**28**

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**MAD UFOs: Magnetically Arrested Discs with persistent Ultra-Fast Outflows**

Suková, Petra; Zajaček, M.; Karas, Vladimír

2023 - English
We study an outflow that develops in the MAD state in 3D GRMHD simulations. We show that the outflow can be accelerated to\nrelativistic velocities and persist over the course of our simulation. We compare the properties of the outflow from MAD discs with those launched by orbiting secondary at close orbit. The main difference is that the orbiting body launches a more coherent, quasiperiodic ultrafast outflow at lower velocities (v < 0.5c) while the outflow launched in the MAD state (without the body) has a stochastic behaviour and has anapproximately flat velocity distribution between lower anf higher outflow velocities, 0.2c < v < 0.3c and v > 0.5c.
Keywords:
*accretion discs; black holes; active galactic nuclei*
Fulltext is available at external website.
MAD UFOs: Magnetically Arrested Discs with persistent Ultra-Fast Outflows

We study an outflow that develops in the MAD state in 3D GRMHD simulations. We show that the outflow can be accelerated to\nrelativistic velocities and persist over the course of our simulation. We ...

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**Astrophysical black holes embedded in organized magnetic fields**

Karas, Vladimír

2023 - English
Large-scale magnetic fields pervade the cosmic environment where the astrophysical black holes are often embedded and influenced by mutual interaction. In thiscontribution, we outline the appropriate mathematical framework to describe magnetized black holes within General Relativity, and we show several examples of how these can be employed in the astrophysical context. In particular, we examine the magnetized black hole metric in terms of an exact solution of electro-vacuum Einstein-Maxwell equations under the influence of a non-vanishing electric charge. New effects emerge: the expulsion of the magnetic flux out of the black-hole horizon depends on the intensity of the imposed magnetic field.
Keywords:
*black holes; electromagnetic fields; general relativity*
Fulltext is available at external website.
Astrophysical black holes embedded in organized magnetic fields

Large-scale magnetic fields pervade the cosmic environment where the astrophysical black holes are often embedded and influenced by mutual interaction. In thiscontribution, we outline the appropriate ...

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**Can extended bodies follow geodesic trajectories?**

Lukes-Gerakopoulos, Georgios; Mukherjee, Sajal

2023 - English
We provide an extension of the analysis on whether an extended test body can follow a geodesic trajectory given by Mukherjee et al. (2022). In particular, we consider a test body in a pole-dipole-quadrupole approximation under the Ohashi-Kyrian-Semer´ak spin supplementary condition moving in the Schwarzschild and Kerr background. Using orbital setups under which a pole-dipole body can follow geodesic motion, we explore under which conditions this can also take place in the pole-dipole-quadrupole approximation when only the mass quadrupole is taken into account. For our analysis, we employ the assumption that the dipole contribution and the quadrupole contribution vanish independentlly.
Keywords:
*Mathisson–Papapetrou–Dixon equations; particle dynamics; black holes; chaos*
Fulltext is available at external website.
Can extended bodies follow geodesic trajectories?

We provide an extension of the analysis on whether an extended test body can follow a geodesic trajectory given by Mukherjee et al. (2022). In particular, we consider a test body in a ...

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**Growth of orbital resonances around a black hole surrounded by matter**

Stratený, Michal; Lukes-Gerakopoulos, Georgios

2023 - English
This work studies the dynamics of geodesic motion within a curved spacetime around a Schwarzschild black hole, perturbed by a gravitational field of a far axisymmetric distribution of mass enclosing the system. This spacetime can serve as a versatile model for a diverse range of astrophysical scenarios and, in particular, for extreme mass ratio inspirals as in our work. We show that the system is non-integrable by employing Poincaré surface of section and rotation numbers. By utilising the rotation numbers, the widths of resonances are calculated, which are then used in establishing the relation between the underlying perturbation parameter driving the system from integrability and the quadrupole parameter characterising the perturbed metric. This relation allows us to estimate the phase shift caused by the resonance during an inspiral.
Keywords:
*geodesic motion; black holes; chaos*
Fulltext is available at external website.
Growth of orbital resonances around a black hole surrounded by matter

This work studies the dynamics of geodesic motion within a curved spacetime around a Schwarzschild black hole, perturbed by a gravitational field of a far axisymmetric distribution of mass enclosing ...

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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 ...

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**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 ...

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**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 ...

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**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 ...

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**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 ...

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**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 ...

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