# RAGtime 23

Europe/Prague
Opava

Hauerova 728
,
Description

## Venue

This year, RAGtime will once again be held in person at Hauerova 728/4, 746 01 Opava. You will be provided with the building entrance cards upon registration.

The 23nd RAGtime workshop will be held in Opava (6-10 September 2021) in hybrid form. Please, follow this website for further informations.

All participants on site must present a proof of vaccination or negative test to the organisers upon arrival and registration to the conference!

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 window 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 the national and international collaboration regarding the present and future cosmic X-ray missions.

## Proceedings of RAGtime

The Proceedings have been regularly published by the Institute of Physics at the  Silesian University in Opava. The next Proceedings will be published by the end of 2023. You can submit your paper following the instructions at the Proceedings website. The deadline for submission is November 30, 2021.

Participants
• Ana Laura Müller
• Anabella Araudo
• Andrea Kotrlová
• Antonina Zinhailo
• Arman Tursunov
• Audrey Trova
• Bakhtinur Juraev
• Bobir Toshmatov
• Daniel Charbulák
• Daniela Pugliese
• David Abarca
• David Alvarez-Castillo
• Debora Lančová
• Deepika Bollimpalli
• Diego Calderón
• Dilshodbek Bardiev
• Dmitriy Ovchinnikov
• Dominik Gronkiewicz
• Ernesto Contreras
• Eva Šrámková
• Evariste Boj
• Fatemeh Kayanikhoo
• Filip Blaschke
• Gabriel Török
• Gabriela Urbancová
• Gregory Vereshchagin
• Guillermo Lara
• Hana Kučáková
• Ioannis Dimitropoulos
• Jan Schee
• Jana Malkrabová
• Jaroslav Vrba
• Javlon Rayimbaev
• Jiří Horák
• Jiří Kovář
• John Miller
• Jorge Ovalle
• Jorge Rueda
• Kateřina Klimovičová
• Kris Schroven
• Lenka Polášková
• Lenka Vozárová
• Luigi Stella
• Lukáš Gráf
• Maciek Wielgus
• Marek Abramowicz
• Martin Kološ
• Martin Urbanec
• Maurizio Falanga
• Michal Bursa
• Michal Zajaček
• Monika Matuszková
• Ondřej Kopáček
• Ondřej Nicolas Karpíšek
• Ozodbek Rakhimov
• Pavlína Jalůvková
• Peter Lichard
• Petr Slaný
• Raj Prince
• Remo Ruffini
• René Šprňa
• Riccardo La Placa
• Ronaldo Vieira
• Ruchi Mishra
• Sajal Mukherjee
• Samaresh Mondal
• Santu Mondal
• Stanislav Hledík
• Sudipta Hensh
• Tayebeh Tahamtan
• Thomas Pappas
• Tomáš Lanča
• Tomáš Stanovský
• Wenda Zhang
• Wlodek Kluzniak
• Zdeněk Stuchlík
• Zuzana Turoňová
• Özlem Taşpınar
Contact
• Monday, 6 September
• 12:00 14:00
Registration 2h
• 14:00 14:15
Conference Opening 15m

Prof. Zdeněk Stuchlík

• 14:15 15:00
Penrose process: its variants and astrophysical applications 45m

The Penrose process governing extraction of energy from black holes by the simple effects related to uncharged or charged particles is discussed in most of the known variants related to Kerr black holes and Kerr naked singularities, or to electrically charged static black holes. Attention is focused on the magnetic Penrose process in its extremely efficient version enabling extreme acceleration of charged particles after ionization of uncharged matter, giving explanation of the ultra-high energy cosmic rays. The effect of radiative self-force of accelerated particles is briefly discussed. It is shown that in the ergosphere of Kerr black holes it could lead to a new verion of radiative Penrose process due to radiation of photons with negative energy as related to distant static observers that cause the extraction of black hole rotational energy in this version of the Penrose process.

Speaker: Prof. Zdeněk Stuchlík (Institute of Physics, Silesian University in Opava)
• 15:00 15:25
Gravitational Waves - Theory and Observations
• 15:00
Detecting gravitational waves from hyperbolic encounters 25m

In this talk, I will discuss the possibilities of detecting
gravitational waves (GW) from hyperbolic encounters within the scope of
the current and future interferometric detectors. Given that the globular
clusters are most favorable to host these events, we study the scattering
events in these dense clusters, and discuss event rates for observing such
encounters. By incorporating the localized effects appearing inside a
cluster, and using realistic cluster models, we argue that our estimations
are on the conservative side, while the actual detection rates may surpass
these average rates. I will discuss these results in detail, and point out
its implication in the GW astronomy and understanding cluster's
properties.

Speaker: Sajal Mukherjee (Astronomical Institute of Czech Academy of Sciences)
• 15:25 15:55
Coffee Break 30m
• 15:55 17:10
Spacetime Solutions for Compact Objects
• 15:55
General parametrization of wormhole spacetimes 25m

The general parametrization for spacetimes of spherically symmetric Lorentzian, traversable wormholes in an arbitrary metric theory of gravity is presented. The parametrization is similar in spirit to the post-Newtonian parametrized formalism, but with validity that extends beyond the weak field region and covers the whole space. Our method is based on a continued-fraction expansion in terms of a compactified radial coordinate. Calculations of shadows and quasinormal modes for various examples of parametrization of known wormhole metrics that we have performed show that, for most cases, the parametrization provides excellent accuracy already at the first order. Therefore, only a few parameters are dominant and important for finding potentially observable quantities in a wormhole background. We have also extended the analysis to the regime of slow rotation.

Speaker: Thomas Pappas (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava)
• 16:20
Gravitationally coupled Electroweak Monopole 25m

We investigate the Cho-Maison magnetic monopole in the presence of gravity. Unlike 't Hooft-Polyakov monopole, this hypothetical particle is a topological offspring of the ElectroWeak theory with a mass estimated around 5-7 TeV. Our goal is to study solutions within a large family of extended Electrowek models coupled minimally or non-minimally to gravity and see what kind of space-time emerges and what are the limitations on the monopole's mass. We focus on exact solutions in the BPS limit and on a novel notion of duality.

Speaker: Filip Blaschke (Silesian University in Opava)
• 17:10 18:10
Individual Meetings, Discussions, Work on Common Projects
• Tuesday, 7 September
• 10:00 10:25
Extreme Environments of AGNs and SMBHs
• 10:00
Non-isotropic structure of the Galactic center S cluster: Implications for its origin 25m

The innermost part of the Nuclear Star Cluster is known as the S cluster that consists mostly of young B-type stars. The origin of these young stars at the scale of 1000 gravitational radii from Sgr A has remained a puzzle. The brightest of those stars have been crucial for determining the mass of Sgr A as well as for testing gravitational theories in the strong-field limit. Recently, Ali et al. (2020) presented the two-disk structure of the S cluster, which is in contrast with its usually assumed isotropic distribution. In the follow-up study, the authors present a four-disk structure in the innermost parts of the Galactic center. Here we review the fundamental dynamics of the S cluster and in particular we analyze the implications of its multi-disk nature for its origin and evolution.

Speaker: Michal Zajaček (Center for Theoretical Physics, Warsaw, Poland)
• 10:25 10:40
Freeman Dyson on Quantum Gravity 15m

Freeman Dyson made these points on Quantum Gravity:
(1.) A single graviton cannot be detected, because a measuring
two-body apparatus size should be smaller that its gravitational radius.
(2.) With no possibility of detecting a single graviton the
very concept of Quantum Gravity is practically meaningless.
(3.) There is nothing wrong with having separate theories
for the large scale (gravity) and for the small scale (quanta). A
unification may be in practice (and in principle) unhelpful or
even meaningless.

Speaker: Marek Abramowicz (Silesian University, Opava)
• 10:40 11:10
Coffee Break 30m
• 11:10 12:20
Extreme Environments of AGNs and SMBHs
• 11:10
Non-thermal emission from Broad Line Region clouds 25m

The unified model of active galactic nuclei includes clouds orbiting closely to the central supermassive black hole with very high velocities. These clouds form the so-called broad-line region, which coexists with the accretion disk. Given the evidence of the infall motion of the clouds, it is expected that they collide with the disk. This scenario might lead to the production of shock waves where particles can be accelerated up to relativistic energies. In our work, we use the Failed Radiatively Accelerated Dusty Outflow (FRADO) model to describe realistically the dynamics of the broad-line region and calculate the expected non-thermal radiation. We found that the impacts of the clouds can produce significant hard X-ray and gamma emission, which could be used to distinguish between BLR models.

Speaker: Ana Laura Müller
• 11:35
Sgr A* flares, an electromagnetic signal and more 20m

Sagittarius A is a super massive black hole in the center of our galaxy observed across the electromagnetic spectrum for decades. Moreover, it is the primary target of the Event Horizon Telescope (EHT) and the GRAVITY collaboration, which observe in the 230 GHz (EHT) and the NIR (GRAVITY). The compact radio emission is expected to be thermal radiation from the vicinity of the black hole. The NIR emission exhibits strong variability and flares which are observed from the Galactic center every 6-8 hours. Flaring activity has been attributed to re-connection events close to the Black hole. We perform general-relativistic magneto-hydrodynamic to model the accretion flow of SgrA, and employ thermal emission models to reproduce the emission at 230 GHz, analyzing the regions close to the black hole that this emission is generated. Furthermore, we monitor the magnetic re-connection events in the simulation and model the non-thermal emission at the NIR. Our investigation focuses on the production of flares from Sgr A* observed in the NIR from GRAVITY, our aim is to reproduce a model that calculates lightcurves at these frequencies.

Speaker: Mr Ioannis Dimitropoulos (Kapodistrian university of Athens)
• 11:55
On the maximum energy of particles in the hotspots of radiogalaxy 3C 445 25m

We examine limits to the energy to which cosmic rays can be accelerated by the
termination shocks in the southern jet of radiogalaxy 3C 445. At high radio frequencies, the southern hotspot shows two sub structures named SE and SW.
By using the observed flux density at 22 GHz in SE and SW, we find that a
hotspot magnetic field of about 500 micro Gauss is required to explain the synchrotron emission. The upper-energy cut-off in the hotspot synchrotron spectrum is at a frequency < 5x10^15 Hz, indicating that the maximum energy of non-thermal electrons accelerated at the jet reverse shock is ~ 1 TeV in a magnetic field of 500 micro Gauss.
Based on the condition that the magnetic-turbulence scalelength has to be larger than the plasma skin depth, and that the energy density in non-thermal particles cannot violate the limit imposed by the jet kinetic luminosity, we show that the electrons maximum energy cannot be constrained by synchrotron losses as traditionally assumed. In addition to that, and assuming that the shock is quasi-perpendicular, we show that non-resonant hybrid instabilities generated by the streaming of cosmic rays with energy
~ 1 TeV can grow fast enough to amplify the jet magnetic field up to 500 micro Gauss and accelerate particles up to the maximum energy observed in 3C445 SE and SW.
This result has important implications for the acceleration of ultra high energy cosmic rays.

Speaker: Anabella Araudo (FZU - Institute of Physics of the Czech Academy of Sciences & Astronomical Institute of the Czech Academy of Sciences)
• 14:00 15:10
Extreme Environments of AGNs and SMBHs
• 14:00
Active Galactic Nuclei as Relativistic Systems 45m

Astrophysical black holes in nuclei of galaxies are of indisputable relevance to the current research. These extreme objects are of immense interest to pure relativists as well as observational astronomers. We will present an overview of selected aspects of physical processes occurring in the inner regions of Active Galactic Nuclei (AGN). Observational evidence strongly suggests that strong gravitational fields play a significant role in governing the energy output of AGN and their influence on the surrounding medium. In order to reduce an unnecessary overlap with numerous reviews and textbooks on the similar subject, we focus on just a few topics, such as the launching scenarios of nuclear outflows and jets that start and pre-collimate in the immediate vicinity of black holes. We will discuss the interaction between magnetic and gravitational fields in the strong-gravity regime of General Relativity.

• 14:45
Stellar winds pump the heart of the Milky Way 25m

The central super-massive black hole of the Milky Way, Sgr A, accretes at a very low rate making it a very underluminous galactic nucleus. Despite the tens of Wolf-Rayet stars present within the inner parsec supplying $\sim10^{−3}\rm M_{\odot}\ yr^{-1}$ in stellar winds, only a negligible fraction of this material ($<10^{-4}$) ends up being accreted onto Sgr A. The recent discovery of cold gas ($\sim 10^4\rm\ K$) in its vicinity raised questions about how such material could settle in the hostile ($\sim10^7\rm\ K$) environment near Sgr A. In this work we show that the system of mass-losing stars blowing winds can naturally account for both the hot, inefficient accretion flow, as well as the formation of a cold disk-like structure. We run hydrodynamical simulations using the grid-based code RAMSES starting as early in the past as possible to observe the state of the system at the present time. Our results show that the system reaches a quasi-steady state in about $\sim500\rm\ yr$ with material being captured at a rate of $\sim10^{−6}\rm\ M_{\odot}\ yr^{-1}$ at scales of $\sim10^{-4}$ pc , consistent with the observations and previous models. However, on longer timescales ( $>3000$ yr ) the material accumulates close to the black hole in the form of a disk. Considering the duration of the Wolf-Rayet phase ( $\sim10^{5}$ yr ), we conclude that this scenario likely has already happened, and could be responsible for the more active past of Sgr A, and/or its current outflow. We argue that the hypothesis of the mass-losing stars being the main regulator of the activity of the black hole deserves further consideration.

Speaker: Dr Diego Calderón (Charles University)
• 15:10 15:40
Coffee Break 30m
• 15:40 16:50
Black Hole Imaging
• 15:40
Inverse problem in black hole imaging 20m Online

#### Online

Recent observations by the Event Horizon Telescope of the image of the supermassive Black Hole at the center of M87 and SagA* provide a window into the strong-field regime of gravity. Several authors have studied the constraints that black hole shadow measurements cast on specific gravitational solutions or on the coefficients of more general parametrized frameworks. When multiple parameters are present, these constraints are usually degenerate. In this work, we tackle the inverse problem of gravitational imaging, i.e. we aim to reconstruct the metric and accretion disk profile from the image seen by a far-away observer. We allow the metric and disk profile to be described by an arbitrary frame or basis in functional space. By performing a Fisher Matrix analysis, and identifying the principal components of the covariance matrix, we extract the most prominent features in the metric and disk profile that produce the image. We test this framework in a simplified model by injecting mock deviations of the Schwarzchild metric surrounded by a spherical accretion disk. Since the problem is highly degenerate, we are able to reconstruct the metric deviations and disk profile provided that we use strong enough theoretical priors on the metric and disk profile.

Speaker: Mr Guillermo Lara (SISSA)
• 16:00
Current and future tests of gravity with black hole shadows 45m Online

#### Online

Speaker: Maciek Wielgus (Black Hole Initiative, Harvard)
• 16:50 17:50
Individual Meetings, Discussions, Work on Common Projects
• Wednesday, 8 September
• 10:00 11:10
Neutron Stars
• 10:00
Braking of pulsars with a time-dependent moment of inertia 25m

We investigate the impact of the time-dependent moment of inertia on the braking index of pulsars. We assume the moment of inertia to be slowly varying in time and we explore how much must the moment of inertia change to explain the currently measured values of the braking index.

Speaker: Martin Urbanec
• 10:25
Binary neutron star coalescence - after the merger 45m Online

#### Online

I will discuss developments with this

Speaker: Prof. John Miller (University of Oxford / SISSA, Trieste)
• 11:10 11:30
Coffee Break 20m
• 11:30 12:35
Neutron Stars
• 11:30
The neutron star equation of state under new constraints 25m

Recent constraints from neutron star measurements and nuclear experiments have narrowed the allowed physical regions for the compact stars equation of state (EoS) resulting in some of the well established models being less probable than before. These new constraints include the NICER mass-radius measurement of the object PSR J0740+6620, whereas laboratory experiments like Spectral Pion Ratio by Radioactive Ion Beams (SπRIT Collaboration) and parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons from $^{208}Pb$ (PREX collaboration) have reported values of the slope of the nuclear symmetry energy at saturation, a quantity strongly correlated to the neutron star radius. In this talk I will review mostly EoS models of hadronic and hybrid stars, and will present the results from a Bayesian Analysis that includes the state-of-the-art astrophysical constraints in order to select the most probable EoS parameters.

Speaker: David Alvarez-Castillo (Institute of Nuclear Physics PAS)
• 11:55
Simulations of levitating atmospheres using KORAL 20m

Some accreting neutron stars may produce near-Eddington luminosity. In general relativity, the atmosphere of a sufficiently luminous neutron star is disconnected from and levitates above its surface. The atmosphere is centered at a radius where the gravitational and radiation forces balance each other. On the other hand, in a number of space-time metrics in the case of naked singularities, there is a sphere within which gravity is repulsive. So any accreting matter does not reach the singularity and hovers over it. A levitating atmosphere can also be formed here entirely supported by gravity. We aim to simulate both these atmospheres using the KORAL code in order to obtain stationary solutions. The simulation with neutron star is with radiation, whereas the one with naked singularity is without radiation. We aim to compare these simulations.

Speaker: Ms Ruchi Mishra (Camk, warsaw)
• 12:15
Beamed super-Eddington emission from a weakly magnetized neutron star - an ultraluminous X-ray source (ULX) 20m

We model an ultraluminous X-ray source using a global 2D axisymmetric general relativistic radiation magnetohydrodynamic simulation of super-Eddington accretion onto a neutron star with a 10 billion Gauss dipolar magnetic field. We found that the magnetic confinement of the gas into accretion columns along with the interaction of the gas with the neutron star surface produce a total luminosity of a few times the Eddington limit. Furthermore, this radiation is beamed by outflows from the accretion disk causing the apparent isotropic luminosity to exceed $10^{40}$ erg/s, showing that low-magnetic field neutron stars can be good candidates for the central engines of ULXs.

Speaker: David Abarca (Nicolaus Copernicus Astronomical Center)
• 14:00 14:45
Derivation of the existence of the Black Holic Quantum from GRBs 45m
Speaker: Remo Ruffini
• 14:45 15:10
Quantifying the Blackholic Quantum in GRBs and AGN 25m

The concept of blackholic quantum is at the base of the emission of high-energy (GeV) electromagnetic radiation and ultrahigh-energy cosmic rays from long GRBs and AGN. I assess from the quantitative viewpoint the blackholic quantum in specific sources and outline astrophysical consequences.

Speaker: Prof. Jorge Rueda (ICRANET)
• 15:10 15:20
Conference Photo 10m
• 15:20 15:50
Coffee Break 30m
• 15:50 17:05
Spacetime Solutions for Compact Objects
• 15:50
Gravitating razor-thin discs around Schwarzschild black holes via the "displace, cut, and reflect" method 25m online

#### online

There is growing interest in the effects of the discs' self-gravity around black holes (BH), both from the theoretical point of view, as exact solutions of Einstein's equations, and from possible implications to black hole astrophysics. Here we apply the well known "displace, cut, and reflect" method, originally proposed by Kuzmin to construct analytical disk-galaxy models, to generate "black hole + disc" exact solutions of Einstein's equations in vacuum. We consider a N-BH (or, more generally, a N-rod) seed solution. By "cutting" the seed spacetime in the middle of the highest BH-rod in Weyl coordinates and reflecting with respect to the corresponding horisontal plane, we obtain a family of self-gravitating razor-thin disks around Schwarzschild BHs.

Speaker: Prof. Ronaldo Vieira (Federal University of ABC, Brazil)
• 16:15
Kerr-de Sitter black hole reexamined 25m

We interpret the cosmological constant as the energy of the vacuum, and under a minimum amount of assumptions, we show that it is deformed in the vicinity of a black hole. This leads us to reexamine the Kerr de Sitter solution. We provide a new solution, simpler and geometrically richer, which shows the impact of the rotation in form of a warped curvature. We carry out a detailed and exact study on the new black hole solution, and we conclude with a conjecture regarding the possible impact of our results on alternative theories.

Speaker: Dr Jorge Ovalle (Silesian University in Opava)
• 16:40
Where is Love? Tidal deformability in the black hole limit 25m

One of the macroscopically measurable effects of gravity is the tidal deformability of astrophysical objects, which can be quantified by their tidal Love numbers. For planets and stars, these numbers measure the resistance of their material against the tidal forces, and the resulting contribution to their gravitational multipole moments. According to GR, nonrotating deformed BHs, instead, show no addition to their gravitational multipole moments, and all of their Love numbers are zero. Here I will discuss different configurations of nonrotating compact and ultracompact objects to bridge the compactness gap between BHs an NSs and calculate their Love number. For the first time we compute the Love number for uniform density ultracompact stars with compactness beyond the Buchdahl limit, and we find that the Love number approaches smoothly to zero as the compactness approaches the Schwarzschild limit. Our results provide insight on the zero tidal deformability limit.

• 18:00 22:00
Conference Dinner 4h
• Thursday, 9 September
• 09:50 11:00
Simulations of Dynamical Accretion Flows
• 09:55
Thick accretion disk in an external gravitational and electromagnetic field 25m Online

#### Online

We present an analytical study on a thick accretion disk model considering a charged fluid encircling a charge or non-charge black hole and endowed with an axisymmetric and stationary magnetic field. Our study is based on the Polish doughnut and provides an extension to this well-known model. Indeed, the effect of the electric and magnetic, and the gravitational fields are known on the accretion flow. The vertical and radial structures of the torus are influenced by these forces. Here our interest is to know how each of the interactions is involved in the equilibrium process as the gravity, the electric, and the magnetic force influence the shape and the density distribution of the fluid. We focus on orbiting structures in the equatorial plane, as single or double tori, on the polar axis, and structures above the equatorial plane as a levitating torus.

Speaker: Audrey Trova (Zarm, University of Bremen)
• 10:20
Does thermal instability affect the warm corona? 20m

Soft X-ray excess, observed in Active Galactic Nuclei and Black Hole Binaries is often explained by the model of warm, optically thick layer. The photons from underlying disk cool the corona, and their energy gain can be observed as an excess over the power law at energy of around 1 keV. Warm corona has been successfuly modeled using many approaches, but the parameters obtained from the observations also are on the edge of region in the parameter space where the structure becomes suspicible to thermal instability and condensation to clumps. Warm corona is observed commonly in AGN, which means that the instability cannot completely hinder its formation. We discuss points for and against occurence of this phenomenon and propose scenarios that could explain the apparent discrepancy between models and observations.

Speaker: Dominik Gronkiewicz
• 10:40
Magnetic reconnection and Orszag-Tang Vortex 20m

Magnetic reconnection is an important phenomenon in the astrophysical plasma, e.g. in solar flares, interstellar medium, stellar evolution and supernovae, accretion discs, and jets. We study magnetic reconnection and plasmoid formation in the Orszag-Tang problem. We show magnetic energy reduction caused by magnetic reconnection. We also study the effect of resistivity and compare the numerical resistivity in 2D and 3D magnetohydrodynamics simulations. We also compare Newtonian code (PLUTO 4.4) and general relativistic code (KORAL) performance in this standard test problem.

Speaker: Ms Fatemeh Kayanikhoo (Nicolaus Copernicus Astronomical Center)
• 11:00 11:30
Coffee Break 30m
• 11:30 12:10
Simulations of Dynamical Accretion Flows
• 11:30
Numerical simulations of multiple accretion tori around black hole. 20m

The matter orbiting around black hole in microquasar or active galactic nuclei forms toroidal accretion disk structures. The main topic of this talk will be general relativistic magnetohydrodynamics (GRMHD) simulations of multiple accretion configurations orbiting around one central black hole. Such structures have been recently described as ringed accretion disks (RADs) in full general relativistic approach. With the use of GRMHD numerical code HARM-COOL the RAD configurations time evaluations will be modeled and RAD stability will be tested. To boost interactions between individual rings from RAD configuration, large scale magnetic filed will be added into RAD model. Accretion configurations with different distribution of angular momentum inside will be also compared.

Speaker: Dilshodbek Bardiev
• 11:50
Puffy Accretion Disk: Observational Properties and Inner Structure 20m

Puffy disk is a novel model of accretion disks resulting only from a complex global GRRMHD simulation. In this talk I will focus on the observational appearance of such type of disk, which is different from standart models of thin or slim disks. Complicated vertical structure of the disk can tell us a lot about the behaviour of plasma in the turbulent environment in the vicinity of compact objects and how it can affect the appearance of such object for distant observers.

Speaker: Debora Lančová (Institute of Physics, Silesian University in Opava)
• 12:10 12:30
Neutron Stars
• 12:10
Neutron star radius-to-mass ratio from accretion disc occultation 20m online

#### online

The equation of state (EoS) of matter at supranuclear densities in the core of neutron stars (NSs) contains key information on the properties of the strong force and the possible existence of exotic states of matter; however it is only poorly constrained from earth-based experiments and theory. NS properties, the precise
values of M and R in particular, encode unique information on the EoS, and different approaches have been proposed to measure, or at least constrain, NS radii. I present a new method to measure the R/M ratio of disk accreting NSs which display a relativistically broadened Fe-Kα profile: this method exploits the occultation of
the inner disk region right behind the NS by the body of the NS itself as seen by an observer at infinity. The
occultation imprints conspicuous features in the line profiles especially for disk inclinations > 65° and NSs of radius larger than R > 6 GM/c^2, and can provide a proxy of the NS R/M ratio. We developed a dedicated model within XSPEC and by fitting it to current data from a few high-inclination NS low mass X-ray binaries,
we found that the signal to noise ratio of the X-ray spectra from present-generation instruments is probably
insufficient to measure the R/M ratio. However, through XSPEC simulations adopting response matrices of the near-future eXTP mission we showed that R/M can be measured to ~3% precision over a range of inclinations.
Such precision in radius determination is required to draw quantitative conclusions on the EoS of ultra-dense matter and represents the goal that other methods too aim to achieve in the future.
If time allows, I will also talk about some factors that can influence the current and future analysis of disk-emitted radiation in the X-ray spectra, focusing in particular on the Fe-Kα line.

Speaker: Riccardo la Placa
• 14:00 14:40
Modelling the Spectra and Time Variability
• 14:00
Oscillations of thick accretion discs in the Hartle-Thorne geometry 20m

We will present our progress in examining the influence of the quadrupole moment of slowly rotating neutron stars on the oscillations of non-slender accretion tori. We assume a perfect fluid, polytropic, constant speciﬁc angular momentum, non-selfgravitating torus, and analytically calculate formulas for the oscillation frequencies.

Speakers: Monika Matuszková, Kateřina Klimovičová (Silesian University in Opava)
• 14:20
Spectral and timing studies ultraluminous X-ray sources 20m Online

#### Online

Ultraluminous X-ray sources (ULXs) provide an ideal laboratory to explore super-Eddington accretion onto stellar-mass compact objects, which may be an important episode in black hole evolution. We present the evidence of super-Eddington accretion in the case of three ULX sources on the base of multi-epoch spectral and timing analysis of X-ray data taken by Chandra, XMM-Newton, Suzaku and NuSTAR satellites. Our spectral modeling reveals the inverse luminosity relation with the inner disk temperature, indicating super-Eddington accretion with emission geometrically beamed by an optically thick massive outflow. In the case of one source, we found clear spectral state transition in analogy to state transition observed in Galactic black hole binaries. Furthermore, short-timescale variability amplitude, found in this source, is correlated with spectral state, i.e. fractional variability amplitude is on the level of ~15% in hard/low state, whereas in the thermal disk-dominated high-flux state, variability drops to ~2%. Finally, we present and discuss the new detection of Fe K_{\alpha} with equivalent with of 300eV, accompanied by new detection of soft X-ray lag with amplitude of the 1300s, in case of another ULX source. This is the second ULX source where the Fe line has been detected, and the fourth, where soft X-ray lag was detected.

Speaker: Mr Samaresh Mondal (Nicolaus Copernicus Astronomical Center)
• 14:40 15:05
Spacetime Solutions for Compact Objects
• 14:40
Gravitational cracking and complexity in the framework of gravitational decoupling 25m Online

#### Online

In this talk, I analyse the stability of self-gravitating spheres in the context of gravitational cracking. Besides exploring the role played by the anisotropy in the occurrence of cracking, we also study the effect of the complexity factor recently introduced in Phys. Rev. D 97, 044010 (2018). The models under study correspond to anisotropic solutions obtained in the framework of the Gravitational Decoupling. The effect that the variation of the decoupling parameter and the compactness of the source have on the behaviour of the radial force is studied in detail.

Speaker: Dr Ernesto Contreras (Universidad San Francisco de Quito)
• 15:05 15:35
Coffee Break 30m
• 15:35 16:50
Modelling the Spectra and Time Variability
• 15:35
Nonlinear evolution of self-trapped g-modes in thin accretion disks 25m

Due to general relativistic effects, accretion disks surrounding black holes can host self-trapped g-modes close to their inner edges. This talk is devoted to their nonlinear evolution. I will discuss change of the trapping region and oscillation frequency of the g-modes when the amplitude of the oscillations is significant. When nonlinearities in the hydrodynamic equations are taken into account, the g-modes are coupled to other oscillation modes. I will show that and their impact on the original g-mode. Finally I will discuss possible excitation mechanisms.

Speaker: Jiri Horak (Astronomical Institute ASCR, Prague)
• 16:00
Resonances around black hole 25m

We will explore dynamics of various nonlinear systems oscillating around black hole (charged particle, spinning particle, string loop, bumpy spacetime) and we will try to search for 3:2 resonances.

Speaker: Martin Kološ (Institute of Physics, Silesian University in Opava)
• 16:25
Epicyclic Oscillations around Simpson--Visser Regular Black Holes and Wormholes 25m

We study epicyclic oscillatory motion along circular geodesics of the Simpson--Visser meta-geometry describing in a unique way regular black-bounce black holes and reflection-symmetric wormholes by using a length parameter $l$. We give the frequencies of the orbital and epicyclic motion in a Keplerian disc with inner edge at the innermost circular geodesic located above the black hole outer horizon or on the our side of the wormhole. We use these frequencies in the epicyclic resonance version of the so-called geodesic models of high-frequency quasi-periodic oscillations (HF QPOs) observed in microquasars and around supermassive black holes in active galactic nuclei to test the ability of this meta-geometry to improve the fitting of HF QPOs observational data from the surrounding of supermassive black holes. We demonstrate that this is really possible for wormholes with sufficiently high length parameter $l$.

Speaker: Jaroslav Vrba
• 17:30 18:30
UNISFERA 1h
• Friday, 10 September
• 09:40 10:25
Modelling the Spectra and Time Variability
• 09:40
Modeling X-ray polarization of accreting BHs with Monk 25m Online

#### Online

Monk (Zhang et al. 2019) is a general relativistic radiative transfer code that is capable of performing polarized radiative transfer. With Monk we systemically calculated the energy spectra and polarization of the X-ray radiation from AGNs and BHXRBs in the hard state. We also couple Monk and Dyplo (Marcel et al. 2018) to evaluate the polarization of BHXRBs in the hard state where we solve the structure of the disc and the inner hot flow instead of taking phenomenological assumptions. In this talk, we will give an short introduction to Monk, and then present the results.

Speaker: Wenda Zhang (National Astronomical Observatories, Chinese Academy of Sciences)
• 10:05
Spinning test particle oscillations and its application to QPOs 20m Online

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We will discuss the oscillations of spinning test particles and data fitting of quasi-periodic oscillations observed in the well known quasars as well as active galactic nuclei in the framework of the model of geodesic oscillations of Keplerian disks modified for the epicyclic oscillations of spinning test particles in the background of Kerr black holes.

• 10:25 11:00
Projects in Strong-Field General Relativity: my worldline with Pavel Bakala 35m online

#### online

In this presentation I will survey the projects that Pavel Bakala and I,
together with students and collaborators, carried out in recent years,
especially during Pavel's sabbatical at the Astronomical Observatory
of Rome in 2019. Strong-field GR in application to astrophysical
effects and problems was the leitmotiv of the projects, with
extreme light bending playing an especially prominent role.

Speaker: Prof. Luigi Stella (Luigi Stella - INAF Astronomical Observatory of Rome )
• 11:00 11:30
Coffee Break 30m
• 11:30 12:40
EM Interactions in GR
• 11:30
Innermost stable circular orbits around charged, rotating black holes 25m

The fact that stable circular orbits cannot be found arbitrarily close around very compact objects -- such as black holes - can only be explained relativistically. The occurrence of an Innermost Stable Circular Orbit has a strong influence the accretion processes, that take place in the very inner regions of an accretion environment. We discuss these orbits in the case of charged, spinning black holes, and find fascinating new properties.

Most astonishingly the radius of the ISCO increases with an increasing particle-black hole charge product |qQ| in both cases of an attractive and repulsive Coulomb interaction for sufficiently big |qQ|. Therefore there exists a 'minimal ISCO' at a certain charge product qQ.

Speaker: Kris Schroven
• 11:55
Particle ionization near a weakly charged black hole 20m

In many astrophysical scenarios, the charge of the black hole is often neglected due to unrealistically large values of the charge required for the Reissner-Nordström spacetime metric. However, black holes may possess a small electric charge due to various selective accretion mechanisms. We investigate the process of ionization of a neutral particle in the vicinity of a weakly charged non-rotating black hole and calculate the energy of an ionized particle in a chosen scenario. High energy obtained by a charged particle after the ionization process can serve as a distinguishing signature of the weakly charged black holes.

Speaker: Bakhtinur Juraev
• 12:15
Role of non-axisymmetry in electro-vacuum magnetospheres of compact objects 25m online

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We discuss some effects of breaking the axial symmetry in vacuum magnetospheres of black holes and neutron stars. In particular, we analyze the emergence of magnetic null points which may occur due to non-axisymmetry, and substantially affect the dynamics of electrically charged particles.

Speaker: Ondrej Kopacek (Astronomical Institute, Czech Academy of Sciences)
• 12:40 13:00
Conference closing 20m
• 13:00 16:00
Individual Meetings, Discussions, Work on Common Projects