RAGtime 27

10 Nov 2025, 08:00 → 14 Nov 2025, 13:00 Europe/Prague

Venue

The 27th RAGtime workshop will be held in a hybrid form in Opava from November 10th to 14th 2025.

The number of registered participants is limited, and registration is moderated. We kindly ask you to wait until you receive a confirmation of your registration to finalize your travel plans. Please note that registrations associated with submitted abstracts will be given priority.

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Conference fee

There will be a conference fee. A reduced fee applies to students and early researchers up to 3 years after finishing their PhD.

Conference fees may be paid by credit card or bank transfer. On-site fees will be accepted only in cash. If you wish to pay the Standard or Late fee in cash, let us know by email to ragtime@physics.cz.

Standart

    Early-bird fee: 5 000 CZK / 200 EUR (until October 10th 2025)

    Regular fee: 7500 CZK / 300 EUR (from October 11th 2025 until November 7th 2025)

Students and ECR 

    Early-bird fee: 2 000 CZK / 80 EUR (until October 10th 2025)

    Regular fee:  3 700 CZK / 150 EUR (from October 11th 2025 until November 7th 2025)

On-site fee: 500€ / 12 500 CZK in cash (during the conference)

Cancellation will be free until August 31st 2025, then 50% fee until 20th October 2025, after which it will be 100%.

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The RAGtime workshops

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

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

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

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

 

 

RAGtime27poster-4K.pdf

RAGtime27poster-A3.pdf

Monday, 10 November

10:30 → 12:00 Coffee & Biscuits

10:30 → 13:00 Registration

13:00 → 13:15 Opening

Speaker: Prof. Zdeněk Stuchlík (SLU)

13:15 → 14:00 Zdenek Stuchlík - Keynote

Keynote

Speaker: Prof. Zdeněk Stuchlík (SLU)

14:00 → 14:30 Coffee break

14:30 → 15:10 Singularity resolution via quantum signal analysis

We propose the novel way to address the coordinate singularity of the classical General Relativity. Through Gabor signal quantisation we explore techniques for space-time metrics. Transforming functions on space-time into operators provides a refined description for a Schwarzschild metrics or for a uniformly accelerated system. In this talk we present a simple toy model based on $(2+1)$ Minkowski written in singular coordinates that has been Gabor regularised and discuss several changes to the original flat model that are brought by smoothing coordinate origin. We also demonstrate that this process can be interpreted as inducing non-trivial curvature and torsion, reflecting a quantum smoothing of geometrical structures defined at singular points.

Speaker: Ewa Czuchry

15:10 → 15:30 On the uplift of 4D wormholes in Braneworld models and their 5D structure

Recent developments for the consistent embedding of general 4D static and spherically-symmetric spacetimes in arbitrary single-brane braneworld models in the form of the General Embedding Algorithm (GEA) [Phys.Rev.D 109 (2024) 4, L041501], initiated the program of studying the bulk structure of braneworld wormholes. In this article, adopting a completely generic approach, we derive the general conditions that the metric functions of any braneworld spacetime must satisfy to describe a wormhole structure in the bulk. Particular emphasis is placed on clarifying the proper uplift of 4D spacetimes, expressed in terms of arbitrary radial coordinates on the brane, and we demonstrate the important role of the circumferential radius metric function r(u) for the embedding. To ensure applicability even when r(u) is non-invertible, we develop an extended formulation of the GEA. Additionally, the flare-out conditions for braneworld wormholes are presented for the first time and are found to differ from the case of flat extra dimensions. To illustrate the method, we first perform the uplift into both thin (Randall-Sundrum II) and thick braneworld models for four well-known 4D wormhole spacetimes: the effective braneworld wormhole solutions of Casadio-Fabbri-Mazzacurati and Bronnikov-Kim, the Simpson-Visser spacetime, and the Ellis-Bronnikov or "anti-Fisher" solution. Subsequently, we study their bulk features by means of curvature invariants, flare-out conditions, energy conditions and embedding diagrams. Our analysis reveals that the assumption of a warped extra dimension has non-trivial implications for the structure of 5D wormholes.

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

15:30 → 16:00 Coffee break

16:00 → 16:20 Stable Naked Singularities in Einstein-Weyl Gravity

We investigate the class of attractive naked singularities arising in Einstein–Weyl gravity. Unlike in General Relativity, where naked singularities are generically unstable and excluded by cosmic censorship, we show that these solutions are linearly stable under tensor perturbations. By numerically evolving the perturbation equations in the time domain, we find that all modes decay, with characteristic oscillatory tails consistent with massive field perturbations in quadratic gravity. These results establish that attractive naked singularities in Einstein–Weyl gravity are dynamically stable configurations. While they lack an event horizon, their external gravitational field closely mimics that of black holes, suggesting that they may represent viable black-hole alternatives in higher-derivative gravity.

Speaker: Andrea Spina (INFN-Università di Catania)

16:20 → 16:40 Telling apart black holes and naked singularities with trajectories of massive particles

I will present a numerical study of the dynamics of massive particles orbiting black holes and naked singularities in the Reissner-Nordström geometry. I modeled a stream of particles with a constant angular momentum and with a range of energies. I then solved the geodesic equation of motion and compared the trajectories around black holes and naked singularities by tuning the charge parameter of the metric. The setup that I used can approximate astrophysical scenarios such as tidal disruption events. I will discuss differences and similarities in the orbital dynamics and deflection angles. I found that particles reflected by a black hole follow a stream-like family of orbits within a narrow range of deflection angles, whereas in the case of naked singularities, particles are scattered in all directions on the plane of motion. I will explain this behavior as an interplay between the presence of a centrifugal barrier at the location of the unstable circular orbit and an absorbing event horizon in the case of a black hole or a reflective zero-gravity sphere in the case of a naked singularity. These qualitative differences are expected to impact the observable signatures of tidal disruption events.

Speaker: Angelos Karakonstantakis (Nicolaus Copernicus Astronomical Center)

16:40 → 17:00 Gravitational decoupling and regular hairy black holes: Geodesic stability and quasinormal modes

In this work, we investigate the stability of geodesic orbits around a regular hairy black hole within the framework of gravitational decoupling. The analysis is performed through Lyapunov exponents, which quantify the divergence rate of nearby trajectories in dynamical systems. Both timelike and null geodesics are considered to explore the impact of the hair parameter on orbital stability. Deviations from the Schwarzschild geometry are shown to significantly affect the dynamics of test particles, potentially leading to observable signatures. Additionally, we compute the quasinormal modes of regular hairy black holes to further probe their stability and dynamical response. Interestingly, we explore the role of the hair parameter as a possible mimicker of the spin parameter in rotating black holes, motivated by the observed similarities between their effects on geodesic motion.

Speaker: Ronaldo César De Paiva

Tuesday, 11 November

10:00 → 10:20 Black hole formation from failed core-collapse supernovae: theory and observations

There are multiple well-studied astrophysical pathways for producing stellar-mass black holes (BHs), and in this talk, I will focus on their formation channel through the failed explosion of collapsing iron cores in massive stars. At the end of core-silicon burning, massive stars develop a an electron-degeneracy supported iron core. The iron core loses stability when it surpasses its effective Chandrasekhar mass. The collapse of the iron-core may result in an explosion, observable as a Type II, Ib or Ic supernova (SN), powered by the neutrino heating mechanism. There are, however, theoretical as well as observational indications that this SN mechanism is not always successful - that iron-cores sometimes fail to explode and continue to fall in until a BH is formed. To discriminate the outcome (successful versus failed supernova), different “explodability” measures based on the stellar structure at the onset of iron-core infall have been introduced, which allow us to make statistical prediction of SN outcomes. The final fate landscape of stars undergoing iron-core collapse has been substantially revised over the past 20 years, and up-to-date the scientific community has not reached a consensus. I will argue that recent advances in the understanding of the late-phase evolution of massive stars as well as the trimodal peak-dearth structure in the low-mass end of the distribution of chirp masses of binary BHs observed by the LIGO-Virgo-KAGRA collaboration after the GWTC-4 gravitational wave data release strongly favour a bimodal pattern in the final fate landscape as a function of core mass.
In a general relativistic treatment of a failed SN, BH formation is typically associated with a singularity forming as matter continues to contract under its own gravity. I will address the question whether singularity prediction indicates a breakdown of General Relativity, while distinguishing between the geodesic incompleteness (GI) and infinite curvature (IC) singularity concepts. My answer will be yes for IC but no for GI. I will close with remarks concerning heat exchange with a quasi-static Schwarzschild BH and the validity of the four laws of thermodynamics.

Speaker: Kiril Maltsev (Heidelberg Institute for Theoretical Studies)

10:20 → 10:40 Relativistic Effects as Constraints on Quadrupole Deformations

We investigate two relativistic effects—the Shirokov effect and the Shapiro delay—within the Zipoy–Voorhees spacetime (q-metric), a generalization of the Schwarzschild solution that incorporates a quadrupole moment. By analyzing the geodesic deviation equations, we demonstrate that the quadrupole parameter induces oscillatory dynamics of test particles, with oscillation periods coinciding with the orbital period in the rotating reference frame. This behavior fundamentally distinguishes the q-metric from the Schwarzschild case and provides new insight into how multipolar structures modify orbital motion. Furthermore, we derive the Shapiro time delay in the q-metric and show its explicit dependence on quadrupole deformations of spacetime, finding a significant first-order contribution in contrast to some recent results. These findings deepen the understanding of how deviations from spherical symmetry affect gravitational phenomena such as orbital dynamics, time delay, and lensing. The results are of interest for astrophysical applications, particularly in the study of neutron stars, naked singularities, and black hole mimickers with strong quadrupole moments. Future observational tests, including pulsar timing and spacecraft tracking, may help to place further constraints on such deviations in strong gravitational fields.

Speaker: Dr Daniya Utepova (Abai Kazakh National University)

10:40 → 11:00 Gabo - Talk

abstarct

Speaker: Gabriel Török (Silesian University in Opava)

11:00 → 11:30 Coffee break

11:30 → 12:10 Plunging to and escaping from a spherical orbit near a rotating black hole

We explore off-equatorial acceleration of electrically charged matter near a magnetized black hole with the aim of understanding the boundaries between the regions of stable, plunging, and escaping motion. As a generalisation of the Innermost Stable Circular Orbit, the concept of the radius of the Innermost Stable Spherical Orbit determines the inner rim of inclined accretion/ejection process. We demonstrate that the region of bound orbits has a complicated structure due to enhanced precession in strong gravity.

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

12:10 → 12:30 High-energy collisions near horizons of axisymmetric spacetimes: generic approach

We present a generic approach to the investigation of collisions of particles near black hole horizons. This takes into account that particles move in a generic axisymmetric spacetime (without specification of the metric functions) and that their motion is affected by the action of a force of unspecified nature. By requiring the horizon to be regular and forces to be finite, we obtain the set of conditions that have to hold to get the high energy of collision. Our algorithm allows to deduce whether the high-energy collision happens or not for various types of horizons and for various types of colliding particles. This analysis is required in the context of the consideration of black holes as particle accelerators.

Speaker: Hryhorii Ovcharenko (Charles University, Faculty of Mathematics and Physics, Institute of Theoretical Physics, V Holesovickach 2, 18000 Prague 8, Czechia)

12:30 → 14:00 Lunch break

14:00 → 14:40 Probing black hole physics with polarimetry

Low luminosity accretion disks around supermassive black holes emit synchrotron radiation all the way to the event horizon. This emission can be probed with very long baseline radiointerferometry. Using this technique, two sources - Sagittarius A and M 87 - were resolved using Event Horizon Telescope at the event horizon scale. In particular, resolved polarized emission from the compact region, constituting a sensitive probe of the magnetic field configuration, was observed. In this talk I will discuss how resolved images of black holes are used to probe the accreting system properties, including black hole spin, plasma velocity profile, and geometry of magnetic field.

Speaker: Maciek Wielgus (Black Hole Initiative, Harvard)

14:40 → 15:00 Polarized emission of orbiting hot-spots near Sagittarius A*: effects of electromagnetic interaction

We investigate the polarimetric signatures of orbiting hot-spots around a Schwarzschild black hole in the presence of an external magnetic field, accounting for the electromagnetic interaction between the charged emitter and the field. Using a general-relativistic model that incorporates synchrotron emission and ray-tracing of light propagation, we analyze how the electromagnetic interaction parameter modifies the observed polarization patterns, with particular emphasis on the behavior of the electric vector position angle (EVPA) and the time-evolving polarization loops in the $Q$-$U$ plane. Applying the model to millimeter wavelength ALMA observations of Sagittarius~A*, we explore the parameter space that best reproduces the asymmetry, time ratio, and area ratio of the observed polarization loops. We find that the inclusion of a small positive interaction parameter increases the symmetry of the loops and
the time ratio, while a negative magnetic parameter introduces strong asymmetry and fails to reproduce the data. Our results indicate that electromagnetic interaction can lead to ambiguity in the estimation of the system parameters such as orbital inclination or hot-spot velocity.

Speaker: Abylaikhan Tlemissov (Silesian University in Opava)

15:00 → 15:25 Coffee break

15:25 → 15:45 X-ray polarimetry in Microquasars

X-ray polarimetry provides a powerful new window into the geometry and physical conditions of the innermost regions of microquasars. I will present recent results from the IXPE mission that have revealed the X-ray polarimetric properties of accreting compact objects across different accretion states. These polarimetric signatures offer direct insight into scattering environments and emission mechanisms near the event horizon, placing constraints on coronal geometry, its optical depth, system inclination, and potential misalignment with the binary orbit. I will also discuss how complementary polarimetric observations in the optical and radio bands contribute to a unified, multiwavelength view of accretion and ejection in microquasars. Case studies of key black hole systems will illustrate how X-ray polarimetry is reshaping our understanding of relativistic accretion flows.

Speaker: Jiri Svoboda (Czech Academy of Sciences)

15:45 → 16:05 Constraining the black bole spin, inclination and orientation of GRS 1739−278 with IXPE

I will present an analysis of the recent IXPE observation of the black hole X-ray binary GRS 1739–278 during its current mini-outburst. The 2–8 keV spectrum is dominated by thermal disc emission with a contribution of thermal reflection component interpreted as caused by the disc self-irradiation. The hard Comptonized contribution is found to be negligible. The IXPE data show an averaged polarization degree of PD=(2.3±0.4)% and polarization angle of PA=62°±5° in the 2–8 keV range. The model-independent PCUBE analysis reveals that PD increases from ∼2% at 2 keV to ∼10% in the highest 6–8 keV bin, while PA remains constant across the IXPE band within statistical uncertainties. Modelling with Novikov-Thorne standard disc including the disc self-radiation indicates that high-spin configurations enhance the contribution of the reflected returning thermal radiation, which dominates the observed polarization properties. We infer an extreme black hole spin of a≃0.994 and a system inclination of θ≃54°. Owing to the large contribution from the reflected returning radiation, the observed polarization direction is nearly parallel to the projected system axis, the position angle of which is predicted to be at ∼58° east of north.

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

16:05 → 16:25 UV and X-ray variability of 1H 0323+342: an X-ray source in outburst as an interesting candidate for multi-band monitoring?

I present ultraviolet, X-ray, and gamma-ray analyses of Swift and Fermi-LAT observations of the narrow-line Seyfert 1 galaxy 1H~0323+342, obtained between late September and mid-October 2025 (MJDs 60939.977–60961.620). The source underwent a recent outburst (ATel 17411), which prompted a multiwavelength investigation. The main goal of this work is to study its X-ray spectral evolution in connection with ultraviolet magnitudes and flux densities, gamma-ray emission, and variability amplitude, in order to explore the interplay between the accretion disk, corona, and relativistic jet. Furthermore, through ultraviolet extrapolation, I assess its suitability as a target for upcoming UV space missions, with particular emphasis on the Czech UV mission QUVIK.

Speaker: Valentina Rosa (Astronomical Institute of the Czech Academy of Sciences)

16:25 → 16:40 Conference Photo

18:00 → 23:00 Conference Dinner

Wednesday, 12 November

10:00 → 10:20 Visualizing 3D Simulations Data with Julia: A Scalable and Efficient Approach

I present an efficient approach to visualizing 3D numerical simulation results using the Julia programming language, highlighting Julia’s growing ecosystem as a powerful tool for modern computational astrophysics. Leveraging Julia's high-level syntax, speed, and packages such as HDF5.jl, GLMakie.jl, and Meshes.jl, using an example from state-of-the-art simulation of a magnetized torus around supermassive black hole with Athena++ code, I demonstrate a workflow for interactive and scalable visualization of 3D simulations data. The presented suite enables intuitive data exploration of magnetic loops and flux ropes, while maintaining computational efficiency, which is essential in work with large-scale simulations. I discuss key implementation details, performance details and potential extensions for real-time analysis.

Speaker: Mr Raman Kumar (SGMK, Torun)

10:20 → 10:40 Star-disk magnetospheric interaction in 3D with tilted stellar field

I present a simple setup for full 3D MHD numerical simulations of star-disk interaction with tilted stellar field. The presented setup is done with publicly available PLUTO code and can serve multiple roles: as a teaching and visualization tool, but also as a serious simulations tool. I show the first results with a cold, thin accretion disk around a magnetized star.

Speaker: Miljenko Cemeljic (SGMK, CAMK, SU, ASIAA)

10:40 → 11:00 Self-similar accretion flows

Many analytic solutions in astrophysics have been found by assuming certain symmetries. A classical example in accretion disk theory is the natural assumption of axial symmetry of the flow, which reduces the dimensionality of the problem from three to two. Additional assumptions -- often physically motivated rather than mathematically correct -- are still required to eliminate one more dimension and reduce the system to a one-dimensional problem that is relatively easy to solve. A typical approach is to assume strict vertical hydrostatic equilibrium, which allows integration of the fluid equations in the vertical direction. In this way, the originally two-dimensional problem is replaced by a one-dimensional one that may (or may not) retain many of the important properties of the former.

In this talk, we will consider a different approach based on assuming another type of symmetry: scaling invariance, which leads to self-similar solutions. After reviewing several existing examples of such solutions, I will focus on a classical case -- a cold, geometrically thin, and optically thick disk. I will show in which respects the structure of such a self-similar flow agrees with and differs from the classical Shakura–Sunyaev solution. The nature of the self-similar solution does not allow its application near compact objects, where general relativity -- naturally breaking scaling invariance -- plays an essential role. Nevertheless, I will briefly outline a perturbative method that may help overcome these difficulties.

Speaker: Jiri Horak (Astronomical Institute ASCR, Prague)

11:00 → 11:40 Coffee break

11:40 → 12:30 Charged particle dynamic in black hole magnetosphere - habilitation

Extreme conditions in the magnetosphere of black holes cause a variety of interesting phenomena, that are subject to intensive studies in modern multi-messenger astrophysics. The central theme of this presentation is the analytical and numerical study of various high-energy radiative processes in the combined gravitational and electromagnetic fields, including the synchrotron radiation reaction and bremsstrahlung of charged particles moving in curved spacetime, scattering and decay of particles in the close vicinity of black holes, black hole energy extraction, and the treatment of chaos in the dynamical environment of black holes. The results of our theoretical studies can make fundamental contributions to understanding various currently investigated phenomena, such as relativistic jets, ultra-high-energy cosmic rays, black hole accretion disks, quasi-periodic oscillations, and others, where general relativistic effects with electromagnetic contributions are important.

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

12:30 → 14:00 Lunch break

14:00 → 14:20 Black hole jet reactivation through magnetic polarity reversal in GRMHD simulations

Polarity inversion events of black hole jets have been proposed to explain state transitions of X-ray binaries or certain changing-look AGNs. Such events can be triggered by magnetic fields of opposite polarity being accreted onto the black hole, causing the jet to shut down and reform with reversed polarity. To investigate this, we conducted axisymmetric general relativistic magnetohydrodynamic (GRMHD) simulations of an accretion torus initially threaded by two magnetic field components of opposite polarity, with the polarity inversion occurring at distinct radial locations. In the early stages, the black hole produces a relativistic Blandford–Znajek jet powered by accretion, which is subsequently quenched and disrupted, followed by the re-emergence of a jet with reversed magnetic polarity as accretion continues.

Speaker: Wen Xuan Sia (Nicolaus Copernicus Astronomical Center (CAMK))

14:20 → 14:40 The SANE, the MAD, and the Magnetized: GRRMHD Tales of Compact Objects

I will present the results of simulations comparing accretion onto a low-mass black hole with accretion onto a neutron star of the same mass. Even when both are initialized with the same conditions, the simulations for the black hole are significantly different from those for the neutron star. In the case of the black hole, the accretion disk becomes magnetically arrested, resulting in lower luminosity.
The result is: Black holes do not eat and they get MAD if you force them to.

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

14:40 → 15:00 Simulations of accretion on compact star: The effect of compactness

I will present results of numerical simulations of accretion on a magnetised star with variable compactness and surface magnetic field strenght.

Speaker: Fatemeh Kayanikhoo (CAMK)

15:00 → 15:30 Coffee break

15:30 → 15:50 Exploring the Dynamics of Magnetically Arrested Disks: The Role of Radiative Cooling

Accretion disks are essential for understanding the dynamics of gas around black holes. The magnetically arrested disk (MAD) state, where the magnetic flux near the event horizon becomes saturated, has garnered significant attention following observations of supermassive black holes in M87 and Sagittarius A by the Event Horizon Telescope (EHT) collaboration, which suggest that this is the preferred accretion state for such systems. In particular, low-luminosity systems like Sagittarius A are significantly influenced by radiative cooling processes, which profoundly affect the thermal, magnetic, and dynamical properties of the accretion disk. In this talk, I will describe how radiative cooling impacts the structure and behavior of MADs at sub-Eddington accretion rates. We analytically identify a critical mass accretion rate below which synchrotron radiation becomes a dominant cooling mechanism, altering the disk's thermal equilibrium and the MAD parameter. Using general relativistic magnetohydrodynamic (GRMHD) simulations from our massively parallel code cuHARM, I will explore how these cooling effects influence force balance, magnetic saturation, and jet efficiency for a range of black hole spins and accretion rates.

Speaker: Akshay Singh (Bar-Ilan University)

15:50 → 16:10 Effects of Strong Gravity on the Warm Corona in AGN

We explore the inner regions of an accretion flow in Active Galactic Nuclei (AGN), where we consider a warm corona to be on top of the accretion disk, acting like a disk atmosphere. The warm corona is assumed to be dissipative, and it is illuminated by the external X-ray lamp at the surface and by the cold disk flux at the bottom. We study this scenario by radially stratifying the accretion disk atmosphere and computing the vertical structure at each radial point using the photoionization code TITAN. Then we feed the whole radial structure into the ray-tracing code GYOTO, where we include the relativistic effects on the emission from these inner regions of the accretion disk atmosphere. This study helps us to compute the spectral emission and features that can be studied from the inner region of the accretion disk. The existence of the warm corona shows the deviation of the emissivity radial profile from that of a bare standard disk. This also shows that the accretion disk atmosphere can be hot, emitting highly ionized Fe Kɑ line that are subjected to relativistic corrections. The main aim of this study is to investigate the effect of these lines and their structural changes with the change in spin of the supermassive black hole and the viewing angle of an observer in the regime of strong gravity.

Speaker: Parikshit Partha Biswas (Nicolaus Copernicus Astronomical Center, Warsaw)

16:10 → 16:30 Magnetic vs. Relativistic Disk Precession Models for QPOs in PULXs

Pulsating Ultraluminous X-ray Sources (PULXs) are extragalactic off-nuclear, point-like objects with extremely high X-ray luminosities that show coherent pulsations typical for neutron stars. Only three such candidates have been reported to show quasi-periodic oscillations (QPOs) in the mHz range to date. We review two of the leading models that describe the QPOs from these sources. In the case of strong-gravity frame-dragging models, we demonstrate that they are unlikely to explain the QPOs in most scenarios. Alternatively, applying Dong Lai’s magnetic precession model, we derive a simple formula that links the QPO and pulsation frequencies of their system. Notably, both the magnetic field and the accretion rate are eliminated from the formula, making it broadly applicable across sources. Our model accurately reproduces the trend between QPO frequency and pulsation frequency to within a factor of order unity that depends on the accretion geometry.

Speaker: Sukalpa Kundu (Szkoła Główna Mikołaja Kopernika / CAMK PAN Warsaw)

Thursday, 13 November

10:00 → 10:20 Towards general relativistic hydrodynamic modelling of tidal disruption events

If a star travels close enough to a massive compact object can cause its disruption due to strong tidal forces overcoming the stellar self-gravity. This scenario results into a bright, characteristic electromagnetic signature known as a tidal disruption event (TDE). The numerical modelling of such phenomena is extremely challenging due to the extreme conditions that the star is subject to, e.g. strong tidal stretching and compression, as well as shocks, all in the vicinity of a massive black hole. In this work, we present our efforts for developing a module for the code SPHINCS that allows performing global hydrodynamic simulations of stellar disruptions in a Kerr spacetime. The code simulates the fluid under the General Relativistic Smoothed-Particle Hydrodynamics approach. We will present the implementation, validation tests, and first applications.

Speaker: Dr Diego Calderón (Max Planck Institute for Astrophysics)

10:20 → 10:40 How does magnetospheric propagation affects the polarised radio emission from pulsars?

Starting with the current understanding of radio pulsar emission, we investigate the polarization resolved pulse profiles of radio pulsar PSR J1900–2600 observed by the MeerKAT Telescope at 1280 and 816 MHz. We apply three novel models: Birefringent filtering, Modified Partial Coherence and Mode Coupling to explain depolarization, mode conversion (emergence of circular polarization) and deviation of polarization position angle (PA) tracks from the Rotating Vector Model. The first model attributes observed polarization features to birefringent filtering by an intervening magnetized plasma screen. Second considers the partial coherence combination of the two orthogonal propagation modes (OPMs) and lastly, the third model incorporates the effect of interdependence of the two OPMs and effect of the polarisation-limiting radius. We evaluate the ability of each model and their possible combinations to explain the observed polarisation characteristics. Our goal is to introduce novel pulsar data analysis methods and identify the dominant physical mechanisms shaping the polarised emission in radio pulsars.

Speaker: Piyush Marmat (Nicolaus Copernicus Superior School (SGMK) and NCAC-PAN, Torun, Poland)

10:40 → 11:00 A simulation of planets in highly relativistic pulsar winds

We investigated a novel pulsar planet detection method based on radio emissions produced by the interaction of a terrestrial planet with the pulsar wind.
Using the PLUTO code, we simulate relativistic magnetohydrodynamics of an Earth-sized planet surrounded by highly relativistic pulsar wind. We achieved a Lorentz factor of approximately 5.8 or a pulsar wind speed of approximately 98.5% of the speed of light in our simulation.
We consider two distinct planetary response regimes: with the planet modeled as a perfect conductor and a ferromagnetic scenario, where the external magnetic field magnetizes the planet in turn.
We examine the effects of changes in pulsar wind densities and external magnetic field strengths on the radio emission characteristics and compare the results with previous studies at lower velocity. The resulting emissions show characteristics similar to those of Alfven wing structures, in which wing-like disturbances in the flow are produced by the interaction of a conducting barrier with a magnetized plasma. We evaluated whether currently known pulsar planets can be found by their radioemissions. For this, we consider two planet classes: terrestrial and diamond planets. We evaluated whether these planets could be detected with current radio telescopes.

Speaker: Tanja Kaister (CAMK)

11:00 → 11:30 Coffee break

11:30 → 12:10 prof. Rueda - keynote

keynote

Speaker: Prof. Jorge Rueda (ICRA)

12:10 → 12:30 prof. Abramowicz - keynote

abstract

Speaker: Marek Abramowicz (Silesian University, Opava)

12:30 → 14:00 Lunch break

14:00 → 14:20 Combining machine learning with recurrence analysis for resonance detection

The width of a resonance in a nearly integrable system can tell us how a perturbation parameter is driving the system away from integrability. Although the tool that we are presenting here is quite generic, and can be used in a variety of systems, our particular interest lies in binary compact object systems known as extreme mass ratio inspirals (EMRIs), when a lighter compact object, like a black hole or a neutron star, inspirals into a supermassive black hole due to gravitational radiation reaction. During this inspiral, the lighter object crosses resonances, which are still not very well modeled. Finding resonances in EMRI models in EMRI models is critical to incorporate them in waveform models. To tackle this issue in our study, we show first that recurrence quantifiers of orbits carry imprints of resonant behavior. As a next step, we apply a long short-term memory machine learning architecture to automate the resonance detection procedure. Our analysis is developed on a simple standard map and gradually we extend it to more complicated systems, until finally we employ it in a generic deformed Kerr spacetime known in the literature as the Johannsen-Psaltis spacetime.

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

14:20 → 14:40 Deterministic and Stochastic Study of the X-ray Emission from the TeV Blazar Mrk 421

We present a comprehensive timing analysis of 50 X-ray light curves of the blazar Mrk 421, obtained over 17 years with the {\it XMM-Newton} satellite. Using both deterministic and stochastic approaches, we investigate the multi-scale nature of its variability. Deterministic dynamics are examined through an adaptive implementation of Recurrence Quantification Analysis (RQA), where systematically varied thresholds reveal scale-dependent structures in the emission. In parallel, autoregressive moving average (ARMA) and autoregressive integrated moving average (ARIMA) models characterize stochastic behavior across temporal scales.
Our results indicate that Mrk 421’s X-ray emission arises from a complex interplay of deterministic and stochastic processes. Deterministic patterns are localized at smaller temporal scales, while stochastic fluctuations propagate across both scales. Noise contributes at all scales, as shown by correlation analysis. This framework not only clarifies the physical processes driving the blazar’s X-ray variability but also demonstrates the potential of adaptive RQA for studying multi-scale behavior in other astrophysical sources.

Speaker: Radim Pánis

14:40 → 15:00 Impact of Cosmic-Ray streaming on the dynamics of magnetic-field in galaxies

We present a new module to the multi–fluid, grid based MHD code PIERNIK, which is based on the HLLD Riemann solver . The original scheme has been extended by an addition of interacting cosmic rays (CR) . We use the two-moment approach by Jiang et.al. (Astrophys.J. 854 (2018) 1, 5) to implement the dynamics of the cosmic ray fluid. In this work we present the implementation and validation of the new module as well as its application to several relevant astrophysical test problems. This extension enables us to study CR-driven dynamics and MHD dynamos in galactic disks, where the interplay of streaming and diffusion of CR regulates gas flows and magnetic-field amplification.

Speaker: Vinod Viswambharan Pisharody (Szkoła Główna Mikołaja Kopernika (SGMK))

15:00 → 15:20 Blueberries and Green Peas as Local Analogues of Early Galaxies

Most galaxies in the early Universe were compact, intensely star-forming dwarf galaxies whose radiation helped reionise the Universe. One of the best local analogues of these high-redshift galaxies are low-mass, low-metallicity star-forming dwarf galaxies, in particular Green Peas and Blueberries. Recent X-ray and radio observations reveal a broad range of luminosities within these populations. While many sources appear underluminous, some exhibit characteristics that may be consistent with low-luminosity active galactic nuclei. Multiwavelength observations are essential for distinguishing between emission driven by star formation and by possible AGN activity, offering new clues for interpreting these signatures in both nearby and high-redshift systems.

Speaker: Barbora Adamcová (Astronomical Institute of the Czech Academy of Sciences)

15:20 → 15:50 Coffee break

15:50 → 16:30 Dark matter versus Baryonic matter Black Holes

Keynote - Dark matter versus Baryonic matter Black Holes

Speaker: Prof. Remo Ruffini (ICRANet)