**ABSTRACTS**

**Author: ** David Abarca (CAMK)

**Title: ***GRRMHD simulations of super-Eddington accretion onto neutron stars *

We simulate accretion at super-critical rates onto magnetized and non-magnetized neutron stars to investigate the properties of matter flow in the vicinity of ULX pulsars. Our magnetohydrodynamic simulations are performed in full general relativity with self-consistent radiation treatment using the M1 closure scheme. The non-magnetized neutron star is modeled using a reflective inner boundary condition. We demonstrate that high rates of accretion alone onto anon-magnetized neutron star are insufficient to produce super-Eddington luminosity, although preliminary results show that the addition of a stellar magnetic field can bring the luminosity up to ten times the Eddington limit.

**Author: ** Marek Abramowicz (Silesian University in Opava; CAMK)

**A popular lecture for a general public: ***How to search for Alien Civilizations in the Galaxy? *

A technologically advanced civilization will signal their existence by a Messenger emitting gravitational radiation. The Messenger is an object of the Jupiter mass located on the innermost stable circular orbit around the SgrA*, the supermassive black hole at the center of the Galaxy.

**Author: ** Marek Abramowicz (Silesian University in Opava; CAMK)

**Title: ***Collisions of primordial black holes with neutron stars may explain the phenomenon of fast radio bursts*

If primordial black holes constitute a (small) fraction of the dark matter halloes of galaxies, their collisions with galactic neutron stars explain the puzzling fast radio burst (FRB) phenomenon, in particular the FRB high occurrence rate, large luminosities and short duration. In these collisions 1.5 M_Sun black holes are formed.

**Author: ** David Edwin Alvarez Castillo (JINR)

**Title: ***Neutron Star Equation of State after the GW170817 event*

In this talk I will review the method of estimation of tidal deformabilities of compact stars and present results for pure hadronic as well as hybrid stars that include the mass twins case. Then I will discuss the impact of the nuclear symmetry energy in the determination of the compact star radius.In particular, the recent detection of gravitational radiation from the GW170817 event shed light on the properties of the neutron star equation of state (EoS), thus comprising both the study of the symmetry energy and stellar radius. Furthermore, I shall address the question of the possibility of a universal symmetry energy contribution to the neutron star equation of state under restricted Direct Urca cooling. When these two aspects are combined, powerful predictions for the stiffness of the neutron star EoS are obtained. Furthermore, I will focus on the case of mass twin compact stars, hybrid compact stars with approximately the same masses but different radii. To qualify the above, I will show a recent developed EoS that features of a color superconducting chiral quark model with nonlocal, covariant interactions bearing density dependent vector meson coupling and a density-dependent bag pressure. This model allows for a scenario where the compact stars of the GW170817 event are either both hadronic, both hybrid, or simultaneously hadronic and hybrid configurations.

**Author: ** Anabella Araudo (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Acceleration of Ultra High Energy Cosmic Rays by relativistic plasmas. The case of AGN jets*

IIt has been suggested that relativistic shocks in extragalactic jets may accelerate the highest energy cosmic rays, with energies of about 100 EeV. The maximum energy to which particles can be accelerated via a diffusive mechanism depends on the magnetic turbulence near the shock but recent theoretical advances indicate that relativistic shocks are probably unable to accelerate particles to energies much larger than a PeV. I will discuss the main reasons that make relativistic plasmas unable to accelerate Ultra High Energy Cosmic Rays (UHECRs). I will focus on relativistic jets in Active Galactic Nuclei (AGN), where the cut-off of the synchrotron spectrum in the hotspots of radiogalaxies indicates that the maximum energy of non-thermal electrons accelerated at the jet termination shock is ~ 1 TeV for a canonical magnetic field of 100 micro Gauss. We show that this maximum energy of electrons cannot be constrained by synchrotron losses as usually assumed. The maximum energy is ultimately determined by the ability to scatter particles downstream of the shock, and this limit applies to both electrons and protons. Therefore, the maximum energy of protons is also ~ 1 TeV, indicating that the termination shocks in AGN jets are very poor UHECR accelerators.

**Author: ** Pavel Bakala (Silesian University in Opava)

**Co-authors: **Debora Lančová, Kateřina Goluchová, Gabriel Török, Luigi Stella, Maurizio Falanga, Vittorio De Falco, Alessandra de Rosa

**Title: ***Thin accretion disc in strong gravity influenced by interaction with radiation field*

Accretion structures in the vicinity of neutron stars radiate a sizable fraction of their Eddington luminosity are significantly influenced by the radiation emitted from the surface of stars and from the boundary layer. Apart from the radiation pressure, the accreted matter is also affected by the Poynting-Robertson effect, which causes angular momentum loss and therefore acts as an additional source of viscosity in the disk. Using numerical simulations, we studied the influence of the Poynting-Robertson drag on the thin accretion disks in close vicinity of a neutron star. We adopted fully relativistic descriptipn of interaction of disc matter with the radiation field. In the parallelized simulation code, we implement the complete general relativistic description of the Poynting-Robertson effect including the influence of density redistribution on the optical depth of the disk. The modelled motion of matter in the disk thus results from a complex interplay of strong gravitational field, the Poynting –Robertson effect, radiation pressure and disk viscosity. We found that two different areas arise in the disc - the inner area driven by the interaction with the radiation field and the outer one which keeps the quasi-Keplerian disc regime.

**Author: ** Dilshodbek Bardiev (Silesian University in Opava)

**Co-authors: **Bahtinur Juraev, Martin Kološ

**Title: ***Charged particle motion around Schwarzschild black hole with split monopole magnetosphere*

We study charge particle dynamics around Schwarzschild black hole with split monopole magnetic field. Charged particle trajectories, position of circular orbits in the equatorial and off the equatorial plane and also frequencies of small harmonic oscillations will be presented. (Tuto prezentaci prosím zařadit do studentské sekce, pokud nic takového nebude, tak někam, kde nebude moc posuchačů - jedná se o prezentaci výsledků studentů z výměného programu Erasmus+.)

**Author: ** Martin Blaschke (Silesian University in Opava)

**Title: ***Classical corrections to black hole entropy in $d$ dimensions: a rear window to quantum gravity?*

We provide a simple derivation of the corrections for Schwarzschild and Schwarzschild-Tangherlini black hole entropy without knowing the details of quantum gravity. We will follow Bekenstein, Wheeler and Jaynes ideas, using summations techniques without calculus approximations, to directly find logarithmic corrections to well-known entropy formula for black holes. Our approach is free from pathological behaviour giving negative entropy for small values of black hole mass $M$. With the aid of "Universality" principle we will argue that this purely classical approach could open a window for exploring properties of quantum gravity.

**Author: ** Deepika Ananda Bollimpalli (CAMK)

**Title: ***A new way to measure the neutron star parameters from atmospheric oscillations*

Neutron stars are excellent astrophysical laboratories for studying matter at very high nuclear densities. To understand the equation of state (EoS) of such dense material, it is very important to know the mass and radius of the neutron star. There is plenty of observational evidence that some neutron star systems reach super-Eddington luminosities, either through the accretion of matter onto the stellar surface or by thermonuclear burning. Stars with such high luminosities are shown to harbor levitating atmospheres, supported at a certain height above the stellar surface by the radiation pressure from the star. We study the oscillations of these radiation-supported atmospheres and find a family of relativistic eigenmodes and eigenfrequencies of the radial oscillations. We find that damping due to radiation drag limits the frequency of these oscillations. Computation of this maximum frequency and how the frequency encodes information about the mass and radius of the neutron star will be discussed.

**Author: ** Michal Bursa (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Relations between slim and thin disks at low mass accretion rates*

Slim disks are a generalization of the thin disk solution to black hole disk accretion equations, but does the slim disk solutions match the thin disk ones at low mass accretion rates?

**Author: ** Michal Dovčiak (Astronomical Institute of the Czech Academy of Sciences)

**Co-authors: **Romana Mikušincová, Jiří Svoboda

**Title: ***Accreting stellar-mass black holes via X-ray polarimetry*

In this talk we present polarimetric simulations of an X-ray binary system GRS 1915+105 and study polarization of binary systems in the soft/high state with the aim to measure the black hole spin and inclination of the system. For this purpose, we use existing models for simulating the polarimetry-mission data and fit these in XSPEC in order to find out how well the system properties are recovered and constrained.

**Author: ** Sudipta Hensh (Silesian University in Opava)

**Title: ***Evolution of Rotating Braneworld Naked Singularities*

Naked singularities come as a solution to Einstein's field equations. Cosmic censorship hypothesis is not proved yet within the framework of general relativity. Therefore, naked singularities cannot be rejected as nonphysical. We study the simple accretion model of the Keplerian thin disk onto a naked singularity described by rotating braneworld metric. Our aim is to investigate the final fate of naked singularities due to matter accreting from the accretion disks. We show that the naked singularities are eventually converting into the extreme back hole as a result of swallowing matter.

**Author: ** Jiří Horák (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***On the disk accretion onto a relativistic radiating star*

Thin accretion disks around radiating nonrotating relativistic stars are revisited. The attention is paid to both energetic and dynamical effects of the radiation on the disk structure. In addition to turbulent angular momentum transport, the radiation drag helps to remove the angular momentum from the matter and changes the structure of the disk.

**Author: ** Vladimír Karas (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Approaches to the mass estimation in supermassive black holes*

Mass is the primary parameter defining the nature of any astronomical body. Astrophysical black holes do not produce detectable radiation of their own (like stars), nor they reflect radiation (like planets). Still, properties of the electromagnetic signal originating from other sources in the vicinity of black hole bear imprints of its presence and can be employed to measure the black hole mass. This is mainly thanks to the influence of strong gravity of the black hole on photons and light rays. In this lecture we will summarize several proven and some promising approaches to the black-hole mass measurements. We focus on

Super-Massive Black Holes (SMBH) that reside in galactic nuclei, nevertheless, the mass-scaling property allows some of these methods to be adapted also for stellar-mass black holes that are found in binary stars. Finally, we propose that an interplay of relativistic effects could be responsible for the shape of the observed light curves and discuss the reliability of the method based on the bright X-ray flares.

**Author: ** Wlodek Kluźniak (CAMK)

**Title: ***Instabilities and oscillations of accretion disks in global GRRMHD simulations*

Inclusion of radiation in GRMHD simulations has allowed us to re-examine the question of stability of black hole accretion disks. Both the thermal instability and the viscous instability are apparent in our global simulations of thin accretion disks in the radiation-pressure dominated regime. I will also present results on the oscillatory behaviour of accretion disks and tori in general relativistic hydrodynamic simulations, which may be relevant to the high-frequency QPOs observed in accreting black holes.

[References:* Mishra et al. 2017, Fragile et al. 3018, Mishra et al. 2018*]

**Author: ** Ondřej Kopáček (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Emergence of magnetic null points in electro-vacuum magnetospheres of compact objects*

Relativistic effects of compact objects onto electromagnetic fields in their vicinity are investigated using the test-field approximation. In particular, we study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma and may lead to violent mass ejection, we show here that strong gravitation of the black hole or neutron star may actively support the process by suitably entangling the field-lines even in the electro-vacuum description.

**Author: ** Debora Lančová (Silesian University in Opava)

**Title: ***GRMHD simulation of thin accretion disc stabilized by magnetic field*

This year we started our collaboration with prof. Marek Abramowicz, Dr. Maciek Wielgus and others on a simulation of a stable geometrically thin accretion disc in vicinity of a black hole. We are using GRMHD code Koral created by A. Sadowski. The starting point for our simulations is a solution already published in paper Sadowski, 2016: Thin accretion discs are stabilized by a strong magnetic field. The thin disc is spontaneously formed from initially thick accretion torus located far from the black hole. The thin disc is stabilized by magnetic field even with accretion rate Mdot = 0.8 MEdd. We plan to perform a deep analysis of this solution, including discussion of stresses on the inner edge, the radial profile of the effective viscous parameter 𝛼𝑒𝑓𝑓, properties of the disc with different values of 𝑀acc, non-zero spin and others following topics.

**Author: ** Georgios Lukes-Gerakopoulos (Astronomical Institute of the Czech Academy of Sciences)

**Co-authors: **Giovanni Acquaviva, Charalampos Markakis

**Title: ***Probing Dark Energy through perfect fluid thermodynamics*

In this talk, the issue of dark energy will be addressed by employing perfect fluid thermodynamics describing baryonic matter. In this discussion we will keep the equation of state quite general, we will just demand that the speed of sound is positive and less than the speed of light. The spacetime background will not be specified either, we will just demand that the Universe is expanding. In this framework, we will discuss some propositions by looking at the asymptotic behaviors of the fluid.

**Author: ** John Miller (University of Oxford; SISSA)

**Title: ***Gravitational wave observations of binary neutron star mergers: some further things we hope to see in the future*

Enormous excitement has been stirred up by the LIGO-Virgo observations of GW170817, associated with a rather close binary neutron star merger which also produced a short gamma-ray burst. However, this is just the start! In this talk I will present a personal view of things to hope for from future observations when sensitivities will be enhanced even further and higher frequencies will be observable.

**Author: ** John Miller (University of Oxford; SISSA)

**Title: ***Neutron star assassins?*

The wild suggestion that some or all of the dark matter might be in the form of tiny black holes produced in the early universe, is not quite dead. A proposal for finally ruling it out comes from the possibility that if they did exist, then they would destroy old neutron stars so that only quite young ones would remain, seemingly contrary to observations. I will give a brief introduction to the ideas involved with this, which have some surprising aspects.

**Author: ** Jorge Ovalle (Silesian University in Opava)

**Title: ***Gravitational Decoupling: generating black hole solutions*

We explain in detail the gravitational decoupling approach (MGD-decoupling), which is being widely used to solve Einstein's field equations. As a direct application, we show how to generate hairy black hole solutions.

**Author: ** Radim Pánis (Silesian University in Opava)

**Co-authors: **Martin Kološ, Zdeněk Stuchlík

**Title: ***Detection of chaotic behavior in time series generated by charged particle motion around magnetized black hole*

Time series, generated by nonlinear particle motion around magnetized black hole, has been studied using standard and machine learning

method for chaos detection. Test particles constituting neutral Kepplerian accretion disc around magnetized black hole can get easily ionized and will start to feel external uniform magnetic field. New type of motion for now charged particle will be possible, ionized particles can leave the circular orbits located in accretion disc infinitesimally thin plane and this ionization proces could leads to the disk thickening or even complete disk destruction. Detection of charged particle trajectory chaoticity can be used in determination of accretion disc fate. The studied methods for chaotic behaviour detection has been used on machine generated data given by particle trajectory, but such methods can be latter used on real observed timing data from various astronomical sources.

**Author: ** Camilo Posada (Silesian University in Opava)

**Title: ***Slowly rotating ultracompact Schwarzschild star*

The Schwarzschild interior solution, or Schwarzschild star, which describes a spheri-cally symmetric homogeneous mass with a constant energy density, shows a divergence in pressure when the radius of the star R = (9=4)M. Recently, Mazur and Mottola showed that this divergence is integrable, inducing non-isotropic transverse stresses on a surface of some radius R0. When this radius approaches the Schwarzschild radius, the interior solution becomes one of negative pressure evoking a de Sitter spacetime. This gravita-tional condensate star, or gravastar, is an alternative solution to the idea of a black hole as the nal state of gravitational collapse. Using Hartles model to calculate equilibrium congurations of slowly rotating masses, we report results of surface and integral proper-ties for a slowly rotating Schwarzschild star in the long-ignored region RS < R < (9=4)M. We found that in the gravastar limit, the angular velocity of the uid relative to the local inertial frame tends to zero, indicating rigid rotation. Remarkably, the moment of inertia I and the mass quadrupole moment Q approach the corresponding values for the Kerr metric to second order in the angular velocity. These results provide a solution to the problem of the source of a slowly rotating Kerr black hole.

**Author: ** Jiří Svoboda (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Unification of black hole across the mass scale*

Astrophysical black holes exist with a wide range of masses, from a few to ten billion times the mass of the Sun. If “fed” by surrounding gas, they become the most efficient X-ray emitters in the Universe. The gas can be accreted from a nearby star (as in stellar-mass X-ray binaries, XRB), or from the material in the nucleus of external galaxies (as in the active galactic nuclei, AGN). The largely different mass between XRB and AGN implies the different size- and time-scales. While XRBs evolve rapidly and change their accretion mode, most AGNs remain in the same state over periods of decades. Study of AGN spectral states is still possible with a large sample of multi-wavelength observations. In this talk, I will review our knowledge about AGN spectral states and their comparison to XRBs. Investigation of what is inherently different between AGN and XRB and what can be scaled up (down) will allow us to use the knowledge of one black-hole class to the other. Vice versa, understanding the differences could help us to explain yet-open problems in AGN and XRB physics.

**Author: ** Arman Tursunov (Silesian University in Opava)

**Co-authors: **Martin Kološ, Zdeněk Stuchlík, Naresh Dadhich, Bobomurat Ahmedov

**Title: ***Ultra-high-energy cosmic rays from supermassive black holes*

Recent neutrino and multimessenger observations strongly established the existence of ultra-high-energy cosmic rays (UHECRs) of energies >EeV, as well as they also indicate their source being extragalactic supermassive black hole (SMBH). Note that this energy range is clearly beyond the GZK-cutoff limit, which points to exotic nature of the phenomena. We have therefore to seek an efficient mechanism of energy extraction from SMBHs. Novel super-efficient regime of the magnetic Penrose process (MPP) could indeed foot the bill. The driving force of the mechanism is in the presence of a stable charge of a black hole, discharge of which by accreting oppositely charged matter converts the mechanical rotational energy of a black hole into extractable electromagnetic energy. We consider a neutron beta-decay skirting very close to horizon. It then turns out that resulting protons can be accelerated to energies above 10^20eV for SMBH of a billion solar masses and magnetic field of 10^4G. This clearly makes the super-efficient regime of MPP the most promising mechanism for fuelling the UHECRs powerhouse.

**Author: **Jaroslav Vrba (Silesian University in Opava)

**Title: ***Trapped null-geodesics in slowly rotating space-time*

The studying of an extremely compact object is a very interesting part of astrophysics. The extremely compact object satisfy R<3GM/c2 and in our work, it is also a homogeneous sphere. Null-geodesics can provide much information about their interior and their properties because gravitational waves, photons and also neutrinos move along them. We focus on trapping of null-geodesics in the framework of linearized Hartle-Thorne space-time in contrast to the non-rotating case. It is not surprising that the rotation breaks down the symmetry of trapping areas. The profile of effective potential changes due to the rotation: locations of the maximum and the minimum are shifted depending on the direction of null-geodesics (co-rotating or counter-rotating). We present effective potentials, escape cones of null-geodesics and the local and the global trapping coefficients. It is impossible to find trapped null-geodesics in objects with R>3GM/c2, however, it is not true for rotating one, where trapped areas can emerge even for R>3GM/c2, but only for

counter-rotating null-geodesics.

**Author: **Vojtěch Witzany (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Hamiltonian formalism for spinning bodies in curved space-time*

The first correction due to the finite size of a small, relativistically rotating body in curved space-time is the so-called spin-curvature coupling as embodied in the Mathisson-Papapetrou-Dixon equations. In this talk, I will present our results on the Hamiltonian formalism for these equations. In particular, I will focus on the motivation and interpretation of the Poisson structure of the problem.

[References:* Witzany et al. 2018*]

**Author: ** Ondřej Zelenka (Astronomical Institute of the Czech Academy of Sciences)

**Co-authors: **Georgios Lukes-Gerakopoulos

**Title: ***Recurrence analysis of spinning particles in the Schwarzschild background*

The Mathisson-Papapetrou-Dixon (MPD) equations are describing the motion of a spinning test particle in a curved spacetime. When the spin is set to zero, the MPD equations reduce to geodesic equations. In the case of the Schwarzschild spacetime the geodesic motion corresponds to an integrable system, but when the spin of the particle is taken into account, the degrees of freedom of the system increase making the system non-integrable. To study the dynamics of the MPD equations in the Schwarzschild case, we employ the recurrence analysis method, which allows to discern chaos from order. The astrophysical motivation behind such study is the fact that the motion of a spinning particle in the Schwarzschild background approximates an Extreme Mass Ratio system, i.e. a system in which a stellar compact object orbits around a supermassive black hole.

**Author: ** Wenda Zhang (Astronomical Institute of the Czech Academy of Sciences)

**Title: ***Constraint on AGN corona size with fully relativistic modeling of 3D corona*

The ``lamp-post'' scenario was usually used to describe AGN corona, in which the corona is assumed to be an infinitesimal point source on the symmetry axis. In some AGNs, especially NLS1s, the broad iron line profile requires extremely compact corona. On the other hand, the corona must be large enough to intercept enough seed photons to be able to produce the observed X-ray continuum and reflection spectrum. One attempt to constrain AGN corona size was made by Dovciak & Done 2016, in which the corona size was estimated by conservation of photons during Comptonization process. They found out that for one NLS1 1H 0707-495, the size of the corona must be substantially larger than 1 GM/c^2. In that work, the Comptonized spectrum was evaluated assuming lamp-post geometry and the corona size was estimated under the assumption that the emission of the corona is homogeneous. To calculate the spectrum of 3D corona (i.e., the corona is extended and with finite height) self-consistently, we perform a fully relativistic Monte Carlo calculation of energy and polarization spectrum of AGNs of disk-corona geometry, taking into account propagation of null geodesic in Kerr spacetime and Compton scattering process.