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 Coffee & Biscuits

10:30 Registration

1 Opening

Speaker: Prof. Zdeněk Stuchlík (SLU)

2 Zdenek Stuchlík - Keynote

Keynote

Speaker: Prof. Zdeněk Stuchlík (SLU)

14:00 Coffee break

3 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

4 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

15:20 Coffee break

5 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)

6 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)

Tuesday, 11 November

10:50 Coffee break

7 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)

12:30 Lunch break

8 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)

9 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)

16:10 Conference Photo

18:00 Conference Dinner

Wednesday, 12 November

10 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.)

10:50 Coffee break

11 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 Lunch break

Thursday, 13 November

10:50 Coffee break

12 John Miller Keynote

Keynote

Speaker: John Miller (University of Oxford)

12:30 Lunch break

13 Dark matter versus Baryonic matter Black Holes

Keynote - Dark matter versus Baryonic matter Black Holes

Speaker: Prof. Remo Ruffini (ICRANet)

14 prof. Rueda - keynote

keynote

Speaker: Prof. Jorge Rueda (ICRA)