Number of found documents: 163
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Recurrence analysis of spinning particles in the Schwarzschild background
Zelenka, Ondřej; Lukes-Gerakopoulos, Georgios; Witzany, Vojtěch
2020 - English
In this work the dynamics of a spinning particle moving in the Schwarzschild background is studied. In particular, the methods of Poincaré section and recurrence analysis are employed to discern chaos from order. It is shown that the chaotic or regular nature of the orbital motion is reflected on the gravitational waves. Keywords: black holes; spinning particles; chaos Fulltext is available at external website.
Recurrence analysis of spinning particles in the Schwarzschild background

In this work the dynamics of a spinning particle moving in the Schwarzschild background is studied. In particular, the methods of Poincaré section and recurrence analysis are employed to discern ...

Zelenka, Ondřej; Lukes-Gerakopoulos, Georgios; Witzany, Vojtěch
Astronomický ústav, 2020

Understanding the iron K alpha line emissivity profile with GR radiative transfer code
Zhang, Wenda; Dovčiak, Michal; Bursa, Michal; Svoboda, Jiří; Karas, Vladimír
2020 - English
In this work, we present calculations of the illumination and the iron K alpha emissivity profiles performed with the GR radiative transfer code Monk that employs the Monte Carlo method. In most previous studies the distinction between the illumination and emissivity profiles was not clearly made. For AGN discs, the emissivity profile has a similar shape with the illumination profile, but in the inner most region the former is steeper than the latter, where as for accretion discs in black hole X-ray binaries, the distinction between the two profiles is more dramatic. We find out that the different behavior between AGN and black hole X-ray binary discs is due to the different energy spectra of the illuminating radiation. This suggests that the emissivity profile of the iron Kαline cannot be determined by black hole spin and corona geometry alone and the energy spectrum of the illuminating radiation has to be taken into account. We also study the dependence of the emissivity profile on the geometry of the corona. Keywords: emissivity profiles; accretion discs; radiative transfer Fulltext is available at external website.
Understanding the iron K alpha line emissivity profile with GR radiative transfer code

In this work, we present calculations of the illumination and the iron K alpha emissivity profiles performed with the GR radiative transfer code Monk that employs the Monte Carlo method. In most ...

Zhang, Wenda; Dovčiak, Michal; Bursa, Michal; Svoboda, Jiří; Karas, Vladimír
Astronomický ústav, 2020

From gappy to ringed: signatures of an accretion disk radial structure in profiles of the reflection line
Štolc, Marcel; Zajaček, M.; Karas, Vladimír
2020 - English
We focus on the theoretical profiles of a spectral line produced by reflection of the surface of both gappy accretion disk and a ring-like structure near a black hole. We describe the relativistic effects in an approximative manner. While a smooth accretion disk leads to a typical, double-horn shape with unequal wings due to Doppler boosting and an additional peak due to the lensing amplification at high inclination angle, the gaps and rings give rise to a more complex dependence which reflects the location and the radial extent of the inhomogeneities in the accretion flow. Keywords: black holes; accretion disks; radiation Fulltext is available at external website.
From gappy to ringed: signatures of an accretion disk radial structure in profiles of the reflection line

We focus on the theoretical profiles of a spectral line produced by reflection of the surface of both gappy accretion disk and a ring-like structure near a black hole. We describe the relativistic ...

Štolc, Marcel; Zajaček, M.; Karas, Vladimír
Astronomický ústav, 2020

Perturbing the accretion flow onto a supermassive black hole by a passing star
Suková, Petra; Zajaček, M.; Witzany, V.; Karas, Vladimír
2020 - English
The close neighbourhood of a supermassive black hole contains not only accreting gas and dust, but also stellar-sized objects like stars, stellar-mass black holes, neutron stars, and dust-enshrouded objects that altogether form a dense nuclear star-cluster.These objects interact with the accreting medium and they perturb the otherwise quasi-stationary configuration of the accretion flow. We investigate how the passages of a star can influence the black hole gaseous environment with GRMHD 2D and3D simulations. We focus on the changes in the accretion rate and the associated emergence of outflowing blobs of plasma. Keywords: black holes; accretion disks; active galactic nuclei Fulltext is available at external website.
Perturbing the accretion flow onto a supermassive black hole by a passing star

The close neighbourhood of a supermassive black hole contains not only accreting gas and dust, but also stellar-sized objects like stars, stellar-mass black holes, neutron stars, and dust-enshrouded ...

Suková, Petra; Zajaček, M.; Witzany, V.; Karas, Vladimír
Astronomický ústav, 2020

Propagation of waves in polytropic disks
Horák, Jiří
2020 - English
We derive an analytic dispersion relation for waves in three-dimensional polytropic disks. The problem can be separated to two one-dimensional problems of radial and vertical wave propagation. For the vertical problem, we use and generalize first-order perturbation method for waves near the vertical resonance introduced previously by several authors. Based on comparison of the analytical solutions with numerical integration, we find a surprisingly large region of validity of our dispersion relation including both p-mode and g-mode oscillations. Keywords: perturbation methods; disk dynamics Fulltext is available at external website.
Propagation of waves in polytropic disks

We derive an analytic dispersion relation for waves in three-dimensional polytropic disks. The problem can be separated to two one-dimensional problems of radial and vertical wave propagation. For ...

Horák, Jiří
Astronomický ústav, 2020

Dynamical analysis approaches in spatially curved FRW spacetimes
Kerachian, M.; Acquaviva, G.; Lukes-Gerakopoulos, Georgios
2020 - English
We summarize two agnostic approaches in the framework of spatially curved Friedmann-Robertson-Walker (FRW) cosmologies discussed in detail in (Kerachian et al., 2020, 2019). The first case concerns the dynamics of a fluid with an unspecified barotropic equation of state (EoS), for which the only assumption made is the non-negativity of the fluid’s energy density. The second case concerns the dynamics of a non-minimally coupled real scalar field with unspecified positive potential. For each of these models, we define a new set of dimensionless variables and a new evolution parameter. In the framework of these agnostic setups, we are able to identify several general features, like symmetries, invariant subsetsand critical points, and provide their cosmological interpretation. Keywords: gravitation; cosmology; dynamical systems Fulltext is available at external website.
Dynamical analysis approaches in spatially curved FRW spacetimes

We summarize two agnostic approaches in the framework of spatially curved Friedmann-Robertson-Walker (FRW) cosmologies discussed in detail in (Kerachian et al., 2020, 2019). The first case ...

Kerachian, M.; Acquaviva, G.; Lukes-Gerakopoulos, Georgios
Astronomický ústav, 2020

Probing dark energy through perfect fluid thermodynamics
Lukes-Gerakopoulos, Georgios; Acquaviva, G.; Markakis, K.
2020 - English
We demonstrate that the thermodynamics of a perfect fluid describing baryonic matter can, in certain limits, lead to an equation of state similar to that of dark energy. We keep the cosmic fluid equation of state quite general by just demanding that the speed of sound is positive and less than the speed of light. In this framework, we discuss some propositions by looking at the asymptotic behaviour of the cosmic fluid. Keywords: dark energy; perfect fluid thermodynamic Fulltext is available at external website.
Probing dark energy through perfect fluid thermodynamics

We demonstrate that the thermodynamics of a perfect fluid describing baryonic matter can, in certain limits, lead to an equation of state similar to that of dark energy. We keep the cosmic fluid ...

Lukes-Gerakopoulos, Georgios; Acquaviva, G.; Markakis, K.
Astronomický ústav, 2020

Missing bright red giants in the Galactic center: A fingerprint of its once active state?
Zajaček, Michal; Araudo, Anabella; Karas, Vladimír; Czerny, B.; Eckart, A.; Suková, Petra; Štolc, Marcel; Witzany, V.
2020 - English
We propose a novel scenario for the bright red-giant depletion based onthe collisions between red giants and the nuclear jet, which was likely active in the Galactic center a few million years ago and could have led to the formation of the large-scaleγ-ray Fermi bubbles. The process of the jet-induced ablation of red giants appears to be most efficient within∼0.04 pc(S-cluster), while at larger distances it was complemented by star–accretion disc collisions and at smaller scales, tidal stripping operated. These three mechanisms likely operated simultaneously and createdan apparent core of late-type stars within∼0.5 pc. Keywords: galaxy center; supergiants; jets Fulltext is available at external website.
Missing bright red giants in the Galactic center: A fingerprint of its once active state?

We propose a novel scenario for the bright red-giant depletion based onthe collisions between red giants and the nuclear jet, which was likely active in the Galactic center a few million years ago and ...

Zajaček, Michal; Araudo, Anabella; Karas, Vladimír; Czerny, B.; Eckart, A.; Suková, Petra; Štolc, Marcel; Witzany, V.
Astronomický ústav, 2020

Magnetically ejected disks: Equatorial outflows near vertically magnetized black hole
Karas, Vladimír; Sapountzis, K.; Janiuk, A.
2020 - English
We show an example of an equatorial outflow driven by a large scale magnetic field. We initiate our computations with a spherically symmetric distribution of gas, which flows on to the domain from a large distance, r R+. After the flow settles in a steady (Bondi) solution, we impose an axially symmetric configuration of a uniform (Wald) magnetic field aligned with the rotation axis of the black hole. Then we evolve the initial configuration numerically by employing the MHD code that approaches the force-free limit of a perfectly conducting fluid. We observe how the magnetic lines of force start accreting with the plasma while an equatorial intermittent outflow develops and goes on ejecting some material away from the black hole. Keywords: black hole physics; magnetic fields; accretion Fulltext is available at external website.
Magnetically ejected disks: Equatorial outflows near vertically magnetized black hole

We show an example of an equatorial outflow driven by a large scale magnetic field. We initiate our computations with a spherically symmetric distribution of gas, which flows on to the domain from ...

Karas, Vladimír; Sapountzis, K.; Janiuk, A.
Astronomický ústav, 2020

On maximum energy cutoff in the hotspot of radiogalaxies 3C 105 and 3C 445
Pulnova, Y.; Araudo, Anabella
2020 - English
The origin of Ultra-High-Energy Cosmic Rays is still unknown, and Active Galactic Nuclei have been proposed as candidates to accelerate these particles. Using thewell-resolved radio emission from radiogalaxies 3C 105 and 3C 445 we investigate the standard assumption that the distribution of non-thermal electrons has a maximum energy cutoff due to the synchrotron cooling. We show that as a consequence this would lead to an unphysically large number density in the hotspot. This result has important implications for the origin of Ultra-High-Energy Cosmic Rays. Keywords: ultra-high-energy cosmic rays; diffusive shock acceleration; synchrotroncooling Fulltext is available at external website.
On maximum energy cutoff in the hotspot of radiogalaxies 3C 105 and 3C 445

The origin of Ultra-High-Energy Cosmic Rays is still unknown, and Active Galactic Nuclei have been proposed as candidates to accelerate these particles. Using thewell-resolved radio emission from ...

Pulnova, Y.; Araudo, Anabella
Astronomický ústav, 2020

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