RAGtime 25



Na Rybníčku 626/1
Gabriel Török (Silesian University in Opava), Zdeněk Stuchlík


The 25th RAGtime workshop will be held in a hybrid form in Opava from November 27th to December 1st 2023. 

The number of registered participants is limited, the registration is moderated. Please wait with your travel plans until you receive the registration confirmation. Registration connected to the submitted abstract will have priority. 

The RAGtime workshops

The series of the annual RAGtime workshops has been held at the Institute by the Relativistic Astrophysics Group (RAG) since 1999. Its scientific scope is devoted to current problems of relativistic astrophysics, mainly focused on the physics of black holes and neutron stars. The workshop is attended by multiple experts in the field and allows for many valuable discussions and interactions between the participants.

The scientific focus of the workshop will traditionally be devoted mainly to problems of relativistic physics of black holes and neutron or quark stars. Large attention will be given to confronting theoretical models with up-to-date observations available through both electromagnetic and gravitational wave windows to the Universe.

Special attention will be devoted to the modelling of astrophysical effects in the combined strong gravity and relevant external magnetic fields. Current issues in accretion theory will be explored. We will furthermore attempt to address problems related to cosmology, mathematical aspects of the theory of relativity, and alternative theories of gravity.

In addition, new methods of generating complex general relativistic solutions containing a black hole will be exposed. We will also focus on national and international collaboration regarding the present and future cosmic X-ray missions.

The EXPRO meeting

The EXPRO group meeting will be held as a part of the workshop on Tuesday, November 28th, from 12:45 at Uni Space Lab (Coffee break place).



  • Abylaikhan Tlemissov
  • Ana Laura Müller
  • Andrea Kotrlová
  • Antonina Zinhailo
  • Bakhtinur Juraev
  • Barbora Bílá
  • Bobir Toshmatov
  • Bobur Turimov
  • Daniel Charbulák
  • Daniela Pugliese
  • David Alvarez Castillo
  • Debora Lančová
  • Denis Musil
  • Dilshodbek Bardiev
  • Dmitriy Ovchinnikov
  • Eva Sramkova
  • Evariste Boj
  • Ewa Czuchry
  • Farukh Abdulkhamidov
  • Fatemeh Kayanikhoo
  • Gabriel Török
  • Hana Kučáková
  • Iryna Bormotova
  • Jan Hladík
  • Jaroslav Vrba
  • Jiri Horak
  • John Miller
  • Karim Abd El Dayem
  • Kateřina Klimovičová
  • Kristína Novotná
  • Krzysztof Hryniewicz
  • Lukáš Rafaj
  • Maciek Wielgus
  • Marek Abramowicz
  • Martin Blaschke
  • Martin Kološ
  • Martin Urbanec
  • Maurizio Falanga
  • Michal Marvan
  • Michal Stratený
  • Milena Skvortsova
  • Miljenko Čemeljić
  • Misbah Shahzadi
  • Monika Matuszková
  • Ondřej Chlopčík
  • Ondřej Nicolas Karpíšek
  • Pavlina Jaluvkova
  • Peggy Varniere
  • Petra Suková
  • Piyush Marmat
  • Prosenjit Paul
  • Radim Pánis
  • Remo Ruffini
  • René Šprňa
  • Riccardo La Placa
  • Roman Konoplya
  • Ruchi Mishra
  • Samaresh Mondal
  • Samik Mitra
  • Thomas Pappas
  • Tomasz Krajewski
  • Tomáš Stanovský
  • Vladimír Karas
  • Vlasios Petousis
  • Wlodek Kluzniak
  • Zdeněk Stuchlík
  • Zhanerke Tlemissova
    • 10:30 AM 12:00 PM
      Coffee & Biscuits 1h 30m

      If you are in Opava, you can join the us for some coffee and cookies in ARMA

    • 1:00 PM 1:15 PM
      Opening 15m
      Speaker: Prof. Zdeněk Stuchlík (Silesian University in Opava)
    • 1:15 PM 1:55 PM
      Dynamics of Charged Particles in Dipole Magnetosphere of Neutron Stars Treated Under Influence of Radiative Back-Reaction 40m

      In the magnetosphere of neutron stars represented by a dipole field in the Schwarzschild spacetime charged test particles influenced by the Lorentz force can follow circular orbits "in" and "off" the equatorial plane coinciding with the symmetry plane of the dipole field. Chaotic motion in belts concentrated around the off-equatorial orbits is possible for particles under strong magnetic attraction. In close vicinity of the circular orbits of both types, the epicyclic motion of harmonic character is possible. Frequencies of the epicyclic and orbital motion can be related to those of the high-frequency quasiperiodic oscillations observed in systems containing a neutron star. Influence of the additional back-reaction force related to the radiation of the particles, appropriately described by the Landau-Lifshitz equation, can cause in dependence on the character of the magnetic force the succesive fall of the orbiting particle onto the neutron star surface, or unexpected widening of the particle orbit.

      Speaker: Zdeněk Stuchlík
    • 1:55 PM 2:15 PM
      Nodal precession model applied for twin HF QPOs observed around magnetized black holes 20m

      The nodal oscillations of matter orbiting compact objects as black holes and neutron stars are usually applied for explanation of the observed low-frequency quasiperiodic oscillations. In the present paper we test applicability of the nodal precession variants of the magnetically modified geodesic model, based on the epicyclic motion of slightly charged matter in magnetosphere of a neutron star represented by a Schwarzschild geometry combined with the dipole magnetic field, for fitting of the twin high-frequency quasiperiodic oscillations observed around magnetized neutron stars. We demonstrate that there is a nodal precession variant giving possibility to fit the data similarly to the standard relativistic precession variant, and we show that the other nodal variant can be clearly excluded.

      Speaker: Jaroslav Vrba
    • 2:15 PM 3:00 PM
      Coffee break 45m
    • 3:00 PM 3:20 PM
      Probing hadron-quark phase transition in twin stars using f-modes 20m

      Although it is conjectured that a phase transition from hadronic to deconfined quark matter is possible in the ultrahigh density environment in Neutron Stars, the nature of such a transition is still unknown. Depending on whether there is a sharp or slow phase transition, one may expect a third family of stable compact stars or "twin stars" to appear, with the same mass but different radii compared to Neutron stars. The possibility of identifying twin stars using astrophysical observations has been a subject of interest, which has gained further momentum with the recent detection of gravitational waves from binary neutron stars. In this work, we investigate for the first time the prospect of probing the nature of hadron-quark phase transition with future detection of gravitational waves from unstable fundamental (f-) mode oscillations in Neutron Stars. By employing a recently developed model that parametrizes the nature of the hadron-quark phase transition via ``pasta phases", we calculate f-mode characteristics within a full general relativistic formalism. We then recover the stellar properties from the detected mode parameters using Universal Relations in GW asteroseismology. Our investigations suggest that the detection of gravitational waves emanating from the f-modes with the third-generation gravitational wave detectors offers a promising scenario for confirming the existence of the twin stars. We also estimate the various uncertainties associated with the determination of the mode parameters and conclude that these uncertainties make the situation more challenging to identify the nature of the hadron-quark phase transition.

      Speaker: David Alvarez-Castillo (Institute of Nuclear Physics PAS)
    • 3:20 PM 3:40 PM
      Relativistic effects on the orbit of the closest stars to the black hole at the center of the Galaxy 20m

      In this presentation we investigate the detection of the angular momentum (or spin) and quadrupole moment of the black hole at the center of the galaxy called Sgr A. These parameters affect the astrometric and spectroscopic observations of stars in the close vicinity of the black hole (S stars). Here, we consider a collection of S stars as well as a putative star called S2/10 that is identical to S2 but 10 times closer to Sgr A, and thus much more affected by the spin effects. Such a star might exist if it is too faint to have been already detected by GRAVITY. It is possible that either future observations of this instrument, or of its update GRAVITY+ that is under development, might detect such a faint inner star. In order to reach our objectives, we use the different relativistic models in order to generate the orbit and radial velocity of the S stars and analyze how they can be affected by the spin and quadrupole moment of Sgr A*. This, allows us to affirm the detectability of these quantities which will enable us to test the no-hair theorem and thus general relativity.

      Speaker: Mr Karim Abd El Dayem (Observatoire de Paris (LESIA))
    • 3:40 PM 4:00 PM
      Constraints for the X17 boson from compact objects observations 20m

      We investigate the hypothetical X17 boson on neutron stars and Quark Stars (QSs) using various hadronic Equation of States (EoSs) with phenomenological or microscopic origin. Our aim is to set realistic constraints on its coupling constant and the mass scaling, with respect to causality and various possible upper mass limits and the dimensionless tidal deformability Λ1.4. In particular, we pay special attention on two main phenomenological parameters of the X17, the one is related to the coupling constant g that it has with hadrons or quarks and the other with the in-medium effects through the regulator C. Both are very crucial concerning the contribution on the total energy density and pressure. In the case of considering the X17 as a carrier of nuclear force in Relativistic Mean Field (RMF) theory, an admixture into vector boson segment was constrained by 20% and 30%. In our investigation, we came to the general conclusion that the effect of the hypothetical X17 both on neutron and QSs constrained mainly by the causality limit, which is a specific property of each EoS, and it depends on the interplay between, the main two parameters, the interaction coupling g and the in-medium effects regulator C. These effects are more pronounced in the case of QSs concerning all the bulk properties.

      Speaker: Dr Vlasios Petousis () Institute of Experimental and Applied Physics, Czech Technical University, Prague)
    • 10:00 AM 10:40 AM
      Energy flow of λ in Hořava-Lifshitz cosmology 40m

      Hořava-Lifshitz gravity has been proposed as a ghost-free and perturbatively renormalizable quantum gravity model candidate with an anisotropic UV-scaling between space and time. I would like to present a cosmological background analysis of the minimal projectable formulations of the theory, with particular focus on the running of the parameter λ with energy. Using a large dataset consisting of Cosmic Microwave Background data from Planck, Pantheon+ supernovae catalogue, SH0ES Cepheid variable stars, Baryon acoustic oscillations (BAO), Cosmic Chronometers, and gamma-ray bursts (GRB), provide new bounds on the cosmological parameters, in particular λ, which describes deviation from the classical general relativity. The obtained bounds differ from the value λ=1 corresponding to Einstein gravity and seem to run with the energies of the data sources.

      Speaker: Prof. Ewa Czuchry (Department of Applied Mechanics and Computer Science Military University of Technology, Warsaw, Poland)
    • 10:40 AM 11:00 AM
      General embedding algorithm and field-theory reconstruction in 5D braneworld models 20m

      We present a general algorithm that enables the consistent embedding of any four-dimensional static and spherically symmetric geometry into any five-dimensional single-brane braneworld model, characterized by an injective and nonsingular warp factor. Furthermore, we supplement the algorithm by introducing a method that allows one to, in principle, reconstruct 5D field theories that support the aforementioned geometries.

      Speaker: Thomas Pappas (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava)
    • 11:00 AM 11:30 AM
      Coffee break 30m
    • 11:30 AM 11:50 AM
      Radiation belts around black hole 20m

      We will discuss formation of zones where energetic charged particles (electrons, protons) can stay in orbit around magnetized black hole. Similar to the particle dynamic in Van Allen radiation belt for dipole Earth's magnetosphere, we will see the motion along magnetic field lines and also radiating charged particle capture by central black hole.

      Speaker: Martin Kološ (Institute of Physics, Silesian University in Opava)
    • 11:50 AM 12:10 PM
      Parameter Constraints of Compact Objects through QPO Data Using MCMC Analysis 20m

      This work delves into the exploration of a wormhole within the framework of Beyond Horndeski Theories. Our investigation encompasses the analysis of crucial components such as the effective potential, the structure of the wormhole throat, and the presence of quasi-periodic oscillations. Specifically, we assess how the wormhole affects the frequency characteristics of X-ray quasi-periodic oscillations (QPOs) observed in four X-ray binaries. To provide context, we compare these findings with established results related to upper and lower frequencies derived from the relativistic precession model. Utilizing a Monte Carlo Markov Chain (MCMC) simulation, we rigorously examine the potential impacts of wormhole parameters on the observed X-ray QPOs.

      Speaker: Farukh Abdulkhamidov (Ulugh Beg Astronomical Institute)
    • 12:10 PM 12:30 PM
      Structure-Preserving Numerical Simulations of Particles Around Slowly Rotating Neutron Stars 20m

      We explore the chaotic signatures of the geodesic dynamics for particles moving in the slowly rotating Hartle-Thorne spacetime. We investigate various phase space structures, including hyperbolic points and chaotic regions in the neighborhood of resonant islands. We present a comparison of several efficient structure-preserving numerical schemes of order four applied to the considered non-integrable dynamical system and we investigate which schemes possess the canonical property of the Hamiltonian flow.

      Speaker: Misbah Shahzadi (COMSATS University Islamabad, Pakistan)
    • 2:00 PM 2:40 PM
      Modulation mechanism of twin-peak quasi-periodic oscillations in neutron star X-ray binaries 40m

      The twin-peak quasi-periodic oscillations (QPOs) observed in neutron star (NS) X-ray binaries, represent a possible tool for probing strong gravity and dense matter equation of state. Nevertheless, the QPO modulation mechanism remains an unresolved puzzle, especially because the QPO amplitudes are sometimes as high as 30% rms. Oscillations of an inner accretion torus can modulate the accretion rate and induce a NS boundary layer (BL) variable luminosity. Using relativistic ray tracing, we investigate light curves originating from variable BL and oscillating toroidal structures of accreted fluid. We identify the scenario which allows for high QPO amplitudes and discuss implications for a variety of QPO models. In the case of simulation including torus exhibiting radial and vertical oscillations, we observe frequency peak corresponding to the vertical oscillations, which has not been identified in previous hydrodynamical simulations of accretion. We also show that when oscillations disrupt the torus stability, the resulting product strongly imprints the Keplerian frequency in the observed light curve.

      Speaker: Gabriel Török (Silesian University in Opava)
    • 2:40 PM 3:00 PM
      X-ray light curves from accreting systems containing inner tori 20m

      Low-mass X-ray binaries containing neutron stars exhibit rapid changes of the X-ray flux on timescales corresponding to hundreds of hertz. It is often assumed that an oscillating accretion torus is responsible for this effect. Within this paradigm, we model and ray-trace the torus in the vicinity of the star and analyze the resulting light curves.

      Speaker: Kateřina Klimovičová (SLU)
    • 3:00 PM 3:20 PM
      Revealing three-dimensional structure of accretion disks using perturbation methods 20m

      Perturbation theory is a powerful tool of the modern fluid dynamics. Various perturbation methods have been successfully applied in many difficult problems of aerodynamics and hydrodynamics. In the context of geometrically thin accretion flows, a regular perturbation method has been recently used by Kluzniak and Kita for consistent treatment of coupled radial and vertical structure of thin disks. Their analysis revealed nontrivial velocity field, including equatorial back-flows and vertical shear of the rotation velocity. The radial structure in their solution has however the same draw back as the standard Shakura-Sunyaev disk model. In the leading order, it is governed by algebraic equations leading to singular behavior of the solution near the radii where the viscous torque vanishes. In our view this problem is related to the use of the regular perturbation method. In this contribution, we use a singular perturbation method based on matched asymptotic expansions to describe the flow in the vicinity of the zero-torque radii. This way we find a global three-dimensional structure of black-hole accretion disks including the flow near the inner edge of the disk and the plunging region, as well as the structure of the flow in boundary layers in the innermost regions of neutron-stars accretion disks.

      Speaker: Jiri Horak (Astronomical Institute ASCR, Prague)
    • 3:20 PM 3:50 PM
      Coffee break 30m
    • 3:50 PM 4:30 PM
      FRBs from neutron star magnetospheres: a unified model 40m

      Fast Radio Bursts (FRBs) have, since their first detection, ranked among the most puzzling events in the sky owing to the extreme energies involved and the presence of both repeating and non-repeating sources: 15 years and thousands of bursts later, their emission mechanism still eludes us. I will present a new unified model for FRBs as magnetospheric events from neutron stars which naturally accounts for both types of behaviours, and show some predictions (and first applications) of the model on the expected energy and redshift distributions, both for the population at large and individual, well-studied sources.

      Speaker: Riccardo La Placa
    • 4:30 PM 4:50 PM
      Connection between accretion disk and line emitting regions in AGNs 20m

      Advancements in both spectral analysis and photoionization simulations helped establish concept of direct connection between accretion disk and line emitting regions in active galactic nuclei (AGN).
      In fact the most promising model of AGN emission lines situates line emitting matter in the accretion disk atmosphere and its extension.
      In this talk I will show how combining computed accretion disk density distribution with photoionization simulations helped explain the typical Seyfert 1 AGN broad emission lines. We used simulated vertical structure of the Shakura & Sunyaev disk with the aim to derive radial emissivity of typical emission lines from AGNs. This result is consistent with the continuum-emission line delay from reverberation mapping measurements.
      Based on this method I will describe how broad and intermediate emission lines can be useful in probing radial distribution of matter in the upper parts of accretion disks. This method may help verify which model of accretion flow is consistent with the observed spectra of particular object. We need to use simulation of low-, sub- and super-Eddington accretion flows to explain population of AGNs. This kind of research is also important for the effort of standardizing AGNs for use in cosmological applications.

      Speaker: Krzysztof Hryniewicz (National Centre for Nuclear Research, Poland)
    • 4:50 PM 5:10 PM
      Beamed emission of super-Eddington accreting magnetized neutron stars 20m

      Studies on ultraluminous X-ray sources (ULXs) became an
      interesting topic in high-energy astrophysics in the 1980s when for the
      first time they were observed during the X-ray surveys conducted by
      Einstein Observatory and the ROSAT satellite. The X-ray emission from
      these sources apparently exceeds the Eddington limit for neutron stars.
      The discovery in 2014 of coherent pulsations in ULX emission revived
      interest in the study of ULXs suggesting that the central object might
      be a neutron star accreting beyond the Eddington limit.
      Both numerical simulations and analysis of observational data are
      necessary to explore the physics responsible for the pulsation and
      high-rate X-ray emission of these objects.
      We simulate accreting magnetized neutron stars and explore the effect of
      the magnetic field on the accretion column structure and luminosity of
      these objects. The simulations are performed in 2.5D with the resolution
      of 512 x 510 using radiative general relativistic magnetohydrodynamics
      code KORAL. We show that large magnetic fields truncate the disk and
      quench the outflows which have an effect on lowering the luminosity. In
      addition, our simulations show that the magnetic field strength affects
      the beamed emission angle.

      Speaker: Fatemeh Kayanikhoo (CAMK)
    • 5:10 PM 5:30 PM
      Conference photo 20m
    • 7:00 PM 11:00 PM
      Conference Dinner 4h
    • 10:00 AM 10:20 AM
      Growth of orbital resonances around a black hole surrounded by matter 20m

      This work studies the dynamics of geodesic motion within a curved spacetime around a Schwarzschild black hole, perturbed by a gravitational field of a far axisymmetric distribution of mass enclosing the system. This spacetime can serve as a versatile model for a diverse range of astrophysical scenarios and, in particular, for extreme mass ratio inspirals as in our work. We show that the system is non-integrable by employing Poincaré surface of section and rotation numbers. By utilising the rotation numbers, the widths of resonances are calculated, which are then used in establishing the relation between the underlying perturbation parameter driving the system from integrability and the quadrupole parameter characterising the perturbed metric. This relation allows us to estimate the phase shift caused by the resonance during an inspiral.

      Speaker: Mr Michal Stratený (ASU AVČR)
    • 10:20 AM 10:40 AM
      Non-conducting tori around black hole immersed in parabolic magnetic field 20m

      We study charged non-conducting fluid tori orbiting in the background given by a Schwarzschild black hole immersed in a parabolic magnetic field. In our study we focus on the off-equatorial charged tori that could be related to creation of jets due to the Blandford-Znajek process.

      Speaker: Dr Martin Blaschke (Institute of Physics, SLU)
    • 10:40 AM 11:20 AM
      A secondary black-hole under collisions with an accretion disc 40m

      The long-term evolution of the orbit of a satellite black hole captured by a massive galactic nucleus is examined. Repetitive transits across the accretion disk slab create a turbulent wake that needs to be further studied in order to estimate the effects of dragging and the amount of material pushed out of the disk plane. The hydrodynamical drag competes with the orbital decay by gravitational waves.

      Speaker: Vladimir Karas (Astronomical Institute)
    • 11:20 AM 12:00 PM
      Coffee break 40m
    • 12:00 PM 12:40 PM
      Radio emission from pulsar-planet magnetospheric interaction 40m

      In analogy with the Solar system planets, we predict the existence of aurora on planets around millisecond pulsars. From our special relativistic magnetohydrodynamic (RMHD) simulations of magnetospheric pulsar-planet interaction, we estimate the radio emission from such systems and provide parameters for such a detection. It would be the first radio detection of an extrasolar planet and the first direct probe in the pulsar wind.

      Speaker: Dr Miljenko Cemeljic (SU, CAMK & ASIAA)
    • 12:40 PM 1:00 PM
      Search for EM counterparts of SMRI - connection to repeating nuclear transients 20m

      I will present different possibilities to discover close small mass ratio inspirals (SMRI) based on the EM variability of the nucleus prior to the merger. The secondary object ($m$), which is several orders of magnitude less massive than the primary supermassive black hole ($M$), i.e. the mass ratio $q = m/M = 10^{-2} - 10^{-8}$, orbits around the primary while transiting through the environment in the vicinity of the SMBH. In the strong gravity regime, the secondary completes about $\sim q^{-1}$ orbits around the primary, with the total duration of the inspiral ranging from weeks to decades. Such inspirals are expected to be one of the main sources of signal for the upcoming space-based gravitational-wave observatory LISA. I will show the possible observational manifestation of such system in the EM spectrum taking into account the repetitive interaction of the secondary with the accreting plasma.

      Speaker: Petra Suková (ASU CAS)
    • 2:30 PM 3:10 PM
      Final results of the first EHT observations of M87 - the end of the saga 40m


      Speaker: Maciek Wielgus (Black Hole Initiative, Harvard)
    • 3:10 PM 3:30 PM
      Accretion onto charged compact objects 20m

      Accretion on massive and rotating compact objects was studied using numerical simulations for many decades. In Einstein's General Relativity gravitational interactions of such objects are described by the Kerr metric. The so-called Kerr-Newman metric pose a generalization to the case of charged compact object. It describes the gravitational forces of (spherically symmetric) objects which carry electric charge.

      In the light of increasing interest in astrophysical properties of charged compact objects we decided to generalize the well-known GRMHD code called KORAL to the case of Kerr-Newman metric. We not only extended already available in the code coordinate systems to the case of the charged object, but we introduced new framework which simplifies inclusion of new metrics.

      During the talk we will motivate extension of accretion studies toward electrically charged objects. Preliminary results of magneto-hydrodynamical simulations in the background of Kerr-Newman metric will be presented.

      Speaker: Tomasz Krajewski (Nicolaus Copernicus Astronomical Center)
    • 3:30 PM 4:00 PM
      Coffee break 30m
    • 4:00 PM 4:20 PM
      Properties of low angular momentum trans-magneto-sonic accretion flows around rotating black holes 20m

      We present the global structure of trans-magnetosonic low angular momentum accretion flow around rotating black holes. Assuming ideal GRMHD conditions and relativistic equation of state (REoS), we solve the governing equations to obtain all possible smooth global accretion solutions. We analyze the dynamic and thermodynamic characteristics of accreting matter in terms of conserved flow parameters, i.e., energy, angular momentum, and magnetic flux. For a thin GRMHD flow, we observe that the toroidal component ($b^\phi$) of the magnetic fields generally dominates over the radial one ($b^r$) at the disk mid-plane even close to the BH. This suggests that the toroidal magnetic field plays a key role in regulating disk dynamics. Additionally, we have observed the development of Maxwell stress within the disk, which results in the transportation of angular momentum. To investigate this phenomenon further, we have calculated the viscosity parameter ($\alpha$), which appears to vary radially. Furthermore, we have analyzed the scaling relation between $\alpha$ and $\beta$ and have found that two accretion domains exist along the radial extent of the disk. Lastly, we discuss the usefulness of this formalism in the context of GRMHD simulation studies.

      Speaker: Samik Mitra (Indian Institute of Technology Guwahati)
    • 4:20 PM 4:40 PM
      Equilibrium tori orbiting Reissner-Nordström naked singularities 20m

      In general relativity the gravitational field of a charged, spherically symmetric, non-rotating body is described by the Reissner-Nordström metric. This metric reveals the existence of a naked singularity when the charge (Q) exceeds the mass (M). For all Reissner-Nordström naked singularities, there exists a zero gravity sphere where a test particle can remain at rest. Outside that sphere gravity is attractive, inside it gravity is repulsive. For large values of Q/M circular test-particle orbits are stable down to the zero-gravity sphere and their angular frequency has a maximum outside the zero-gravity radius. We construct polytropic tori with uniform values of specific angular momentum in the naked singularity regime of the Reissner-Nordström metric (Q/M>1).

      Speaker: Ruchi Mishra
    • 4:40 PM 5:00 PM
      Radiation of particles in the vicinity of the weakly charged Schwarzschild black hole 20m

      It is commonly known that charged particles can be accelerated to extremely high energy by supermassive black holes at the centers of galaxies. Black holes accelerate particles electromagnetically through an electric field that the source induces. Under such circumstances, the accelerated particles radiate electromagnetic waves, which cause the backreaction force to appear. We study the dynamics of radiating charged particles in the vicinity of a weakly charged Schwarzschild black hole. Starting with the description of a flat spacetime, we solve the Lorentz-Dirac equation in its simplified Landau-Lifshitz form. In curved spacetime, we use the DeWitt-Brehme equation and discuss the effect of the self-force, also known as the tail term.

      Speaker: Bakhtinur Juraev
    • 10:00 AM 10:20 AM
      High-energy radiation from jet-cluster interactions 20m

      The collision of stellar winds with astrophysical plasmas could lead to the formation of strong shocks where particles could be accelerated. In this work, we study the interaction between the wind of a young stellar cluster and the jet of an active galactic nucleus. We present our results for the maximum energies that particles could achieve in this scenario, and discuss the production of non-thermal signatures and the possibility of detecting them with current and future instruments.

      Speaker: Ana Laura Müller (Astronomical Institute of the Czech Academy of Sciences)
    • 10:20 AM 10:40 AM
      From Electromagnetic to Gravitational radiation in STVG 20m

      t is well-know that from the both theoretical and practical point of view nature of gravitational and electromagnetic field are totally different and
      even there is not interaction between these two fields. However, recent developments in the STVG theory challenge this notion.
      According to this theory, a black hole serves as a mediator, translating electromagnetic radiation into gravitational radiation.
      In this theory massive do not follow geodesic line unlike photons. Therefore two massive particles occurs in pair production from a
      photon can be accelerated by the black hole and produce gravitational radiation.

      Speaker: Bobur Turimov (Silesian University in Opava)
    • 10:40 AM 11:40 AM
      Coffee break 1h
    • 11:40 AM 12:20 PM
      Interaction of gravitational waves with the pulsar magnetosphere 40m

      he magnetosphere of a pulsar is one the most extreme places in the universe due to its properties,
      like magnetic fields, plasma composition, and several mechanisms to produce electromagnetic emission.
      New open questions lie in how gravitational waves (GWs) can influence the magnetospheres of compact stars,
      resulting in potentially observable changes in the electromagnetic signatures due to the Gertsenshtein-Zel'dovich effect.
      Though these, we emphasize the need for a generalized theoretical formalism when the GW polarisation state and direction of propagation are incident at any arbitrary
      orientation and, most notably, the detailed simulations to test it with a broader range of observations.
      We present our preliminary results of the simulated expected radio-band spectrum observed for a canonical pulsar
      with a small time-dependent addition to a large static, transverse magnetic field. We consider the gravitational radiation
      as the first-order perturbations in nearby spacetime. We also indicate how observation of the direct impact of gravitational waves
      on a neutron star's intrinsic spin is highly unlikely, highlighting magnetospheres as the promising probes of
      high-frequency gravitational wave interactions with compact objects.

      Speaker: Prof. Maurizio Falanga (International Space Science Institute)
    • 12:20 PM 12:40 PM
      Spectrum of the LMXB with NS and iron line modeling 20m

      The iron emission line is the most dominant emisson line observed in x-ray sources like Low Mass X-ray Binary (LMXB). Relativistic effects broaden the otherwise narrow spectrum profile. From the boundaries of the energy spectrum and the shape of the spectral profile one can extract valuable informations about the X-ray source such as inclination angle or inner edge radius of the accretion disk.

      Speaker: Rene Sprna
    • 2:10 PM 2:50 PM
      The sound of the event horizon 40m

      During the ringdown phase of a gravitational signal emitted by a black hole, the least damped quasinormal frequency dominates. If modifications to Einstein’s theory induce noticeable deformations of the black-hole geometry only near the event horizon, the fundamental mode remains largely unaffected. However, even a small change near the event horizon can significantly impact the first few overtones, providing a means to probe the geometry of the event horizon. Overtones are stable against small deformations of space-time at a distance from the black hole, allowing the event horizon to be distinguished from the surrounding environment. In contrast to echoes, overtones make a much larger energy contribution. These findings open up new avenues for future observations.

      Speaker: Roman Konoplya
    • 2:50 PM 3:10 PM
      Quasinormal ringing of Bardeen spacetime 20m

      We review recent calculations of quasinormal modes and asymptotic tails of the Bardeen spacetime interpreted as a quantum corrected Schwarzschild-like black holes. Massless electromagnetic and Dirac fields and massive scalar field are considered. The first few overtones are much more sensitive to the change of the quantum correction parameter reflecting the near-horizon deformation of the spacetime due to quantum corrections. While the asymptotic tails of massless fields are identical to those for the Schwarzschild case, the tails for a massive field differs from the Schwarzschild limit at both intermediate and asymptotic times.

      Speaker: Dmitriy Ovchinnikov (Silesian University in Opava)
    • 3:10 PM 3:30 PM
      Quasinormal spectrum in the asymptotically safe gravity 20m

      Three models of black holes depending on the choice for the identification of the cut-off parameter are known in the asymptotic safe gravity. Here we show that for all three models the same feature takes place: high sensitivity of overtones to small deformation of the near horizon zone due to quantum corrections.

      Speaker: Antonina Zinhailo (Institute of Physics, Silesian University in Opava)
    • 3:30 PM 4:10 PM
      Coffee break 40m
    • 4:10 PM 4:50 PM
      Primordial black holes in the Galaxy dark matter 40m

      If there are PBH in the dark matter of our Galaxy,
      they must collide with the Galactic neutron stars,
      turning them into light black holes, with masses
      smaller than that of the NS-BH mass gap. We argue
      that some consequences of this process may be
      directly observed and therefore the question
      whether PBH do exist may be definitely answered in
      a few years.

      Speaker: Marek Abramowicz (Silesian University, Opava)
    • 4:50 PM 5:10 PM
      Dynamical and observational properties of weakly Proca-charged black holes 20m

      The most straightforward approach to account a mass into the vector potential of electromagnetism involves modification of the Einstein-Maxwell action into the Einstein-Proca form. By employing perturbation theory, where both the Proca mass and the black hole’s charge serve as small parameters, it becomes possible to find an exact analytical solution. In this solution, the metric tensor remains unchanged, but the vector potential deviates from the Coulomb potential. Notably, even when the Proca mass lies within the range below the current experimental upper limit for photon mass, it contributes significantly to the dynamical equations. Further, we examine the motion of neutral and charged particles in the vicinity of a weakly charged Proca black hole. We also discuss the observational implications of the solution on the gravitational bending, black hole shadow, and fitting of the Galactic Center flare orbits.

      Speaker: Mr Abylaikhan Tlemissov (Silesian University in Opava)
    • 5:10 PM 5:30 PM
      Characterizing X-ray point sources in the Galactic center and disk using XMM-Newton observation 20m

      For many years, it has been claimed that the Galactic ridge X-ray emission is truly diffuse in nature. However, with the advance of modern X-ray satellites, it has been found that the diffuse emission is comprised of thousands of X-ray point sources. Further, many studies suggest that a vast majority of these X-ray point sources are magnetic cataclysmic variables (CVs) and active binaries. One unambiguous way to identify these magnetic CVs is by detecting their X-ray periodicity. Therefore, we systematically searched for X-ray periodic sources in the inner Galactic disk, including the Galactic Center region.

      We have used data from our ongoing XMM-Newton Heritage survey of the Galactic disk plus the XMM-Newton archival observation of the GC to search for periodic sources. We detected a sample of 27 periodic sources, of which more than 50% are intermediate polars and 22% are polars. Both the IP and polar type sources display strong iron K emission in the 6-7 keV band. The X-ray spectral studies suggest that the IP-type sources display relatively harder spectra compared to Polars. In this talk, I will briefly overview the X-ray emission from accreting white dwarfs and how to utilize the X-ray observation to identify these sources.

      Speaker: Dr Samaresh Mondal (Osservatorio Astronomico di Brera - INAF, via E. Bianchi 46, 23807 Merate (LC))
    • 10:00 AM 1:00 PM
      Individual discussions 3h