Opening

• Zdeněk Stuchlík

Photon Shells, Black Holes Shadows, and Accretion Toroids

• Zdeněk Stuchlík (Silesian University in Opava)

We analyze the properties of the Kerr black hole (BH) photon shell, focusing on the influence of aggregates of corotating and counterrotating toroids (ringed accretion disks - RAD) orbiting in the BH photon shell. The particular case of a corotating accretion disk orbiting in the BH ergoregion is also investigated. We study influence on the BH shadow boundary, fixing the conditions under which it is possible to observe the ergoregion and RAD "imprint" on the shadows boundary. In general, remarkable differences appear between the counterrotating photon components of the boundary with respect to the corotating ones. In particular, we investigate the bound unstable spherical photon orbits int he ergoregion, or at the inversion point,

Relativistic jets versus ambient magnetic fields

• Arman Tursunov (Silesian University in Opava)

Relativistic jets observed in active galactic nuclei and produced by supermassive black holes are highly sensitive to the structure and dynamics of their surrounding magnetic fields. In this talk, I will argue that these magnetospheres are shaped not only by internally generated magnetic fields - such as those from the accretion disk, as typically theorized - but also by external fields. Interactions with ambient magnetic fields, whether from neighboring astrophysical objects or the host galaxy itself, can lead to significant changes in jet properties. Specifically, the interplay between the field components can reorient jets, cause them to dissipate, or even quench their propagation entirely. I will present findings demonstrating that even the relatively weak magnetic field of the host galaxy can profoundly impact jet dynamics at parsec scales. This mechanism offers a potential explanation for the observed diversity in active galactic nucleus (AGN) jet activity, including the distinction between radio-loud and radio-quiet AGNs.

Exploring the Electromagnetic Field Around a Rotating Black Hole

• Pakhlavon Yovkochev (Institute of Physics in Opava)

We investigate various electromagnetic field configurations around a rotating Kerr black hole, plotting the electric and magnetic vector fields for different observers. The study employs a black hole magnetosphere model that combines an external uniform magnetic field with an internal split magnetic monopole configuration. Additionally, we aim to address the Meissner effect, characterized by the expulsion of magnetic field lines, and the induced electric Wald charge on the black hole.

Chaotic dynamics of pulsating objects around black holes

• Ronaldo Vieira (Federal University of ABC, Brazil)

An extended test body in Newtonian gravity usually deviates from its corresponding point-particle motion: its center-of-masss trajectory is affected by its own internal structure, generating quite diverse phenomena. Here we consider small test bodies with symmetries such that their spin are identically zero along their orbits, and work in the quadrupole approximation. We show that in this approximation small pulsating spheres may acquire chaotic behavior even in spherically symmetric gravitational fields due to a time-dependent term in the Hamiltonian governing their center-of-mass dynamics. In general relativity, by applying Dixon's formalism to the test body up to quadrupole order and keeping symmetries such that the body's spin remains identically zero, we extend these results to pulsating spheroids in Schwarzschild spacetime and also in a general class of black-hole spacetimes describing either Reissner-Nordström metric or spacetimes arising from modified (metric) theories of gravity. The overall picture is the ubiquity of chaos in these systems when the body is approaching the black hole, however small the pulsating body is.

Magnetized particle dynamics around compact objects: Orbital frequencies and their astrophysical applications

• Farukh Abdulkhamidov (Silesian University in Opava)

We explore the dynamics of magnetized test particles in the Schwarzschild spacetime under the influence of a large-scale magnetic field. This work is well-motivated astrophysically by the dynamics of magnetized inhomogeneities in accretion flows around compact objects such as black holes or neutron stars. It could also be motivated by the dynamics of compact, magnetized neutron stars around supermassive galactic black holes. We calculate the fundamental frequencies of small harmonic oscillations around a stable circular orbit in the equatorial plane, providing a basic timescale for astrophysical processes in magnetized plasma. The frequencies obtained are then applied to various astrophysical problems, such as the inner stability of accretion flows, flare dynamics around the supermassive black hole in the SgrA* source, and the observed quasi-periodic oscillations around neutron stars and black holes.

Radiation Reaction in Charged Particle Dynamics within the Dipolar Magnetosphere of Neutron Stars

• Jaroslav Vrba

The dynamics of charged particles in neutron star magnetospheres, represented by a dipolar magnetic field in Schwarzschild spacetime, are influenced by an effective potential governing circular orbits on and off the equatorial plane. Focusing on radiation reaction, we use the Landau-Lifshitz approximation to analyze how it modifies particle motion across equatorial, off-equatorial, and chaotic orbits. We show that under an attractive Lorentz force, radiation reaction drives particles toward the neutron star surface. For repulsive forces, it broadens equatorial orbits and shifts off-equatorial paths toward the equatorial plane if near enough; otherwise, they evolve toward the surface. The critical latitude for this transition is determined numerically as a function of electromagnetic coupling strength.

Numerical simulations with pseudo-potential for Reissner-Nordstrom naked singularity

• Miljenko Cemeljic (SGMK, CAMK & ASIAA)

We perform the first numerical simulations of a thin accretion disc in pseudo-potential for Reissner-Nordstrom naked singularity. The obtained results share the positions of the zero gravity sphere and maximal rotational velocity with the analytical solutions and the general relativistic simulations. They also show the characteristic shape of the innermost part of the disc, matching the analytical solutions.

Non-Spherically Symmetric Models of Compact Objects with the Salpeter Equation of State

• Amina Sadu (Al-Farabi Kazakh National University)

[Abstract][1] Non-Spherically Symmetric Models of Compact Objects with the Salpeter Equation of State This research explores the gravitational behavior of compact astronomical bodies, such as white dwarfs and neutron stars, under the framework of Einstein's general relativity. A simplified mathematical model incorporating the quadrupole moment to a first-order approximation is applied to investigate deviations from spherical symmetry. The study focuses on interior solutions for white dwarfs, employing the Salpeter equation of state (EoS) to analyze equilibrium between gravitational pull and internal pressure. Comparative analysis is performed between the Chandrasekhar and Salpeter EoS, providing insights into the influence of quadrupole deformations on structural properties. Numerical solutions reveal significant differences in pressure and mass distributions for the selected EoS models, offering a deeper understanding of the interior dynamics of compact objects. The results contribute to the broader study of astrophysical phenomena under extreme conditions, enabling refinements in modeling techniques and future applications to non-spherically symmetric configurations. Keywords: white dwarfs, neutron stars, quadrupole moment, equations of state, Einstein's field equations. References: 1. Abishev, M., Beissen, N., Belissarova, F., Boshkayev, K., Mansurova, A., Muratkhan, A., Quevedo, H., & Toktarbay, S. (2021). Approximate perfect fluid solutions with quadrupole moment. International Journal of Modern Physics D, 30(13), 2150096. 2. Salpeter, E. E. (1961). Energy and Pressure of a Zero-Temperature Plasma. Astrophysical Journal, 134, 669. [1]: https://www.overleaf.com/project/6736ef4707a5e9837bc90711

Dynamics of oscillons and oscillon/Q-ball correspondence

• Filip Blaschke (Silesian University in Opava)

Using a renormalization-inspired perturbation expansion we show that oscillons in a generic field theory in (1+1)-dimensions arise as dressed $Q$-balls of a universal complex field theory. This theory is very close to the integrable complex sine-Gordon model which possesses exact multi-$Q$-balls. We show that excited oscillons, with their characteristic amplitude modulations, are two-oscillons bound states generated from a two $Q$-ball solution.

Pions within the Hartree approximation and the application to astrophysics

• Kamil Sokołowski (University of Wroclaw)

Pions play an essential role in the strong interactions experienced by nucleons, the main components of the nuclear matter occurring in neutron stars or supernova explosions. The main problem regarding pions is how to implement them in the equations of state to properly incorporate the additional pion abundance, pressure, energy, and potential occurrence of the pion condensate. In my talk, I will address some of these issues based on the description of pions within the Hartree approximation (also known as the Walecka model) to see how the equations of state are modified when one includes these mesons.

Multi-Messenger Signals from Magnetorotational Stellar Core Collapse

• Shota Shibagaki (University of Wrocław)

Multi-messenger signals of gravitational waves and neutrinos from supernovae carry information about properties of supernova cores, which cannot be directly observed with electromagnetic waves. To maximize impacts of future detection of these multi-messenger signals, it is important to understand the relationship between the characteristics of the multi-messenger signals and the properties of the supernova cores. We performed fully general relativistic three-dimensional neutrino-radiation magneto-hydrodynamic simulations of stellar core collapse with spectral neutrino transport to explore the impacts of characteristic fluid motions on the gravitational wave and neutrino signals from progenitors with various rotation speeds and magnetic field strengths. In this talk, I show the results of the time-frequency analysis of the gravitational wave and neutrino signals obtained from our simulations. In the non-magnetized rapidly rotating models, non-axisymmetric instabilities develop and generate the characteristic gravitational wave and neutrino signals that are correlated in the time-frequency plane. The highly magnetized rapidly rotating model shows that, for an observer on the equatorial plane perpendicular to the rotation axis, the low-frequency gravitational wave amplitude from anisotropic neutrino emission from deformed proto-neutron star becomes more than one order-of-magnitude bigger than that from the bipolar magnetohydrodynamic jets. I also discuss the detectability of their characteristic features.

Probing strong gravity with black hole images

• Maciek Wielgus (Institute for Astrophysics in Andalusia, Granada, Spain)

The Event Horizon Telescope (EHT) recently enabled imaging two supermassive black holes, M87 and Sagittarius A*, with the near event horizon scale resolution. Since the observed emission originates very close to the black hole, where trajectories of photons are subject to large deflection related to spacetime curvature, we can use these observations to test gravity in the strong regime. However, there are important limitations related to astrophysical uncertainties that have to be carefully addressed before unlocking precise constraints on the theory of gravity or on the nature of the central supermassive compact object. In this talk I will present the current landscape of using EHT's black hole images to test gravity, current constraints, caveats, as well as some potential future improvements and research avenues.

Outflows from naked singularities, infall through the black hole horizon

• Tomasz Krajewski (Nicolaus Copernicus Astronomical Center)

We present general-relativistic hydrodynamical simulation results of accretion from an orbiting accretion torus (with a cusp) onto a Reissner-Nordström (RN) black hole and a RN naked singularity. The results could not be more different for the two cases. For a black hole, just as in the familiar Kerr/Schwarzschild case, matter overflowing the cusp plunges into the black hole horizon. For the naked singularity, the accreting matter forms an inner structure of toroidal topology and leaves the system via powerful outflows.

Lense-Thirring Precession as a Source of Low-Frequency Variability

• Chris Fragile

Oscillations and precessions of accretion flows: Effects of the neutron star quadrupole moment

• Gabriel Török (Silesian University in Opava)

Misalignment of the Lense-Thirring precession by an accretion torque

• Jiri Horak (Astronomical Institute, Academy of Sciences)

Type-C QPOs in low-mass X-ray binaries are most commonly explained by the Lense-Thirring precession of the innermost hot and geometrically-thick component of the accretion flow. Most models however ignores any influence of the outer cold geometrically-thin accretion disk. We will discuss how this simple picture changes when the accretion torque by the outer disk is taken into account.

Compact Star Twins

• David Alvarez-Castillo (Institute of Nuclear Physics PAS)

In this talk I shall present the mass twins compact star phenomenon together with the idea behind their existence and the motivation for searches. I will briefly review the special features in the equation of state (EoS) of the twins as well as their properties as compact stars. Such macroscopical properties can be tested with state-of-the-art multi-messenger astronomical observations. I will discuss the possibility that the HESS J1731-347 object is a compact star twin and the corresponding implications. Furthermore, I will address the study of gravitational wave signals from mergers and from f-modes of twins, as well as their existence as an explanation of very eccentric orbits of binary systems and production of isolated neutron stars. The compact star twins idea can be naturally be generalized to more than two compact stars of the same mass which will feature several phase transitions in the EoS that describes them.

Optical studies of double tori structures around rotating black holes

• Dilshodbek Bardiev

We are studying the evolution of multiple tori structures with 3D GRMHD simulations. The output from the simulations can be used in post-processing analysis to create synthetic ray-tracing images using well-known tools. Here I present our latest simulation results and their optical properties.

The IXPE Perspective on Accreting Stellar-Mass Black Holes

• Michal Dovčiak (Astronomický ústav AV ČR, v.v.i.)

Accreting stellar-mass black holes in X-ray binaries (XRBs) exhibit different states, each characterized by distinct spectral properties. The two fundamental states are the high/soft state, dominated by the thermal multi-color blackbody emission from the accretion disk, and the low/hard state, dominated by non-thermal, power-law-like Comptonization emission originating from the hot corona. Over its first 2.5 years, the Imaging X-ray Polarimetry Explorer (IXPE) mission has observed 10 XRBs in various accretion states. This talk will review these polarization observations and discuss their implications for measuring the spins of accreting black holes and constraining the geometry of the X-ray corona.

X-ray polarization models of reflection in active galactic nuclei and X-ray binary systems

• Jakub Podgorny

I will present a series of numerical X-ray spectro-polarimetric models that are designed to study reprocessing of coronal radiation from equatorial components of active galactic nuclei and X-ray binary systems. First, local reflection tables for a constant density slab with precomputed ionization structure are introduced. These are then used inside a fully relativistic code KYNSTOKES that integrates reflected radiation from a geometrically thin disc, assuming a lamp-post or sandwich emitting corona geometry, and that allows to fit X-ray data in XSPEC. The local reflection tables are also used inside the xsstokes codes suitable for XSPEC fitting of reflected emission from nearly neutral regions of the accretion disc, from the broad line regions or from the dusty tori of active galactic nuclei. Examples of usage of all the models in interpreting the latest X-ray polarization data from the IXPE mission operating in 2-8 keV will be provided.

Vibrating ring around black hole

• Martin Kološ (Institute of Physics, Silesian University in Opava)

A thin circular structure vibrating in the central plane of a black hole will be investigated. This circular ring (string loop) can be considered a simplified model for thin magnetic flux tubes (plasma physics), and connections to accreting fluid structures around the black hole will be demonstrated. The stability of the string loop and the frequencies of its vibrational modes will be provided and compared with the vibrational modes of thick toroidal fluid structure around black holes, which is the standard analytical model for the temporal properties of accretion flow.

Modulation of X-ray flux by obscuration of neutron star boundary layer

• Debora Lančová (Institute of Physics, Silesian University in Opava)

TBA

Relevant episodes in Beppo-SAX and IXPE

• Remo Ruffini (ICRANet)

Oscillating advective accretion discs: structure and dynamical properties from the hydrodynamic simulations

• Sanjit Debnath (Aryabhatta Research Institute of Observational Sciences (ARIES), INDIA)

We investigate the time evolution of two-dimensional sub-Keplerian transonic accretion flow onto a non-rotating black hole. We simulate the shocked accretion flow by using boundary values from semi-analytical analysis. Our focus is primarily on the regime of viscosity parameters that exhibit shock oscillation within the disc. By varying both viscosity and radiation cooling, we explore the dynamics of this specific disc configuration. Our findings reveal that turbulence can develop in the post-shock region, with the intensity of turbulence increasing as the viscosity parameter rises. However, the presence of radiation cooling can suppress this turbulent structure. Viscosity tends to push the shock surface outward by redistributing angular momentum, while cooling drives the shock inward by reducing the temperature gradient force, ultimately influencing the stability and dynamics of the shock. Additionally, we also evaluate the luminosity of our models. The oscillations of the post-shock disc lead to quasi-periodic oscillations in the synthetic light curve. Lastly, we highlight that this type of flow may offer a potential explanation for the complex observational features of stellar-mass black hole binaries.

Impact of non-zero net-flux vertical magnetic field on thick accreting torus around black holes

• Samik Mitra (Indian Institute of Technology Guwahati)

We study a global, two-dimensional (2D) general relativistic magnetohydrodynamics (GRMHD) simulation of an accreting torus around a non-rotating black hole using Athena++. Our initial configuration is threaded with a net-vertical magnetic flux. This study investigates the effects of initial field strength onto the disk dynamics. We find that the initial net vertical magnetic field significantly enhances its amplification over a few dynamical time scales. The behavior of MRI turbulence is regulated by the initial plasma-$\beta$ (i.e., the ratio of gas-to-magnetic pressure) at the inner edge of the torus. Therefore, we perform simulations with different $\beta$ spanning from weakly-to-strongly magnetized cases, $\beta^{\rm in}_{\rm edge}$ = 2800, 700, 350, respectively. The shear flow amplifies a strong toroidal magnetic field within the torus via the dynamo process. Consequently, the magnetic fields are sufficient to provide magnetic pressure support for the elevated accretion, $z/R \sim 0.2$. Furthermore, we identify two more regions, (1) gas-pressure dominated dense mid-plane, and (2) moderately magnetized low dense polar region. Finally, over the duration of our simulations, we find evidence that the net flux attains a quasi-steady state and can stably maintain an inner disk with surface accretion.

Power of outflows in neutron star ultraluminous X-ray sources

• Fatemeh Kayanikhoo (CAMK)

Through numerical simulations, we investigated accreting neutron stars with apparent luminosities consistent with ultraluminous X-ray sources (ULXs). We examined the effects of magnetic field strength and accretion rate using the radiative general relativistic magnetohydrodynamics code, Koral. Our findings show that the power of outflows increases as the magnetic dipole strength decreases and the accretion rate increases. In simulations with a magnetic dipole strength of 10^{10} G, the apparent luminosity exceeds 100 Eddington with an accretion rate of 300 Eddington units and reaches 250 Eddington with an accretion rate of 1000 Eddington units. Conversely, simulations with a dipole strength on the order of 10^{11} G with an accretion rate of 300 Eddington units result in an apparent luminosity of only 40 Eddington, which is inconsistent with typical ULXs.

Jean-Pierre Lasota: X-ray Binaries, Accretion Disks, Black Holes and ULXs

• Marek Abramowicz (Silesian University, Opava)

Palatini gravity and its implications for cosmic inflation

• Thomas Pappas (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava)

We present a brief introduction to cosmic inflation in the framework of Palatini gravity, which provides an intriguing alternative to the conventional metric formulation of gravity. In the latter, only the metric specifies the spacetime geometry, whereas in the former, the metric and the spacetime connection are treated as independent variables-an option that can result in a gravity theory distinct from the metric one. In scenarios where the field(s) responsible for cosmic inflation are non-minimally coupled to gravity or the gravitational sector is extended, assumptions about the underlying gravitational degrees of freedom can have substantial implications for the observational effects of inflation. We examine this explicitly by discussing various compelling scenarios.

Possible influence of observed cosmological constant on large-scale jets: geometric arguments

• Petr Slaný

Positive, although very small, value of the cosmological constant changes behaviour of a central gravitational field at large distances. Crucial for our discussion is existence of a static radius where gravitational attraction of a centre is just balanced by cosmic repulsion. Analysis of radial timelike geodesics in the Schwarzschild-de Sitter spacetime reveals that particles with specific energy close to unity have tendency to slow down and cluster near the static radius, forming clumps which, subsequently, start to expand due to cosmic repulsion. For central masses of $(10^6$-$10^{11})\,{\rm M_{\odot}}$ and current value of the cosmological constant $1.1\times 10^{-52}$ ${\rm m^{-2}}$, this phenomenon takes place at distances of tens to hundreds of kiloparsecs from the centre, which are distances in which large-scale jets and huge radio-lobes were observed in some active galaxies.

CMB power spectrum modelling

• Denis Musil

Cosmic Microwave Background radiation (CMB) is considered as first light that travelled freely through space after the Big Bang. After probes like COBE, WMAP and Planck, it was confirmed that temperature of the CMB is not uniform. Computing power spectrum for given cosmological model one can determine cosmological parameters of the Universe by comparing it to the observed power spectrum. In the talk I will present methods and approximations used to create program to compute CMB power spectrum and talk about results for different cosmological models.

Neutron Star Deathlines and Gravity Tests

• Javlon Rayimbaev (Ulugh Beg Astronomical Institute)

TWO-TEMPERATURE ACCRETION FLOWS AROUND COMPACT OBJECTS

• Shilpa Sarkar (Harish Chandra Research Institute (HRI), India)

Accretion mechanism is one of the most effcient process by which the gravitational potential energy of the matter can be converted into energy. This phenomenon is responsible for the exotic processes happening in the Universe. It provides us with an explanation of the huge amount of energy liberated and high luminosities observed in AGN's, X-ray binaries, etc. Therefore, modelling these accretion flows is necessary to obtain a proper picture of the underlying physical processes and phenomena present in these systems. Since electrons are the ones that radiate via processes like synchrotron, bremsstrahlung and inverse-Compton scattering, therefore the electron gas and proton gas, present in the ionised plasma of the accretion disk, are supposed to settle down at two different temperatures; hence the name two-temperature accretion flows. Not much work has been done in two-temperature accretion flows, so we addressed this problem in greater details in the pure general-relativistic regime. The problem with two-temperature ow is that, there is one more variable than the number of equations. Solving the equations of motion for a given set of constants of motion, we find that no unique solution exists, unlike in the case of one-temperature flows. In other words, the solutions are degenerate. So, we get different kinds of transonic solutions with drastically different topologies but for the same constants of motion. In addition, there is no known principle dictated by plasma physics that may constrain the relation between these two-temperatures in any of the boundaries. We removed the degeneracy with the help of second-law of thermodynamics. We show that only one of the solutions among all, has the maximum entropy and therefore is the correct solution, thus eliminating degeneracy. As far as we know, no methodology of obtaining unique transonic two-temperature solutions has been reported so far in the literature. This is the first time we have attempted towards getting the general picture of the physical solutions.

Resonance in a disc-like structure - the Fröyland model

• Filip Janoštík

The work explores resonant phenomena in disc-like structures around celestial bodies using a simple Newtonian model - the Fröyland map. The Fröyland map can be used to simulate behaviour of particles in a disc-like structure due to presence of a gravitational force of a perturbing body, but only below the orbital radius of the perturbing body. A concrete examples of predictions given by the Fröyland model will be explored and compared to observational data. There is a certain similarity between the force due to the perturbing body in the Fröyland model and a force that would correspond to Paczynski-Wiita potential approximation, but an attempt to simulate resonant phenomena for orbits beyond the orbit of a perturbing body leads to rather uninteresting results, which likely makes the P-W similarity useless for exploration of accretion proccesses.

Public talk - Hosted by the Institute of Physics, Unisphere Room

• Maciek Wielgus (Black Hole Initiative, Harvard)

Join us for an engaging public talk in the Unisphere Room at the Institute of Physics.