Ano (31)
Počet nalezených dokumentů:
31
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Probing dark energy through perfect fluid thermodynamics
Lukes-Gerakopoulos, Georgios; Acquaviva, G.; Markakis, K.
2020 - anglický
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.
Klíčová slova:
dark energy; perfect fluid thermodynamic
Dokument je dostupný na externích webových stránkách.
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 ...
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 - anglický
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.
Klíčová slova:
galaxy center; supergiants; jets
Dokument je dostupný na externích webových stránkách.
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 ...
Magnetically ejected disks: Equatorial outflows near vertically magnetized black hole
Karas, Vladimír; Sapountzis, K.; Janiuk, A.
2020 - anglický
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.
Klíčová slova:
black hole physics; magnetic fields; accretion
Dokument je dostupný na externích webových stránkách.
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 ...
On maximum energy cutoff in the hotspot of radiogalaxies 3C 105 and 3C 445
Pulnova, Y.; Araudo, Anabella
2020 - anglický
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.
Klíčová slova:
ultra-high-energy cosmic rays; diffusive shock acceleration; synchrotroncooling
Dokument je dostupný na externích webových stránkách.
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 ...
Emergence of magnetic null points in electro-vacuum magnetospheres of compact objects: The case of a plunging neutron star
Kopáček, Ondřej; Tahamtan, T.; Karas, Vladimír
2020 - anglický
We study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma and may lead to violent mass ejection, we show here that strong gravitation of the supermassive black hole may actively support the process by suit-ably entangling the field lines even in the electro-vacuum description. In this contribution we further discuss the case of a dipole-type magnetic field of the neutron staron the plunging trajectory to the supermassive black hole. While we have previously shown that given model in principle admits the formation of magnetic null points, here we explore whether and where the null points appear for the astrophysically relevant values of the parameters.
Klíčová slova:
black hole; neutron star; plunging trajector
Dokument je dostupný na externích webových stránkách.
Emergence of magnetic null points in electro-vacuum magnetospheres of compact objects: The case of a plunging neutron star
We study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma ...
Analytical solution for charged fluid pressure profiles - circulation in combined electromagnetic field
Kovář, J.; Slaný, P.; Stuchlík, Z.; Karas, Vladimír; Trova, A.
2017 - anglický
We introduce a general transformation leading to an integral form of pressure equations characterizing equilibrium configurations of charged perfect fluid circling in strong gravitational and combined electromagnetic fields. The transformation generalizes our recent analytical treatment applicable to electric or magnetic fields treated separately along with the gravitational one. As an example, we present a particular solution for a fluid circling close to a charged rotating black hole immersed in an asymptotically uniform magnetic field.
Klíčová slova:
charged perfect fluid; toroidal structures; compact object
Dokument je dostupný na externích webových stránkách.
Analytical solution for charged fluid pressure profiles - circulation in combined electromagnetic field
We introduce a general transformation leading to an integral form of pressure equations characterizing equilibrium configurations of charged perfect fluid circling in strong gravitational and combined ...
Hysteresis behavior of shocks in low angular momentum flows
Suková, Petra
2017 - anglický
In this work we present the GRMHD 1D simulations of accreting matter with variable angular momentum. We focus on the existence and behaviour of the shock in the flow. We show that the location of the shock front responds to the change of the angular momentum, which also causes the accretion rate onto the black hole to vary on different time scales. We study the possible hysteresis behaviour of the shock\nfront during the time evolution of the flow. We discuss the potential observational effects of this phenomenon.
Klíčová slova:
black hole; microquasars; accretion flows
Dokument je dostupný na externích webových stránkách.
Hysteresis behavior of shocks in low angular momentum flows
In this work we present the GRMHD 1D simulations of accreting matter with variable angular momentum. We focus on the existence and behaviour of the shock in the flow. We show that the location of the ...
Truncation of AGN jets by their interaction with a passing star cluster
Araudo, Anabella; Karas, Vladimír
2017 - anglický
We study the interaction of a cluster of massive stars with AGN jets as a continuous process that must occur repetitively in galactic nuclei and a promising mechanism that can trigger production of high–energy particles.
Klíčová slova:
active galaxies; jets; clusters
Dokument je dostupný na externích webových stránkách.
Truncation of AGN jets by their interaction with a passing star cluster
We study the interaction of a cluster of massive stars with AGN jets as a continuous process that must occur repetitively in galactic nuclei and a promising mechanism that can trigger production of ...
Wave excitation at Lindblad resonances using the method of multiple scales
Horák, Jiří
2017 - anglický
In this note, the method of multiple scales is adopted to the problem of excitation of non–axisymmetric acoustic waves in vertically integrated disk by tidal gravitational fields. We derive a formula describing a waveform of exited wave that is uniformly valid in a whole disk as long as only a single Lindblad resonance is present. Our formalism is subsequently applied to two classical problems: trapped p–mode oscillations in relativistic accretion disks and the excitation of waves in infinite disk.
Klíčová slova:
perturbation methods; disk dynamics; Lindblad resonances
Dokument je dostupný na externích webových stránkách.
Wave excitation at Lindblad resonances using the method of multiple scales
In this note, the method of multiple scales is adopted to the problem of excitation of non–axisymmetric acoustic waves in vertically integrated disk by tidal gravitational fields. We derive a formula ...
Epicyclic oscillations of thick relativistic disks
Horák, Jiří; Straub, O.; Šrámková, E.; Goluchová, K.; Török, G.
2017 - anglický
We study epicyclic oscillations of thick relativistic tori with constant specific angular momentum distribution using the finite element numerical method. We have compared frequencies of the axisymmetric and non-axisymmetric modes with the analytic formulae obtained by Straub and Šrámková (2009) and Fragile et al. (2016). We have found excellent agreement in the case of axisymmetric radial epicyclic modes. In the case of the axisymmetric vertical epicyclic modes and non-axisymmetric modes in general, the analytic approximation agrees with numerical results only for tori of moderate thicknesses. Our analysis also revealed an instability of the thick constant angular momentum tori with respect to the radial epicyclic oscillation.
Klíčová slova:
black hole physics; accretion disks; oscillation
Dokument je dostupný na externích webových stránkách.
Epicyclic oscillations of thick relativistic disks
We study epicyclic oscillations of thick relativistic tori with constant specific angular momentum distribution using the finite element numerical method. We have compared frequencies of the ...
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