of the workshop is announced.


Invited speakers


Author: Eva Hackmann (ZARM, University of Bremen)

Title: Effects of weak electromagnetic fields in analytical accretion disk models

Analytical models of accretion disks have been an important tool to understand the basic underlying principles of accretion. Classically, these models are constructed using an isolated Kerr black hole. However, astrophysical black holes are usually surrounded by electromagnetic fields. Here, we consider the presence of electromagnetic test fields that are weak in the sense that they do not influence the spacetime geometry. The origin of these fields could be internal, like a tiny electric charge of the black hole, or external, like the galactic magnetic field. Here we focus on some important aspects for accretion disk models, like for instance the marginally bound radius, that is considered to mark the inner edge of a thin accretion disk.


Author: Vladimír Karas (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Tidal disruption events with spectral line reverberation mapping as a probe of accretion geometry

For an inclined accretion flow associated with a Tidal Disruption Event (TDE) near a rotating black hole, the inner parts of the flow of remnant material tend to align with the black hole rotation axis. We demonstrate that the time-resolved spectrum can reveal distinct features of TDEs just before and after the critical time, which is a function of Bardeen-Petterson radius for the disc warping. This suggests that time-resolved X-ray spectroscopy can be developed as a tool to probe these effects in TDE flares and to constrain the black hole mass and spin.

Title: Effects of Strong Gravity in X-rays from inner regions of Active Galactic Nuclei -- Selected Chapters (Poster)

Supermassive black holes (SMBH) reside in the inner regions of many galaxies. They become bright during the periods of enhanced accretion. Quasars, Seyfert galaxies and radio galaxies are among the most prominent examples of highly active objects, where the most of action occurs in the regime of strong gravity near SMBH horizon. Processes of interaction between gravitational, electromagnetic and radiation fields along with their cosmic environment and geometrical orientation determine the observational properties of these sources. Here we update our lecture notes aimed to introduce several unexpected and even bizarre effects that can be associated with magnetized black holes. We concentrate ourselves on electromagnetic signatures that can be studied with via various techniques of light curve photometry and spectro-polarimetry over different spectral bands.

Title: Black holes and neutron stars as an exciting tool for teaching physics of extreme gravity (Poster)

Compact objects - black holes and neutron stars - have been subject of theoretical investigations in astrophysics since 1960 and, more recently, also very intense observational effort. Nowadays, these objects belong to an arena of supercomputing and observational astronomy with high-tech X-ray satellites, the largest optical and radio telescopes, interferometers, and very-high energy experiments. Objects were discovered in the sky that can only be black holes — unless they are something even more exotic. Super-massive black holes are common in nuclei of many galaxies, including the Milky Way. Some of them power quasars, the most brilliant steady sources in the Universe. Stellar-mass black holes have been found in compact binary systems. Some of them, microquasars, represent a scaled-down version of quasars. Finally, black holes have helped us to open an entirely new window of gravitational waves. Lectures on black hole astrophysics offer a way to explain a link between interesting theoretical considerations and existing objects in the Universe.


Author: Włodek Kluźniak (Nicolaus Copernicas Astronimcal Center, Warsaw, Poland)

Title: Diskoseismic modes in radiative general-relativistic hydrodynamical  simulations of thin disks

High frequency QPOs in accreting black hole sources are thought to reflect the underlying space-time metric. A detailed theory of normal modes of thin disks has been worked out and can be found in books (e.g. Kato 2016) and review papers (e.g., Wagoner 2008). Other mechanisms, such as resonance resulting in a 3:2 ratio of frequencies have been proposed by Abramowicz and Kluźniak. QPOs seem to be absent from MHD simulations, however evidence for disk modes was reported in 2009 in pseudo-Newtonian hydro simulations of Reynolds et al. and O’Neill et al. Here, I am reporting on a study of the variability of global GR hydro simulations in the Schwarzschild an Kerr metric (Mishra, Klużniak, Fragile 2019 (submitted). We find that a g-mode is present as well as other modes. For the first time, evidence for a 3:2 resonance between the vertical and radial epicyclic motions is also clearly present.


Author: Jorge Rueda (ICRANet, Pescara, Italy; ICRA, Dipartimento di Fisica, Università di Roma Sapienza, Rome, Italy and INAF, Istituto de Astrofisica e Planetologia Spaziali, Rome, Italy)

Title: The physical ingredients of a binary-driven hypernova (BdHN) and their role in the explanation of a long gamma-ray bursts

Gamma-ray bursts (GRBs) originate in subclasses with specific energetics, spectra, duration, and binary progenitors. We discuss the binary-driven hypernova (BdHN) model for long GRBs. The progenitor of a BdHN is a carbon-oxygen core (CO core)-neutron star (NS) binary. The CO core explodes as a Type Ic supernova (SN) starting a hypercritical accretion process onto the NS companion. The NS then becomes more massive and faster or, in the case, it reaches the critical mass for gravitational collapse, forms a rotating BH. We focus on the theoretical results on BdHNe up to recent 3D SPH numerical simulations and on how the different components of the system play a role in the explanation of the variety of phenomena observed in a GRB: X-ray precursors, the ultrarelativistic gamma-ray (MeV) prompt emission, the GeV-TeV emission, the mildly-relativistic X-ray flares in the early afterglow, and the X-ray late afterglow and its power-law regime.


Author: Remo Ruffini (ICRANet, Pescara, Italy; ICRA, Dipartimento di Fisica, Università di Roma Sapienza, Rome, Italy and INAF, Rome, Italy)

Title: On the BdHNI GRB 190114C and its energetics

Almost fifty years after the paper "Introducing the Black Hole" by Ruffini and Wheeler and the Black Hole (BH) mass energy formula by Christodoulou Ruffini and Hawking, we can finally assert that we have been observing the moment of creation of a BH in GRB 190114C. The predicted properties of the BdHNI have been now observed: both in this source and in the twin source GRB 130427A, the first appearance of the Supernova the SN rise triggering the BdHN has been identified and followed all the way to the appearance of the optical SN. The onset of the GeV radiation coinciding with the BH formation has revealed self similar structures in the time resolved spectral analysis of both sources. For the first time, we find evidence for quantized-discrete-emissions in both sources. The BdHNI morphology was been finally confirmed by the afterglow analysis in both sources and verified in additional BdHNI. These results have been made possible by an attentive analysis and comprehension of high quality data reached by an unprecedented observational multi-messenger effort by the astronomical, the physical and the space research communities well epitomized by the original Vela Satellites, the NASA Compton space mission (CGRO), the Italo-Dutch Beppo SAX satellite, The Russian Konus Wind Satellite, the NASA Niels-Gehrels SWIFT satellite, the Italian AGILE satellite and the NASA FERMI mission. All globally assisted in the radio and in the optical by equally outstanding observational facilities.


Author: Maciek Wielgus (Black Hole Initiative, Harvard University, USA)

Title: Probing GR with the Event Horizon Telescope

I will talk about the general relativity tests enabled by the EHT observations, including promising possible future tests involving polarization and time dependent modeling of Sgr A*. This involves measuring masses, constraining spins, testing the Blandford-Znajek mechanism and no-hair theorem.



Registered speakers


Author: David Abarca (CAMK, Warsaw, Poland)

Title: Simulating super-Eddington accretion onto magnetized neutron stars in GRRMHD

We perform 2D axisymmetric radiative general relativistic magnetohydrodynamic simulations of super-Eddington accretion onto magnetized neutron stars. We have implemented a method which allows the code to handle the highly magnetized regions near the neutron star, which would ordinarily be governed by force-free electrodynamics. The main application of our simulations is to ultra-luminous X-ray sources which have been shown to harbor neutron stars. We demonstrate how magnetic channeling of the flow into accretion columns leads to beamed emission appearing significantly super-Eddington when viewed along the poles of the dipolar magnetic field.


Author: Anabella Araudo (ELI Beamlines, Institute of Physics, Czech Academy of Sciences, Czech Republic)

Title: Truncation of AGN jets by their interaction with an stellar cluster

We study the effects of interaction of jets in Active Galactic Nuclei with various obstacles, namely, stars in Nuclear Star Cluster surrounding the nucleus and globular clusters passing across the inner jet, as well as dense clouds from the Broad Line Region. The interaction provides a scenario to address cosmic ray acceleration and magnetic field amplification. In jet–star interactions a double bow–shock structure is formed where particles get accelerated via diffusive mechanism. Individual encounters have a limited effect, however, dense clusters of massive stars can truncate the jet as the cluster crosses the jet line near the jet launching region. Much of the jet kinetic energy density is transferred to the shock and it becomes available to accelerate particles. We conclude that the interaction of jets with clusters of massive stars is a promising way to explain detectable levels of gamma rays from Fanaroff–Riley class I radio galaxies.

Title: On the maximum energy of accelerated particles in the hotspots of AGN jets (Poster)

We study particle acceleration and magnetic field amplification in the termination shocks (hotspots) of radiogalaxy jets. The cut-off of the synchrotron spectrum in the hotspots of powerful radiogalaxies is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the jet termination shock is about 1 TeV for a canonical magnetic field of 100 μG. Based on theoretical considerations and observational data we show that the maximum energy of electrons cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonable large and most of the jet kinetic energy goes to non-thermal electrons. The maximum energy is ultimately determined by the ability to scatter particles back and forth the shock, and this limit applies to both electrons and protons. Therefore, the maximum energy of protons is also about 1 TeV when radiative cooling is not efficient. We show that non-resonant hybrid (Bell) instabilities generated by the streaming of cosmic rays can grow fast enough to amplify the jet magnetic field up to 100 μG and accelerate particles up to the maximum energies observed in the hotspots of radiogalaxies. 


Author: Martin Blashke (Silesian University in Opava, Czech Republic)

Title: Evolution of Braneworld Kerr-Newman Naked Singularities (poster)

We study the evolution of the Kerr-Newman naked singularities and black holes due to presence of Keplerian accretion disk. We focus on mining unstable classes of the classical Kerr-Newman spacetimes and also on the Kerr-Newman solution under the braneworld paradigm.


Author: Filip Blashke (Silesian University in Opava, Czech Republic)

Title: Finite Electroweak monopole from braneworld (poster)

We present a minimalistic and self-contained braneworld scenario where all SM fields are localized on a single domain wall. In this model, a five-dimensional Higgs field simultaneously provides i) localization of gauge fields on the wall, ii) spontaneous symmetry breaking of electroweak symmetry, iii) emergence of the SM Higgs and iv) regularization of electroweak monopole’s mass. In this way, our model is maximally efficient of reproducing SM in four-dimensions as a low-energy effective theory while being most conservative regarding the number of additional fields. The regularization of the mass of electroweak Cho-Maison monopole comes out as an unexpected bonus. This work is based on hep-ph:1802.06649.


Author: Michal Bursa (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Energy balance between accretion disk and corona

Assuming the disk is the only source of energy in an accreting system and that its internal energy is partly radiated and partly used to support magnetic fiedls, what are the constraints on the corona from the energy conservation point of view?


Author: Claudio Cremaschini (Silesian University in Opava, Czech Republic)

Title: The quantum origin of cosmological constant in manifestly-covariant quantum gravity

In this presentation, two results will be discussed:

1) The connection between the quantum-gravity wave equation and the Einstein field equations of General Relativity. This is obtained starting from the quantum hydrodynamic equations without performing the semiclassical limit, but using only the quantum Hamiltonian structure at the basis of manifestly-covariant quantum gravity (CQG-theory).

2) The quantum prescription of the cosmological constant. This is shown to be ascribed to the non-linear Bohm quantum interaction of the gravitational field with itself in vacuum and to depend generally also on the realization of the quantum probability density for the quantum gravitational field tensor. The emerging physical picture predicts a generally non-stationary quantum cosmological constant which originates from fluctuations (i.e., gradients) of vacuum quantum gravitational energy density and is consistent with the existence of quantum massive gravitons.


Author: Kateřina Goluchová (Silesian University in Opava, Czech Republic)

Title: Power density spectra of modes of orbital motion in strongly curved space–time: obtaining the observable signal (Poster)

We explore the appearance of an observable signal generated by accretion tori and small radiating circular hot spots moving along quasi-elliptic trajectories close to the innermost stable circular orbit in the Schwarzschild spacetime. Our consideration takes into account the capabilities of observatories that operated during the past decades represented by the Rossi X-ray Timing Explorer (RXTE) and the proposed future instruments. The ability to recognize the harmonic content of the signal can help to distinguish between the different proposed physical models.


Author: Sudipta Hensch (Silesian University in Opava)

Title: Gravitational lensing around Kehagias–Sfetsos compact objects surrounded by plasma (poster)

We study the optical properties of the Kehagias–Sfetsos (KS) compact objects, characterized by the “Hořava” parameter, in the presence of plasma, considering its homogeneous or power-law density distribution. The strong effects of both “Hořava” parameter and plasma on the shadow cast by the KS compact objects are demonstrated. Using the weak field approximation, we investigate the gravitational lensing effect. A strong dependence of the deflection angle of the light on both the “Hořava” and plasma parameter is explicitly shown. The magnification of image source due to the weak gravitational lensing is given for both the homogeneous and inhomogeneous plasma.


Author: Piotr Homola (Institute of Nuclear Physics PAN)

Title: Cosmic Ray Extremely Distributed Observatory: a novel astrophysical instrument the theorists might want to keep in mind

The Cosmic-Ray Extremely Distributed Observatory (CREDO) is an international scientific programme dedicated to global studies of yet not observed extremely extended cosmic-ray phenomena, the cosmic-ray ensembles (CRE), possibly beyond the reach of existing detectors and observatories. Up to date cosmic-ray research has been focused on detecting single air showers, while the search for ensembles of cosmic-rays, which may spread over a significant fraction of the Earth, is a scientific terra incognita. The key idea of CREDO is to combine existing cosmic-ray detectors (large professional arrays, educational instruments, individual detectors, such as smartphones, etc.) into a worldwide network, thus enabling a global analysis. The second goal of CREDO is to engage a large number of participants (citizen science!), assuring the geographical spread of the detectors and managing manpower necessary to deal with vast amount of data to search for evidence for cosmic-ray ensembles. In the talk I'll highlight the potential of the CREDO concept and infrastructure as a unique and novel instrumentation to confront theoretical models with up-to-date multi-messenger astrophysical observations.


Author: Jiří Horák (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Corotation instabilities in accretion flows

We discuss different scenarios of transfer of rotational energy from an accretion flow to its global coherent oscillations. The oscillation modes can gain energy either in a linear processes by means of the corotation resonance, or in a nonlinear interaction with other oscillation modes. Both ways will be discussed in thin and thick accretion flow geometries.

Title: Polarization from an oscillating magnetized torus (Poster)

We study oscillations of accretion torus with azimuthal magnetic field. For several lowest-order modes we calculate eigen-frequencies and eigenfunctions and also corresponding intensity and polarization light curves using advanced ray-tracing methods.


Author: Martin Kološ (Silesian University in Opava, Czech Republic)

Title: Accretion disks in black hole magnetosphere

Using general relativistic magnetohydrodynamics simulations we study evolution of antisymmetric accretion torus around black hole endowed with different initial magnetic field configurations. During the evolution the magnetic field inside accretion torus will be irregular and turbulent, while regular parabolic magnetic field will develop in accretion torus funnel around vertical axis.


Author: Roman Konoplya (Silesian University in Opava, Czech Republic)

Title: General parametrization for spherical and axial black holes and representation of numerical black-hole solutions in analytical form

We propose a new parametric framework to describe the spacetime of axisymmetric black holes in generic metric theories of gravity. In this case, the metric components are functions of both the radial and the polar angular coordinates, forcing a double expansion to obtain a generic axisymmetric metric expression. In particular, we use a continued-fraction expansion in terms of a compactified radial coordinate to express the radial dependence, while we exploit a Taylor expansion in terms of the cosine of the polar angle for the polar dependence. These choices lead to a superior convergence in the radial direction and to an exact limit on the equatorial plane. As a validation of our approach, we build parametrized representations of Kerr, rotating dilaton, and Einstein-dilaton-Gauss-Bonnet black holes. We also construct relatively compact analytical representations for metric of spherical black holes in the Einstein-scalar-Gauss-Bonnet, Einstein-Weyl and Einstein-scalar Maxwell theories. Based on: R. Konoplya, L. Rezzolla, A. Zhidenko, Phys.Rev. D93 (2016) no.6, 064015, R. Konoplya, K.Kokkotas, A. Zhidenko, Phys.Rev. D96 (2017) no.6, 064007, Phys.Rev. D96 (2017) no.6, 064004, R. Konoplya, A. Zhidenko, Phys.Rev. D100 (2019) no.4, 044015, R. Konoplya, T. Pappas, A. Zhidenko, arXiv:1907.1011


Author: Ondřej Kopáček (Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic)

Title: Near-horizon escape zones of electrically charged particles around weakly magnetized rotating black hole (Poster)

An interplay of magnetic fields and gravitation drives accretion and outflows near black holes. In particular, for the acceleration of particles and their collimation in jets, an ordered component of the magnetic field seems to be essential. Here we discuss the role of large-scale magnetic fields in transporting the charged particles and dust grains from the bound orbits in the equatorial plane of a rotating (Kerr) black hole and the resulting acceleration along jet-like trajectories escaping the system in a direction parallel to the symmetry axis (perpendicular to the accretion disk). We consider a specific scenario of destabilization of circular geodesics of initially neutral matter by charging (e.g., due to photoionization). Some particles may be set on escaping trajectories and attain relativistic velocity. It appears that the chaotic dynamics controls the outflow and supports the formation of near-horizon escape zones. We investigate the system numerically and construct the basin-boundary plots, which show the location and the extent of the escape zones. The effects of black hole spin and magnetic field strength on the formation and location of escape zones are discussed, and the maximal escape velocity is computed.


Author: Aleksandra Kotek (CAMK, Poland)

Title: Evaporating primordial black holes as sources of ultra-high energy cosmic rays

For nearly a century scientist were speculated about the origin of ultrahigh-energy cosmic rays (UHECR). We established a connection between the origin of UHECR and the Hawking radiation of primordial black holes. It has been discovered that there is a narrow range for masses of primordial black holes that still exist and radiate today. The analysis of flux was carried out with the assumption of extragalactic origin at different distances of 100 Kpc. Some fractions of the galactic halo of the dark matter probably consist of evaporated black holes where Hawking radiation causes an increase of high-energy cosmic ray flux.


Author: Andrea Kotrlová (Silesian University in Opava, Czech Republic)

Title: Non-geodesic corrections to mass-spin estimates for Galactic microquasars implied by QPO models (poster)

We study frequencies of axisymmetric and non-axisymmetric epicyclic modes of accretion disc oscillations and explore the influence of pressure forces present in the disc. We discuss its implications for models of 3:2 high-frequency quasi-periodic oscillations (HF QPOs) that have been observed in the X-ray fluxes of accreting black holes (BHs). We find that in some situations, especially for slowly rotating BHs, the non-geodesic influence is negligible. Nevertheless, in general, it needs to be taken into account and, for several QPO models, it can bring significant impact even for low values of BH spin. Our investigation implies consequences for estimations of black hole spin in the three Galactic microquasars, namely GRS 1915+105, GRO J1655-40, and XTE J1550-564.


Author: Debora Lančová (Silesian University in Opava, Czech Republic)

Title: Puffy accretion disks: sub-Eddington, optically thick, and stable

I will present results of simulations of accretion disk we performed using GRRMHD (general relativistic radiative magnetohydrodynamical) code Koral. Our solution, which we named Puffy accretion disk, combines properties of a thin, slim and thick disk, has high-density core in the equatorial plane with half-thickness h/r ~ 0.1 and optically thick corona with H/r ~ 1. Most of the inflow occurs through a turbulent highly advective Keplerian region surrounding the core. The disk is radiation pressure dominated and thermally stable due to radial net flux of magnetic field. A significant part of the radiation from the disk is captured by the central black hole, so the disk is less luminous than a thin disk would be at the same accretion rate.


Author: Rahim Moradi (ICRANet, Pescara, Italy; ICRA, Dipartimento di Fisica)

Title: Self-consistent solutions for obtaining Mass and spin of BH in GRBs 130427A and 190114C utilizing GeV emission energetic.

We propose that the inner engine of a type I binary-driven hypernova (BdHN) is composed of auniform background magnetic field B0 aligned with the rotation axis of a Kerr black hole (BH) modeled by the Wald solution. Using GeV emission in GRB 130427A and GRB 190114C as prototypes we show that this inner engine acts in a sequence of elementary events. Electrons are accelerated to ultra-relativistic energy near the BH horizon. When propagating with θ< 60 through the magnetic field B0, they give origin by synchrotron emission to GeV and TeV radiation. The mass and spin of BH in these two GRBs are obtained within this model.


Author: Camilo Posada (Silesian University in Opava, Czech Republic)

Title: Stable Schwarzschild stars as black-hole mimickers

The Schwarzschild star is an ultracompact object beyond the Buchdahl limit, which has Schwarzschild geometry outside its surface and positive pressure in the external layer which vanishes at the surface. Recently, it has been shown that the Schwarzschild star is stable against spherically symmetric perturbations. Here we study arbitrary axial nonspherical perturbations and show that the observable quasinormal modes can be as close to the Schwarzschild limit as one wishes, what makes the Schwarzschild star a very good mimicker of a black hole. The decaying time-domain profiles prove that the Schwarzschild star is stable against nonspherical perturbations as well. Another peculiar feature is the absence of echoes at the end of the ringdown. Instead we observe a nonoscillating mode which might belong to the class of algebraically special modes. At asymptotically late times, Schwarzschildian power-law tails dominate in the signal.


Author: Hernando Quevedo (National Autonomous Universtiy of Mexico, Ciudad de México, Mexiko)

Title: The matching problem in relativistic astrophysics

We propose a criterion for finding the minimum distance at which an interior solution of Einstein's equations can be matched with an exterior asymptotically flat solution. It is based upon the analysis of the eigenvalues of the Riemann curvature tensor and their first derivatives, implying $C^3$ differentiability conditions. The matching itself is performed by demanding continuity of the curvature eigenvalues across the matching surface. We apply the $C^3$ matching approach to spherically symmetric perfect fluid spacetimes and obtain the physically meaningful condition that density and pressure should vanish on the matching surface. Several perfect fluid solutions in Newton and Einstein gravity are tested.


Author: Kris Schroven (Astronomical Institute of the Academy of Sciences, Prague)

Title: The role of electric charge in relativistic accretion onto compact objects is discussed by means of analytic models.

Many high-luminosity phenomena in the observed universe can be traced back to accretion processes, in which electromagnetic fields play an important role. These fields are either produced within the accreted matter or they enter the stage as external fields like interstellar magnetic fields or fields, produced by the accreting object. Two analytic models are applied to examine the effects of a realistically small electric BH charge and the effects of a charge distribution in the accreted matter onto the accretion process and accretion disc structure.


Author: Zdeněk Stuchlík (Silesian University in Opava, Czech Republic)

Title: Magnetized black holes: ionized Keplerian disks and acceleration of ultra-high energy particles

Properties of charged particle motion in the field of magnetized black holes (BHs) imply four possible regimes of behavior of ionized Keplerian disks: survival in regular epicyclic motion, transformation into chaotic toroidal state, destruction due to fall into the BHs, destruction due to escape along magnetic field lines (escape to infinity for disks orbiting Kerr BHs). The regime of the epicyclic motion influenced by very weak magnetic fields can be related to the observed high-frequency quasiperiodic oscillations. In the case of very strong magnetic fields particles escaping to infinity could form UHECR due to extremely efficient magnetic Penrose process -- protons with energy $E>10^{21}$~eV can be accelerated by supermassive black holes with $M\sim10^{10}$~M$_{\odot}$ immersed in magnetic field with $B\sim10^{4}$~Gs.


Author: Petra Suková (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Revealing traces of deterministic chaos in the accreting black holes (poster)

The high energy radiation emitted by black hole X-ray binaries originates in an accretion disk. Most of the sources undergo fast and complicated variability patterns on different timescales. The variations that are purely stochastic in their nature, are expected since the viscosity of the accretion disk is connected with its turbulent behaviour induced by magnetic instabilities. The variability of the disk that reflects its global evolution governed by the nonlinear differential equations of hydrodynamics may not be only purely stochastic. Instead, if the global conditions in the accretion flow are such that the system finds itself in an unstable configuration, the large amplitude fluctuations around the fixed point will be induced. The observed behaviour of the disk will then be characterized by the deterministic chaos. The recent hydrodynamical simulations of the global accretion disk evolution confirm that the quasi-periodic flare-like events observed in couple sources are in a good quantitative agreement with the radiation pressure instability model of the disc coupled with strong outflows in form of a wind. At least 8 of the known BH X-ray binaries should have their Eddington accretion rates large enough for the radiation pressure instability to develop. In the current work, we aim to tackle the problem of stochastic versus deterministic nature of the BH accretion disk variability from the analytic and observational point of view.


Author: Odele Straub (Max Planck Institute for extraterrestrial Physics, Garching, Germany)

Title: GRAVITY: scalar field effects on the orbit of S2 star

GRAVITY is a near-infrared instrument at the European Southern Observatory's Very Large Telescope that enables ground-based high angular resolution interferometry on a level of a few tens of micro-arcseconds. This allows to detect relativistic effects like gravitational redshift and pericentre shift on the orbit of the S2 star around the Galactic centre massive black hole, Sgr A*. S2 has the shortest known orbit around Sgr A* and has been observed regularly and with increasing precision since its pericentre passage in 2002. It is by now well established that Sgr A* is a black hole, less clear, however, is its true nature and the properties of its neighbouring environment. The black hole may be a solitary point source or, perhaps more likely, it may be surrounded by some dark material. The presence of dark matter, e.g. in the form of stellar dark remnants or diffuse dark matter (the exact nature is unknown) in the central parsec affects the trajectories of stars that pass though it, in particular it adds a Newtonian component to the precession of stellar orbits. Orbital precession is in fact a critical test of gravity theories. Here, we report the orbital signatures of ultra-light dark matter described by a (hairy) scalar field on the orbit of the S2 star.


Author: Tayebeh Tahamtan (Silesian University in Opava, Czech Republic)

Title: Scalar field with Nonlinear Electrodynamics

We study influence of scalar fields on Nonlinear Electrodynamics spacetimes. The investigation is carried out using both test and gravitating scalar fields. The main motivation being to understand whether certain specific signatures of scalar fields are preserved in this case. One such characteristic is regularity of horizon which is spoiled by scalar field in spherically symmetric static scalar-vacuum spacetimes.


Author: Massimo Tessarotto (University of Trieste, Italy)

Title: Role of Quantum Entropy and Establishment of H-Theorems in the Presence of Graviton Sinks for Manifestly-Covariant Quantum Gravity

Based on the introduction of a suitable quantum functional, identified here with the Boltzmann–Shannon entropy, entropic properties of the quantum gravitational field are investigated in the framework of manifestly-covariant quantum gravity theory. In particular, focus is given to gravitational quantum states in a background de Sitter space-time, with the addition of possible quantum non-unitarity effects modeled in terms of an effective quantum graviton sink localized near the de Sitter event horizon. The theory of manifestly-covariant quantum gravity developed accordingly is shown to retain its emergent-gravity features, which are recovered when the generalized-Lagrangian-path formalism is adopted, yielding a stochastic trajectory-based representation of the quantum wave equation. This permits the analytic determination of the quantum probability density function associated with the quantum gravity state, represented in terms of a generally dynamically-evolving shifted Gaussian function. As an application, the study of the entropic properties of quantum gravity is developed and the conditions for the existence of a local H-theorem or, alternatively, of a constant H-theorem are established.


Author: Arman Tursunov (Silesian University in Opava)

Title: On the recent hot-spots detected around Galactic centre

There are strong observational indications of the presence of a highly aligned magnetic field in the vicinity of supermassive black hole at the center of our Galaxy. Relativistic motion of a plasma hot-spot in magnetic field can lead to the charge separation and resulting non-negligible net charge density in the plasma. This talk is devoted to the investigation of the effect of electromagnetic interaction on the properties and dynamics of flare components around Sgr~A*. The results are compared with the recent near-infrared GRAVIY@ESO observations of bright flares in the vicinity of the Galactic center black hole.


Author: Gabriela Urbancová (Silesian University in Opava)

Title: Epicyclic oscillations in the Hartle-Thorne external geometry (poster)

X-ray observations of binary systems containing a rotating compact star accreting matter from its binary companion are showing twin peaks in their power density spectra. The frequencies of these high-frequency quasi-periodic oscillations can be related to characteristic frequencies of quasi-circular geodesic motion.
We will present a detailed analysis of properties of the frequencies of the radial and the vertical epicyclic motion and the frequency of the orbital motion in the Hartle-Thorne geometry. The external Hartle-Thorne geometry is characterized by gravitational mass M, angular momentum J and quadrupole moment Q and describes properly the external spacetime of rotating stars in general relativity.


Author: Vojtěch Witzany (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Space-based gravitational-wave detectors for 2050

I present the science case for 4 variants of future space-based gravitational-wave detectors as submitted by members of the LISA Consortium to the ESA Voyage 2050 call. The detectors promise to reach deep into the gravitational-wave sky, to unexplored frequency bands, or to resolve the position of the sources with exquisite precision. As a result, they would bring a wealth of opportunities for scientific discovery in black hole physics and relativistic astrophysics and cosmology in general.


Author: Michal Zajacek (Centrum Fizyki Teoretycznej Polskiej Akademii Nauk, Warsaw, Poland)

Title: Shadow in X-ray emission around charged black holes

We will review basic arguments why astrophysical black holes are expected to possess a small (positive) electric charge. We showed that for the Galactic centre black hole associated with the compact radio source Sgr A* the upper limit for the electric charge is 10^15 Coulombs, which directly follows from the Wald induction mechanism (twisting of magnetic field lines due to the black hole rotation). Although this charge value (less than 12 orders of magnitude than the extremal value) is generally negligible for the dynamics of neutral matter and photons, it can affect the dynamics and radiation properties of charged particles. We will build on our previous results (Zajacek, Tursunov, Eckart, Britzen MNRAS 2018) and present an observational test for the presence of even small electric charges associated with compact objects in galactic nuclei. This novel test is based on the modified brightness profiles of thermal bremsstrahlung of hot plasma around charged black holes. In comparison with neutral black holes, thermal bremsstrahlung is expected to flatten or even decrease when black holes are even slighly charged. We will present basic predictions for Sgr A* and M87 black holes.

Title: Modelling the jet kinematics of OJ287: paradigm for other blazars? (Poster)

OJ287 belongs to one of the best candidates among active galactic nuclei to host the binary supermassive black hole at very close separation. Based on a recent analysis of 120 VLBA (Very Long Baseline Array) observations at 15 GHz by Britzen et al. (2018), we set up a basic kinematical model to explain the motion of jet components. The model involves the jet precession, on top of which we find an indication of additional nutation-like motion for the first time. The ratio of the precession and nutation periods is about 20. The precessing motion is quite natural in the supermassive black hole binary system, due to the torques exerted by the companion black hole on the accretion disc around the primary. The other explanation is provided by the disc misalignment with respect to the spin of a single black hole. The jet precession then occurs due to the Lense-Thirring effect. Both scenarios indicate a rich merger history of OJ287, which may be a general characteristic of blazars.


Author: Wenda Zhang (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic)

Title: Probing the Bardeen-Petterson effect in TDEs with iron line reverberation mapping (poster)

For an inclined accretion flow around a rotating black hole, the combined effect of the Lense-Thirring precession and viscous torque tends to align the inner part of the flow with the black hole spin, leading to the formation of a warped disc, known as the Bardeen-Petterson effect (Bardeen & Petterson 1975). In tidal disruption events (TDEs) in which a super-massive black hole tidally disrupts a nearby star and accretes the bound stellar debris, if the black hole is spinning, in general the stellar orbit is inclined with the black hole spin. Therefore TDEs are ideal objects for detecting the Bardeen-Petterson effect. We investigate the properties of time-resolved fluorescent iron line originating from a warped disc that is irradiated by the initial X-ray TDE flare, and find that the time-resolved X-ray spectroscopy can be a powerful tool to probe the Bardeen-Petterson effect in TDE flares and can be used to measure the Bardeen-Petterson radius as well as put constraint on the black hole mass and spin.




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