RAGtime 24

10 Oct 2022, 09:00 → 14 Oct 2022, 19:25 Europe/Prague

Opava

Gabriel Török (Silesian University in Opava), Zdeněk Stuchlík

Venue

The 24nd RAGtime workshop will be held in a hybrid form in Opava (10-14 October 2022). 

The registration is limited to 70 on site participants, the registration in moderated. Please wait with your travel plans until you receive the registration confirmation. Registartion connected to submitted abstract will have priority.

The conference is free of charge. 

There will be at maximum 10 online talks accepted, the conference will be streamed on Zoom. 

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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. 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.

The EXPRO meeting

The EXPRO group meeting will be hold as a part of the workshop on Monday, October 10th.

 

 

binarni systemy_plakat_FINAL kopie kopie.pdf

closing.pptx

poster_comp.pdf

UNISFERA.pdf

Monday, 10 October

09:00 → 14:45 Registration

10:00 → 10:15 Opening

10:15 → 10:55 The Penrose process: its variants and their role in astrophysics

The basic adea of the Penrose process is given and its variants are introduced along with describing the role in astrophysical phenomena. These phenomena illustrate interplay of the regular and chaotic motion allowed for ionized matter. We focus on demonstration of possible occurence of ionized Keplerian disks orbiting a magnetized black hole and the situations giving rise to interesting astrophysical phenomena, occuring due to relevant variants of the Penrose process. In dependence on the intensity of the electromagnetic interaction of slightly charged matter of the orbiting disk with the external magnetic field, there are three possible outcomes of the ionized disks, namely, creation of quasiperiodic epicyclic oscillations of the orbiting matter for weak intensity, creation of winds for mediate intensity, and, due to the strong Magnetic Penrose process, creation of jets and highly ultra-relativistic particles for large intensity of the interaction. In active galactic nuclei the accelerated particles can obtain, due to the Magnetic Penrose process, energy corresponding to the most energetic particles observed in cosmic rays. These processes can occur in the effective ergosphere of the magnetized black hole. Highly accelerated particles could be, however, created also due to the Electric Penrose process where no rotation of the black hole is involved. Of potentially high astrophysical interest is the Radiative Penrose process of acceleration of charged particles due to their synchrotron radiation inside the black hole ergosphere that can occur for counter-rotating radiating particles due to concentration of photons with negative energy relative to observers at infinity at the direction of their motion. Such an effect can be important at the ergosphere boundary, where the energy increase of the accelerated ionized particle can reach one order.

Speaker: Zdeněk Stuchlík

10:55 → 11:25 Coffee Break

11:25 → 12:55 Primodial Black Holes

Convener: John Miller (University of Oxford)

11:25 Primordial black holes and the gravitons cosmological background

Minuscule primordial black holes produced just after inflation can serve as time capsules" bringing back energy from the past to a later epoch when they evaporate. As these black holes behave like matter, while the rest of the Universe content behaves like radiation, the mass fraction of these black holes, that is tiny at formation, becomes significant later. If sufficiently small, these black holes will evaporate while the Universe is still radiation dominated. I discuss this process and in particular the prospect that BH evaporation can producedark radiation" in the form of gravitons that is homogeneous and free of Silk damping. With suitable but extreme parameters, this background graviton field can resolve the Hubble tension. I contrast this idea with another mechanism for the formation of gravitons background field, annihilation of particles at the Planck era and discuss possible ways to distinguish between the two backgrounds.

Speaker: Tsvi Piran (Racah Institute of Physics, The Hebrew University, Jerusalem)

12:05 A Robust Test of the Existence of Primordial Black Holes in Galactic Dark Matter Halos

We argue that if in the next few years LIGO-Virgo-KAGRA interferometers would not detect a sufficient number of events involving light black holes, i.e. with masses smaller than 1.5 solar mass, this would be an unambiguous proof that the primordial black holes in the asteroid mass range are NOT present in the dark matter halo of the Galaxy.

Speaker: Marek Abramowicz (Silesian University, Opava)

12:25 Formation of primordial black holes during the QCD phase transition

The formation of Primordial black holes is naturally enhanced during the quark-hadron phase transition, because of the softening of the equation of state: at a scale between 1 and 3 solar masses, the threshold is reduced of about 10% with a corresponding abundance of primordial black significantly increased by more than 100 times. We show that a sub-population of primordial black black holes formed in the solar mass range is compatible with the current observational constraint and could explain some of the interesting sources emitting gravitational waves detected by LIGO/VIRGO in the black hole mass gap, such as GW190814, and other light events.

Speaker: Ilia Musco (INFN, La Sapienza University off Rome)

Musco RAGTIME24.pdf

12:55 → 14:45 Lunch Break

14:45 → 16:10 Processes in Strong Gravitational Field

Convener: Vladimír Karas

14:45 Accretion states in active galactic nuclei

Active Galactic Nuclei (AGN) are powered by accretion onto a super-massive black hole. Despite of different mass and size-scale, the accretion processes might be similar to accretion on stellar-mass black holes in X-ray Binaries (XRBs). In this talk, I will review the observational studies aimed at comparison of accretion in AGN and XRB.

Speaker: Jiří Svoboda (Czech Academy of Sciences)

15:25 Effects of relativistic precession on tidal-disruption-event light curves

Most galaxies harbour a super-massive black hole (SMBH) in their centre but deriving its physical properties is not straightforward, especially if it is in quiescent state. If a star falls inside the tidal radius of the black hole its disruption can set on the accretion, which produces a distinctive observational signature. However, for characterising the SMBH from the observed tidal disruption event (TDE) it is necessary to understand the physical mechanisms acting and their impact on the observational light curve. In this work, we study that the effect that general relativistic precession of the tidal stream of the disrupted star imprints on the observed light curves of tidal disruption events. We conduct moving-mesh radiation-hydrodynamic simulations of an isotropic wind and irradiation that follows the analytical TDE fallback rate interacting with the stream of the disrupted star after winding around the SMBH before triggering the event. We explore the impact of the SMBH mass, accretion efficiency parameter $\eta$, and line-of-sight angle of the event. We present first results and effects on the observed light curves.

Speaker: Diego Calderón (Charles University)

15:45 Bremsstrahlung of the charged particle in the vicinity of the weakly charged Schwarzschild black hole

The radiation reaction force acting on charged particle cannot be neglected in many astrophysically relevant scenarios. The charge of a black hole is negligible in astrophysically scenarios due to the unrealistically large charge values required for the Reissner-Nordström spacetime metric. However, such kind of dilemma is not valid for black holes carrying small electric charge due to numerous mechanisms of selective accretion. The Lorenz-Dirac (LD) equation describes the motion of a point charge with radiation reaction in flat spacetime, while the DeWitt-Brehme(DWB) equation - in curved spacetime. We use DWB equation simplified to the covariant form of the LD equation for the motion of charged particles in the vicinity of the weakly charged Schwarzschild black hole. We study depending on sign of Coulomb force charged particle spirals down to the black hole or stabilizes the circular orbit.

Speaker: Bakhtinur Juraev

16:10 → 16:40 Coffee Break

16:40 → 18:55 EXPRO meeting

A meeting of the EXPRO group members from Opava and Prague. Closed
Uni Space Room

16:40 → 18:55 Solutions for Compact Objects in GR

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

16:40 An 'exact' modified Tolman VII solution

The Tolman VII (TVII) solution is considered by some as one of the few analytical solutions to Einstein's equations, which describes approximately well the interior of neutron stars (NSs). This solution is characterized by the mass $M$, radius $R$, and an energy density that varies quadratically with the radial coordinate $r$. Recently, Jiang and Yagi proposed a modification of this solution, the so-called modified Tolman VII (MTVII) solution, by introducing an additional quartic term to the energy density radial profile. The MTVII solution is an approximate solution to Einstein's equation, which includes a new parameter $\alpha$ that allows the solution to have a better agreement with the energy density profiles for realistic NSs. Here we consider the MTVII solution, showing that for certain values of the parameter $\alpha$ and compactness $\mathcal{C}$ this solution manifests a region of negative pressure near the surface which leads to negative values of the tidal Love number. To alleviate these drawbacks, we introduce an improved version of the MTVII solution obtained by solving numerically Einstein's equations for the MTVII energy density profile. As an application of our new 'exact' MTVII (EMTVII) solution, we calculate the tidal Love number and tidal deformability, as a function of $\mathcal{C}$, for different values of the parameter $\alpha$. We find that the EMTVII solution predicts a positive tidal Love number for the whole range of allowed values of parameters $(\mathcal{C},\alpha)$, in agreement with previous results for realistic NSs.

Speaker: Camilo Posada

emt.pdf

17:05 Singularities in general relativity: regular black holes without Cauchy horizon

General relativity predicts the loss of causal structure of spacetime. This is clearly seen with the appearance of singularities in the gravitational collapse (Penrose 65), which when removed lead to the appearance of a Cauchy horizon, a null hyper-surface beyond which predictability breaks down (Poisson-Israel 89). So far no counterexample to the above is known, leading to the belief that asymptotically flat regular black holes lacking a Cauchy horizon do not exist. In this talk, contrary to the general belief, we will show that such solutions do exist.

Speaker: Jorge Ovalle (Institute of Physics, Silesian University in Opava)

17:45 Can Sagittarius A* be a PeVatron of charged particles?

A compact supermassive source Sagittarius A* located at the centre of our Galaxy has been observed at different wavelengths across the electromagnetic spectrum, being currently the best known black hole candidate. It is also the closest and largest in projection supermassive black hole candidate. At the same time, its particle acceleration capability related cosmic ray and neutrino messengers were not yet experimentally probed. In this talk, I present several scenarios of particle acceleration by rotating black hole, involving electromagnetic energy extraction mechanisms and their observational signatures in the case of the Galactic centre.

Speaker: Arman Tursunov (Silesian University in Opava)

19:00 → 20:00 Public talk: M87* and Sgr A*: Imaging supermassive black holes

I will briefly discuss how the first images of the supermassive black holes M87 and Sgr A were obtained by the EHT collaboration. In particular, I will describe the theoretical aspects that have allowed us to model the dynamics of the plasma accreting onto the black hole and how such dynamics was used to generate synthetic black-hole images. I will also illustrate how the comparison between the theoretical images and the observations on a broad range of frequencies has allowed us to deduce the presence of supermassive black holes and to extract information about the accretion process. Finally, I will describe the lessons we have learned about strong-field gravity and alternatives to black holes.

Speaker: Prof. Luciano Rezzolla (Goethe University of Frankfurt)

Opava_SMBHs_Rezzolla.pdf

Tuesday, 11 October

09:00 → 10:40 Galactic Center

Convener: Luciano Rezzolla (Goethe University of Frankfurt)

09:00 Modelling the flares of Sagittarius A*

For the past two decades, flares (i.e. outbursts of radiation) have been observed from the centre of the Milky Way where a massive compact object of 4.3 millions solar masses resides at only 8.3 kpc. This makes this object called Sgr A the closest supermassive black hole candidate to Earth and an unique laboratory for relativistic astrophysics. Recent observations have shown that the source of these outbursts is close to the event horizon and has an orbital motion around the black hole. Many scenarios are envisaged to explain this phenomenon without reaching a consensus. Among these scenarios, magnetic reconnection is one of the most promising, supported by many GRMHD and PIC studies. During this presentation I will present a large plasmoid model based on magnetic reconnection results with
a kinetic approch. I will examine the diversity of observables associated to these models and discuss them in the light of the recent VLTI/GRAVITY observations of Sgr A flares.

Speaker: Nicolas Aimar (LESIA - Observatoire de Paris)

09:20 Observing hot spots orbiting Sagittarius A* with ALMA

We report on a first detection of an orbiting hot spot in ALMA polarimetric light curves of Sagittarius A*, manifesting in the loop-like patterns on the linear polarization Q-U plane following the X-ray flare detected by Chandra. The observed hot spot period corresponds to about 70 minutes, and the implied orbital radius corresponds to about 5 Schwarzschild radii. These new observations constrain magnetic field to be predominantly vertical and the viewing angle of the hot spot orbit to be close to face-on. We discuss the impact of black hole spin, hot spot motion Keplerianity, and other model parameters on the interpretation of these results.

Speaker: Maciek Wielgus (Black Hole Initiative, Harvard)

10:00 Dust-enshrouded Objects in the Galactic Center: Stars, Clouds or Something More Exotic?

I will summarize our current understanding of dust-enshrouded sources that are observed in the S cluster around the supermassive black hole (SMBH). Currently, we observe a population of about 10 sources that share common characteristics: a prominent near-infrared excess and rather broad emission lines (Br-gamma, HeI). These sources are quite distinct from the majority of the surrounding B-type main-sequence stars. The most intensively monitored source has been G2/DSO moving on a highly eccentric orbit. I will discuss potential formation channels of these sources: dust-enshrouded young stars, binary merger products, or other, more exotic scenarios. I will analyze how their appearance and the evolution depend on the stellar wind properties and the distance from the SMBH.

Speaker: Michal Zajaček (Masaryk University)

10:20 Alternative theories of gravity and observed flares from Sgr A*

We are fitting dynamics of electrically neutral hot-spot orbiting around Sagittarius A* source in Galactic centre, represented by various modifications of the standard Kerr black hole, to the three flares observed by the GRAVITY instrument. We consider stationary, axisymmetric, and asymptotically flat spacetimes describing charged black holes in general relativity combined with nonlinear electrodynamics, or reflecting the influence of dark matter, or in so-called parameterized dirty Kerr spacetimes, and test them using the hot-spot data.

Speaker: Misbah Shahzadi (COMSATS University Islamabad, Pakistan)

RAGtime2022_Misbah-3.pdf

10:40 → 11:10 Coffee Break

11:10 → 12:30 Black Holes and Magnetic Field

Convener: Romana Mikusincova (Universita degli Studi Roma Tre)

11:10 Astrophysical black holes embedded in organized magnetic fields

Large scale magnetic fields pervade the cosmic environment and the astrophysical black holes are often embedded and influenced by the mutual interaction. We will outline the appropriate mathematical framework to describe magnetized black holes within General Relativity and we will show several examples how these can be employed in the astrophysical context.

Speaker: Vladimír Karas

karas-RAG24-2022.pdf

11:50 Parabolic black hole magnetosphere and charged particle dynamic

The study of charged test particle dynamic in the combined black hole gravitational field and magnetic field around it could provide important theoretical insight into astrophysical processes around such compact objects. We will examine simple but astrophysically relevant parabolic magnetic field configuration as a toy model for magnetosphere around Schwarzschild black hole. Bounded orbits and their stability will be examined using effective potential approach and the existence of stable charged off-equatorial structures close to axial symmetry axis will be proven. Influence of radiation reaction damping force on particle nonlinear chaotic dynamics will be tested.

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

RAGtime2022_Kolos.pdf

12:10 Charged non-conducting tori around Schwarzschild black hole immersed in parabolic magnetic field

We study charged non-conducting fluid tori orbiting in the background given by a Schwarzschild
black hole immersed in a parabolic magnetic field introduced in the context of the Blandford–Znajek
process. In our study we focus on the off-equatorial charged tori that could be related to creation
of jets due to the Blandford-Znajek process.

Speaker: Martin Blaschke (Institute of Physics, SLU)

12:30 → 14:30 Lunch Break

14:30 → 16:00 Neutron Stars

Convener: Maciek Wielgus (Black Hole Initiative, Harvard)

14:30 Binary Neutron Stars: from macroscopic collisions to microphysics

I will argue that if black holes represent one the most fascinating implications of Einstein's theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the gravitational signal can provide tight constraints on the equation of state and sound speed for matter at nuclear densities, as well as on one of the most important consequences of general relativity for compact stars: the existence of a maximum mass. Finally, I will discuss how the merger may lead to a phase transition from hadronic to quark matter. Such a process would lead to a signature in the post-merger gravitational-wave signal and open an observational window on the production of quark matter in the present Universe.

Speaker: Luciano Rezzolla (Goethe University of Frankfurt)

Opava_BNSs_Rezzolla.pdf

15:10 The power of weakly magnetized neutron stars

Weakly magnetized stars lead much more interesting and active lives than their more glamorous cousins (the magnetars and canonical TeraGauss radio and X-ray pulsars), accreting huge amounts of mass and angular momentum, emitting copious amounts of power in energetic particles and quanta, and destroying their companions. I will give a short overview of the highlights of their observed history, from the discovery of the first neutron star in 1963, through eclipsing ("black widow") pulsars and the maximum mass measured of 2.3 solar masses, to ultrahigh frequency X-ray modulation (kHz QPOs), and ultraluminous sources.

Speaker: Wlodek Kluzniak

15:40 Measuring mass and radius of neutron star using kHz QPOs.

We report our recent results on measuring mass and radius using kHz quasi-periodic oscillations. We focus on the mass and radius of 4U1608-52 where independent measurement comes from X-ray bursts. We show that the cusp-torus model is preferred over the relativistic precession model. The cusp torus model gives better fits to the observed frequencies and predicts values of mass and radius that are in better agreement with measurements from X-ray bursts.

Speaker: Martin Urbanec

16:00 → 16:30 Coffee Break

16:30 → 16:45 Conference photo

16:45 → 18:30 X-ray Variability

Convener: Michal Bursa (ASU)

16:45 Eccentric accretion flows: a simple model of the stationary structure and oscillations

We present a simple semi-analytic model of geometrically-thin accretion flows with significant eccentricity. The structure of the flow, including three-dimensional density profile and velocity field is described by three azimuthally-dependent quantities giving a thickness of the flow in the radial and vertical direction and radial distribution of the angular momentum. These quantities are a set of nonlinear ordinary differential equations. We show that the radial and vertical structure of the flow depart significantly from hydrostatic equilibrium that is commonly assumed in the case of circular flows. We also study oscillations of these flows and find both, the radial and vertical epicyclic modes, at frequencies corresponding to multiples of the flow orbital frequency. Indeed, the amplitude of oscillations change significantly with azimuth; the most of the variability occurs at apocenter.

Speaker: Jiří Horák (Astronomical Institute ASCR, Prague)

horak-eccentric-tori.pdf

17:25 The mass, spin and rapid X-ray variability of accreting compact objects

Following the previous research on epicyclic oscillations of accretion disks around black holes (BHs) and neutron stars (NSs), a new model of high-frequency quasi-periodic oscillations (QPOs) considering radial precession of accretion flow, has been proposed (CT model). Within the model the configuration of accreted fluid is approximated by marginally overflowing accretion tori predicted by general relativity (tori terminated by cusps).} According to preliminary investigations based on Kerr spacetimes, the model provides overall better fits of the NS QPO data compared to the relativistic precession (RP) model often considered in the context of NS and BH parameters estimations. It also implies a significantly higher upper limit on the Galactic microquasars BH spin. A short analytic formula has been noticed to well reproduce the model's predictions on the QPO frequencies in Schwarzschild spacetimes. We derive an extended version of this formula that applies to rotating compact objects and include effects given by NS oblateness. For a particular choice of a single parameter, our relation with high accuracy provides frequencies predicted by the CT model. For another value, it provides frequencies predicted by the RP model. Considering this relation along with the data of several atoll sources we find that the CT model is compatible with realistic values of the NS mass and provides better fits of data than the RP model.

Speaker: Gabriel Török (Silesian University in Opava)

18:05 Oscillations and precession of fluid accretion disks around neutron stars

In addition to the Keplerian frequency of orbital motion, the four characteristic oscillatory frequencies of the accretion flow are often discussed in the context of models of quasi-periodic oscillations (QPOs) observed in the neutron star (NS) low-mass X-ray binaries. These are, namely, the vertical and radial epicyclic frequencies, and the frequencies that correspond to the periastron (radial) precession and the Lense-Thirring (vertical) precession of the perturbed orbital motion. We examine the influence of quadrupole moment of a slowly rotating NS on the oscillations of the fluid flow in the innermost region of accretion disks. We apply previously developed analytical methods assuming the background of Hartle-Thorne geometry. We provide explicit formulae for the oscillation and precession frequencies but also their simplified practical versions that allow for an expeditious application of the universal relations determining NS properties. We apply our formulae for precession frequencies to different NS configurations and show that difference of the accretion disk precession frequencies for NSs of the same mass and angular momentum but different oblateness can reach tens of percent. Finally, we briefly discuss the relevance of our findings in the context of QPO models.

Speaker: Eva Šrámková

19:00 → 23:00 Conference dinner

Wednesday, 12 October

09:25 → 10:30 Neutron Stars

Convener: Maurizio Falanga

09:30 Aurora on planet around pulsar

Interaction of a planetary magnetosphere with the magnetic field
of its host star defines the atmospheric outer boundary conditions on the planet and modulates the radio emission from the system. Using state-of the-art numerical simulations, we investigate magnetospheric interaction of a planet orbiting different types of stars including pulsars.

Speaker: Miljenko Čemeljić (CAMK / ASIAA)

10:00 Can extended bodies follow geodesic orbits?

We shall discuss whether a pole-dipole-quadrupole body described by the Mathisson-Papapetrou-Dixon equations can move on geodesic trajectories in a black hole spacetime background. We shall use the Ohashi-Kyrian-Semerak spin supplementary condition to fix the centroid of the body, since under this condition the four-momentum can be set parallel to the four-velocity as holds for geodesic motion.

Speaker: Georgios Lukes-Gerakopoulos (Astronomical Institute, Czech Academy of Sciences)

Ragtime24_ggl.pdf

10:30 → 10:40 Director's award for student's publications in theoretical physics

10:40 → 11:10 Coffee Break

11:10 → 12:35 X-ray Observations

Convener: Martin Blaschke (Institute of Physics, SLU)

11:10 The X-ray polarization view of black hole system, Cyg X-1, in its hard state

Accreting stellar-mass black holes in X-ray binaries (XRBs) can be found in
different states that are characterized by different spectral properties.
The two basic states are high/soft and low/hard states in which either the
thermal multi-colour black-body component originating in the accretion disc or
non-thermal power-law like Comptonisation component originating in the hot
corona dominates.

The NASA Imaging X-ray Polarimetry Explorer (IXPE) mission has recently observed
one of the brightest persistent XRB, Cyg X-1, in its low/hard state. The 242ks
observation of polarization properties of this source in 2-8keV energy band
resulted in highly significant measurement of linear polarization. In this talk
I will summarize this observational result and discuss its implications for
the models of X-ray emission from this source.

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

Opava-2022_Dovciak.pdf

11:50 X-ray polarimetry as a tool to measure the black hole spin in microquasars: simulations of IXPE capabilities

Observing the x-ray black hole binary source GRS1915+105 has been classified as a top priority during the first year of IXPE operation. The primarily occurrence of the source in thermal state makes it the perfect candidate for black hole spin measurements. To credibly assess the source polarimetric properties, we use a multicolor black body emission model accounting for thermal radiation from the disk accretion complemented by self-irradiation of the accretion disk. We simulate the future observations of GRS1915+105 with IXPE and study the impact of a constant albedo on the polarization properties of the source. We demonstrate the capabilities of the mission and the precision of the black hole spin constraints. GRS1915+105 has been in an obscured state since 2018, and is therefore used as a test source in this study. However, the results obtained from our analysis are applicable to any bright source in thermal state.

Speaker: Romana Mikusincova (Universita degli Studi Roma Tre)

Mikusincova_RAGtime2022.pdf

12:10 GRRMHD simulation of sub-Eddington accretion onto stellar mass black hole

A widely accepted picture of an accretion flow in a soft spectral state X-ray binary system is a geometrically thin disk structure much alike the classic analytic solution of Shakura and Sunyaev. Despite the fact that the analytic models are troubled by instabilities and miss important aspects of physics such as magnetic fields, they are successfully used as a framework for interpreting observational data through continuum spectral fitting. We present results of general relativistic radiative magnetohydrodynamic simulations of sub-Eddington optically thick accretion on a stellar mass black hole with a mildly sub-Eddington luminosity. We find the accretion flow stabilized by magnetic field, with a puffed-up optically thick region, resembling a warm corona surrounding denser disk core. We analyze the inner structure and properties of the puffy disk and compare it with analytical models.

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

puffy.pptx

12:35 → 14:35 Lunch Break

14:35 → 16:20 Accretion Discs

Convener: Jiří Horák (Astronomical Institute ASCR, Prague)

14:35 Radial structure of thin accretion disks in a general metric

TBA

Speaker: Michal Bursa (ASU)

14:55 Indication of SMBH - IMBH binary: ultra-fast outflows from AGN

Based on the high cadence X-ray monitoring of a flare coming from the nucleus of a galaxy located cca 250 Mpc away and discovered by ASASSN in December 2020, the existence of ultra-fast outflows in the source was revealed. The outflows were quasiperiodically launched from near the SMBH with variable column density and ionization, but constant velocity, for more than three months. We interpret the observed phenomenon as outflows induced by an object periodically transiting through the accretion flow onto the SMBH. Such scenario is supported by our GRMHD simulations of stellar transits through accretion disc. The strength of the outflow suggests large influence radius of the perturber, hence indicating it to be an intermediate-mass black hole rather than a usual star.

Speaker: Petra Suková (ASU CAS)

15:15 Black widow pulsars and the maximum mass for neutron stars

A review will be presented of the idea of black widow pulsars and of recent literature which aims to give a lower limit for the maximum neutron star mass using observational data for these objects. Comparison will be made with other related work.

Speaker: John Miller (University of Oxford)

RAGtime24_jm.pptx

16:00 Accretion disk backflow in resistive MHD simulations

Backflow is a part of the flow in the accretion disk which is directed away from the star. Although it is well established in purely hydro-dynamical framework, it is only recently that it has been identified in magnetohydrodynamics (MHD) simulations. We perform resistive magnetohydrodynamics (MHD) simulations of an accretion disk with alpha-viscosity, accreting onto a slowly rotating star endowed with a magnetic dipole. We find backflow in the disk-midplane for a range of conditions when the Prandtl number is less than a critical value. Close to the critical Prandtl number value the backflow shows non-stationary behavior. We compare the results with hydrodynamics simulations. We find that in the MHD case the distance from the star at which backflow begins, the stagnation radius, is different than in the hydrodynamic case.

Speaker: Ruchi Mishra (CAMK)

RAGtime_Ruchi.pdf

16:20 → 16:50 Coffee Break

16:50 → 18:30 Observable Signatures of Compact Objects

Convener: Jorge Ovalle (Institute of Physics, Silesian University in Opava)

16:50 Post-merger outflows from magnetized, neutrino-cooled accretion disks and their contribution to the kilonovae emission

Kilonovae are transient events observed in Optical-Near IR band, powered by radioactive decay of elements produced through r-process nucleosynthesis. They form in the ejecta emitted in binary neutron star mergers. The kilonova signal may accompany the gravitational wave emission and potential gamma-ray burst signal, such as in the well known event discovered as GW-GRB 170817. I will discuss the kilonova phenomenon and present some results of the currently modeled outflows and kilonova lightcurves.

Speaker: Agnieszka Janiuk (CTP PAS)

17:30 Test of geodesics model of HF QPOs in the spacetime of black holes surrounded by the dark matter halo

Using the simple but robust model of a shell of dark matter (DM) around a Schwarzschild black hole (BH), represented by the mass ratio of the shell and BH $\mathrm{\Delta}M/M$, the shell extension $\Delta r_\mathrm{s}$ and its inner radius $r_\mathrm{s}$, we study the influence of DM on the spacetime structure and geodesic motion and provide a classification of the BH+DM shell spacetimes according to the properties of the stable circular geodesics governing Keplerian disks. We focus our attention on the epicyclic motion around circular geodesics that can be related to observational phenomena in X-ray radiation from Keplerian accretion disks, assumed to be influenced by the DM shell only gravitationally. We give the frequencies of the orbital and epicyclic motions and discuss their properties in terms of the parameters governing the DM shell. Using the frequencies in relevant variants of the standard geodesic model of high-frequency quasiperiodic oscillations (HF QPOs), we test the role of DM by fitting the HF QPO data from some microquasars and active galactic nuclei with supermassive BHs where no variant of the geodesic model applied in the standard vacuum BH background is able to explain the data. We thus provide a robust review of the applicability of the geodesic model of HF QPOs, and also provide limits on the amount of DM around a BH. We demonstrate that the geodesic model could be well applied to most observations of active galactic nuclei, with strong restrictions on the amount of invisible matter around BHs.

Speaker: Jaroslav Vrba

17:50 Black hole encircled by a thin disc

Disc structures around black holes are of great importance in astrophysics. The usual simplification of most accreting models is neglecting the disc's self-gravitation. However, in many situations, numerous disc properties may be highly sensitive to the gravitational field. We present a full metric describing the field of a thin disc and a Schwarzschild black hole. The disc density profiles are astrophysically realistic, stretching from the horizon to radial infinity, yet falling off quickly at both these locations. The main advantages of the solution are that (i) the discs have no edges, so their fields are everywhere regular (outside the horizon), and that (ii) both non-trivial metric functions are provided analytically and in closed forms. We examine and illustrate the basic properties of the black-hole—disc space-times.

Speaker: Petr Kotlařík (Institute of Theoretical Physics, Charles University)

ragtime22presentation.pdf

18:10 Dark Matter Searches Through Multi-Messenger Observations of Compact Stars

The purpose of this talk is discuss the possibility of detection of dark matter through multiple observations of compact stars and related phenomena. Recent scientific and technological developments have allowed for a better study of the nature of these astrophysical objects, in particular of the equation of state (EoS). As we advance on the quest for clarification of the neutron star internal content, we will be able to reveal or discard the existence of dark matter in the corresponding stellar environments. In one hand, new channels of multi-messenger observations like gravitational radiation from merger events of binary systems of compact stars or radio and X-ray signals from isolated pulsars have revealed their most basic structural properties like mass, radius, compactness, cooling rates and
compressibility of their matter. In the other hand, nuclear measurement and experiments have narrowed the EoS uncertainty in the lowest to intermediate density range. Importantly, there exist several types of violent, transient energetic emissions associated not only with the strong magnetic fields and extreme gravity in the proximity of these objects but with explosive, evolutionary stages often triggered by mass accretion from companion stars. Therefore, we expect that the presence of dark matter will leave an imprint in the many kinds of detected signals from compact stars.

Speaker: David Alvarez-Castillo (Institute of Nuclear Physics PAS)

Alvarez_RagTime_2022.pdf

Thursday, 13 October

09:00 → 10:45 Compact Objects: Theory and Astrophysics

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

09:00 Energetic and Optical Properties of Gravitational Compact Objects

Modern astronomical observations on the international level on the ground and space telescopes, and recent discoveries have provided convincing evidence that black holes have a significant impact on nearby objects, emitting powerful gamma-ray bursts, absorbing the next star, and stimulating the growth of newborn stars in the surrounding areas.
Study the motion of the photon around rotating black holes, in particular, the discovery and analysis of the form of silhouettes of these objects, setting and effective implementation of relevant radio astronomical observations on the proof of the existence of the black hole horizon and retrieval of information events on the central object in our galaxy within the Event Horizon Telescope (EHT) international project is one of the most important tasks of modern astrophysics.
Then gravitational lensing in the vicinity of a slowly rotating massive object surrounded by plasma has been studied. The obtained deflection angle of the light ray in the presence of plasma depends on (i) the frequency of the electromagnetic wave, due to the dispersion properties of the plasma; (ii) the gravitational mass M; and (iii) the angular momentum of the gravitational lens. We have studied photon motion around axially symmetric rotating (i) Kerr black hole, (ii) wormhole in the presence of a plasma with radial power-law density. It is shown that in the presence of a plasma, the observed shape and size of the shadow of rotating (i) Kerr black hole, (ii) wormhole changes depending on the (i) plasma parameters, (ii) gravitational object spin, and (iii) inclination angle between the observer plane and the axis of rotation of the black hole/wormhole.
Finally, I will discuss the end state of evolution of massive stars, and various observational properties of magnetized neutron stars. The energetics of rotating black holes and neutron stars is also in the scope of my talk.

Speaker: Bobomurat Ahmedov

09:40 Observational phenomena in axially symmetric spacetimes with non-negative cosmological constant

We study the general non-equatorial motion of photons in the field of the Kerr and Kerr~de-Sitter spacetimes and using the results of this analysis we propose a classification of these spacetimes, considering the structure of the event horizons, ergosphere and the spherical photon orbits (SPOs), that play crucial role for construction of the light escape cones and related shadows of the central gravitating body. For selected astrophysically relevant classes of the Kerr (-de Sitter) spacetimes we construct and compare the shadows of black holes, naked singularities and superspinars, which we assume to be located between the observer and bright starry background, as they appear for an observer in a different state of motion and with different radial and latitudinal coordinates. The existence of photons with negatively valued energy and their relevance for the radiative Penrose process in the ergoregion is outlined. In the Kerr case, we also mention the existence of a closed stable SPOs in region of possible occurence of thin Keplerian disc and discuss the possibility of some self-occult and self-illumination effects manifesting itself as periodic light echo.

Speaker: Daniel Charbulák

Observ_phenomena.pptx

10:15 Exact axially symmetric stationary electromagnetic field on Kerr black hole background in Debye potential formalism

We provide an explicit, closed and compact expression for the Debye superpotential of a circular source. This superpotential is obtained by integrating the Green function of Teukolsky Master Equation (TME). The Debye potential itself is then, for a particular configuration, calculated in the same manner as the $\phi_0$ field component is calculated from the Green function of the TME --- by convolution of the Green function with sources. This way we provide an exact field of charged ring and circular current on the Kerr background, finalizing thus the work of Linet.

Speaker: David Kofron (Institute of Theoretical Physics, Prague)

10:45 → 11:15 Coffee Break

11:15 → 12:40 Compact and Magnetized Stars

Convener: Martin Urbanec

11:15 Relativistic Iron Line Emission from the Neutron Star Low‐Mass X‐Ray Binary

Relativistic, broad-asymmetric Fe K emission lines are observed in many accreting systems from black holes in active galactic nuclei and X-ray binaries to neutron star low-mass X-ray binaries. The origin of the line broadening is often interpreted as due to dynamical broadening and relativistic effects of matter in accretion disks around the compact object. In the case of neutron stars, iron emission lines are weaker and still modelled by relativistic effects of matter in accretion disks. However, we study an alternative model, that the iron emissions line rising from the accretion column or boundary layer above a neutron star, however, the iron line emission from the disk is also considered. I will review the fundamental observations and the link to modelling them with the aim also to give some limits of neutron star mass-radii

Speaker: Maurizio Falanga (ISSI, Bern)

11:35 Complex modeling of the radiation spectrum of accreting neutron stars

Complex modeling of the radiation spectrum of accreting neutron stars.

Speaker: René Šprňa (Institute of physics in Opava)

Rag Time 24.pdf

Rag Time 24.pptx

11:50 Maximum mass of the rotating dense stellar cores with strong magnetic fields

Strange quark stars (SQS) are a type of compact object that may be created through a second explosion after the proto-neutron star state. The magnetic field of the quark star core may reach 10^{18} G. Using the MIT bag model with the density-dependent bag constant for the equation of state, we study the structure and total energy of the SQS in the presence of the strong magnetic fields with different rotational frequency. The equation of state is computed considering the Landau quantization effect. We calculate the structural parameters of SQSs for 24 configurations (6 Magnetic fields × 4 spin frequencies) by using the LORENE library, investigating our models for the deformation of the star, the binding energy, and compactness. Our results show the maximum gravitational mass of the star increases with the magnetic field and binding energy changes as a linear function of compactness with the negative slope.

Speaker: Fatemeh Kayanikhoo (CAMK)

SQS_RAGtime24.pdf

12:05 Stellar magnetism and variational principle

Magnetars and pulsars are known for their extreme magnetic field. However, magnetism seems to be the common attribute of almost all other astrophysical objects. Can this universality of astrophysical magnetism be related to the other universal principle, that of least action? We derive (simplified) stellar structure equations from a variational principle minimizing the total energy of a star including magnetic energy. Does the simplest possible realization of this model, that of incompressible magnetic fluid present a clue for a plausible explanation of astrophysical magnetism?

Speaker: Andrej Čadež (University of Ljubljana)

12:40 → 14:40 Lunch Break

14:40 → 16:15 Analytical and Numerical Approaches in Relativistic Physics

Convener: Nelson Camilo Posada Aguirre

14:40 Well-posed electromagnetic solutions in Robinson--Trautman geometry

Linear perturbations of Robinson-Trautman radiative solutions with the
new nonlinear electrodynamics model are investigated. The results show
that unlike the solutions of Maxwell electrodynamics in the
Robinson-Trautman class, our new solutions are well-posed.

Speaker: Tayebeh Tahamtan

15:00 Models of inflation in the Palatini formalism

We demonstrate how the inclusion of a R^2 term and the use of the Palatini formalism allows models of inflation that have been excluded by the Planck data in their simplest realizations to become viable once again. We also discuss how the inclusion of a non-minimal derivative coupling term in the action allows the predictions of Palatini-Higgs inflation to be in contact with observations in the near future. Finally, in the framework of Palatini quadratic gravity we construct a two-field scalar-tensor theory that is classically scale invariant and complies with the latest bounds set by the Planck collaboration for a wide range of parameters.

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

15:40 Geodesic chaos- stationary case

Motivated by processes happening around accreting black holes,a series of papers entitled "Free motion around black holes with discs or rings: Between integrability and chaos" were published in order to classify and evaluate the chaotic behaviour that may occur in geodesic motion around static black holes if these are perturbed by gravity of some additional matter. However the static and axisymmetric space-time superpositions, which, at least in the vicinity of sources, is likely not adequate since accreting compact objects are rather supposed to rotate rapidly. Actually, it is interesting to examine how the rotation-induced space-time dragging affects the geodesic dynamics. For this purpose, we use a recently derived metric (Čížek & Semerák 2017) describing a linear perturbation of a Schwarzschild black hole due to a rotating light finite thin disc. The work is still in progress, we will present some preliminary results.

Speaker: Caputo Claudia (Charles University)

caputoRAGtime.pdf

15:55 Energy dissipation in astrophysical simulations: results of the Orszag-Tang test

In magnetized astrophysical plasma magnetic reconnection is the accepted mechanism that heats and accelerates plasmoids and affects the dynamics of the plasma. This is thought to be the mechanism for the dissipation of magnetic energy in the still poorly understood phenomena of solar flares, magnetic substorms in the Earth magnetosphere, jets, and relativistic ejections from the accretion discs of compact objects. In this work, we study energy dissipation by solving the Orszag-Tang test problem with different resolutions and numerical implementations in two astrophysical simulation codes, PLUTO and KORAL. We quantitatively compare results obtained between relativistic and non-relativistic, resistive and non-resistive, as well as 2D and 3D setups. The results can be of practical use for computational astrophysicists wanting to select the most appropriate numerical codes and modules in astrophysical simulations.

Speaker: Fatemeh Kayanikhoo (CAMK)

OT_RAGtime2022.pdf

16:15 → 16:45 Coffee Break

16:45 → 18:40 Observable Signatures of Compact Objects

Convener: Agnieszka Janiuk (CTP PAS)

16:45 Numerical Simulation of accretion tori around black hole

The matter orbiting black hole (BH) in micro-quasar or active galactic nuclei forms toroidal accretion disk structures, and multiple tori structures have been recently described as ringed accretion disks (RADs) in full general relativistic approach. We realize a full general relativistic magnetohydrodynamic (GRMHD) numerical simulations related to double toroidal structure immersed into the equatorial plane of gravitomagnetic field of central Schwarzschild BH in an asymptotically uniform magnetic field. We study the collisional dynamic of an initial RAD structure constructed by two co-rotating or counter-rotating tori, where the accretion of matter from the outer torus is assumed onto the inner torus, using the 2.5D GRMHD simulation schemes with HARM numerical code. We study the dynamics of the system assuming various initial conditions and we have demonstrated that initial matter density is the relevant factor governing the system evolution.

Speaker: Dilshodbek Bardiev

Materials

17:50 Modeling the hard state of MAXI J1820+070 from the perspective of energy balance

From the discovery of the low-mass X-ray binary (LMXB) MAXI J1820+070, it has been observed almost simultaneously by several telescopes, e.g. NICER, NuSTAR, INTEGRAL during different epochs of its outburst. Due to the very good quality of the data, several groups have analysed the timing as well as spectral properties of the data till date. Amazingly it is found that hard and hard-intermediate phase (rising phase) data can be explained by two different scenarios, one with the disk extending to the ISCO and other with the disk trucated quite far from ISCO.

We use our recently developed KYNSED model that couples the source of the soft photons, the disk, and the source of the hard photons, the corona, through their mutual interaction - the corona up-scatters the seed photons from the disc while the accretion disc is heated by the illumination from the corona. The model takes into account the photon number conservation and energy balance. We are trying to fit the hard state of MAXI J1820+070 with this model and this is the first attempt to fit any X-ray binary observation with this model. Assuming the corona is powered by the inner accretion flow and the disk's thermal emission is enhanced due to being heated by the corona, we investigate these two scenarios, disc reaching down to the ISCO and truncated disk. However, we find that the observation can not be matched with any of the scenarios. Even with truncated disk, illumination of the disk by the hot corona is leading to much larger temperature than observed. We believe that the overestimation of temperature by the model is generic and will be inconsistent for observations of other X-ray binaries in similar state. The above mentioned effect depends strongly on the fraction of the illumination which is reflected back as the rest will be absorbed and heat up. Currently we are investigating the reason for such inconsistency.

Speaker: Sudeb Ranjan Datta

18:15 Theory Meets Reality: Testing Accretion Disk Models with LMC X-3 and GRO J1655-40

We will present a detailed investigation of X-ray spectral models used to fit the thermal accretion disk emission in black-hole X-ray binaries. We fit ~2000 RXTE observations of GRO J1655-40 and LMC X-3 to explore the nature of the accretion disk in strong gravitational fields via non-relativistic and relativistic disk models. We find that the non-relativistic multi-color disk blackbody model (diskbb) gives significantly (about ~50-60%) higher values for disk temperatures and lower (often unphysical) inner disk radii than relativistic disk models. We find that this difference in the disk temperature and inner disk radius cannot be explained by only changing spectral hardening factor and changing inner disk radius in combination with relativistic treatment of the disk is a more probable explanation for the previously reported deviations in the L-T relationship in a subset of black hole X-ray binaries. Our study clearly shows the importance of self-consistent modeling of the thermal emission in accreting compact objects, especially when estimating the black hole spin with the continuum-fitting method.

Speaker: Anastasiya Yilmaz

19:00 → 22:00 Social evening

Arma

Friday, 14 October

10:00 → 11:00 UNISPHERE

Building of the IoP! Bezrucovo namesti 13!
A short movie by the students of the Institute of Physics and talk by TG or PH

10:00 Journey to the binary stars with AIDA

A movie by students of the Institute of Physics in Opava
(in English)

10:20 Autumn nigh sky in Opava

10:35 Talk by Tomáš Gráf

11:00 → 11:30 Coffee Break

11:30 → 12:45 Waves and Fields in GR

Convener: Daniel Charbulák

11:30 Phantom scalar field solution in the Weyl class

We derive and analyze particularly simple phantom scalar field solution within the Weyl geometry comparing it with the Curzon--Chazy spacetime. We show that this solution contains a wormhole throat which connects to a region that behaves like a one-directional time machine. We investigate its conformal structure and analyze a non-scalar singularity appearing on the boundary of the spacetime inside the wormhole. We compare various definitions of mass of the spacetime in the presence of phantom matter.

Speaker: Otakar Svítek (Institute of Theoretical Physics, Charles University)

11:50 String loop vibration

In the vicinity of the black hole where the gravitational and
electromagnetic fields play important role, relativistic
magnetohydrodynamics (GRMHD) should be used as a good model for the accreting plasma. There is a formal equivalence between GRMHD equations and equations for dynamics of relativistic string: the current-carrying string loops could represent plasma exhibiting a string-like behavior due to dynamics of the magnetic field lines, or due to the thin flux tubes of magnetized plasma can be simply described as 1D strings. Unfortunately, circular configuration located around central gravitational attractor (star or compact object) is unstable against some vibrational modes, see example in Larry Niven
book Ringworld. In this presentation I would like to show our outgoing research on string loop model dynamics, its stability and vibrational modes.

Speaker: Mariia Churilova

String_loop_RAGtime2022.pdf

12:15 Distinguishing regular and singular black holes in modified gravity

This paper is devoted to investigate the possible ways of distinguishing regular and singular black holes (BHs) in modified gravity (MOG) called regular MOG (RMOG) and Schwarzschild MOG (SMOG) BHs through observational data from twin peak quasiperiodic oscillations (QPOs) which are generated by test particles in stable orbits around the BHs. The presence of MOG field causes to sufficiently the mpeak in effective potential for a radial motion of test particles. The effect of MOG parameter on specific angular momentum and energy has also studied. As a main part of the paper, we focus on investigations of QPOs around SMOG and RMOG BHs in RP model and the relations of upper and lower frequencies of twin peak QPOs in SMOG and RMOG BH models together with extreme rotating Kerr and Schwarzschild BH. Moreover, possible parameters for the central BHs of the objects GRS J1915 + 105 and XTE 1550 – 564 have also obtained numerically in the relativistic precession (RP) model. Finally, we provide comparisons of the innermost stable circular orbit (ISCO) and the orbits where twin peak QPOs with the ratio 3:2 taken place and show that QPOs can not be generated at/inside ISCO and there is a correlation between the radius of ISCO and QPO orbits.

Speaker: Aleksandra Demyanova (Ulugh Beg Astronomical Institute Uzbekistan Academy of Sciences)

12:30 Recurrence quantification analysis of observed gravitational wave data generated by compact object mergers

Astonishing precision of gravitational wave detector LIGO
requires careful analysis of the signal with new data analysis
techniques. Recurrence quantification analysis is nonlinear method for
data analysis based on number and duration of recurrences in observed
data. It is especially useful for detection of deterministic behaviour
in apparently randomly distributed data generated by nonlinear chaotic
systems. Within this contribution the recurrence quantification
analysis will be applied to the signal from gravitational wave
detectors.

Speaker: Lenka Vozárová (Fyzikálny ústav v Opave)

12:45 → 13:25 Lunch break

13:25 → 18:25 Individual discussions