No (217)
Yes (31)
Number of found documents:
248
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to
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 ...
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 ...
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 ...
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 ...
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 - English
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.
Keywords:
black hole; neutron star; plunging trajector
Fulltext is available at external website.
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 ...
Automation of processing and photometric data analysis for transiting exoplanets observed with ESO NIR instrument HAWK-I
Blažek, M.; Kabáth, Petr; Klocová, Tereza; Skarka, Marek
2018 - English
Nowadays, when amount of data still increases, it is necessary to automatise their processing. State-of-the-art instruments are capable to produce even tens of thousands of images during a single night. One of them is HAWK-I that is a part of Very Large Telescope of European Southern Observatory. This instrument works in near-infrared band. In my Master thesis, I dealt with developing a pipeline to process data obtained by the instrument. It is written in Python programming language using commands of IRAF astronomical software and it is developed directly for “Fast Photometry Mode” of HAWK-I. In this mode, a large number of data has been obtained during secondary eclipses of exoplanets by their host star. The pipeline was tested by a data set from sorting of the images to making a light curve. The data of WASP-18 system contained almost 40 000 images observed by using a filter centered at 2.09 μm wavelength and there is a plan to process other data sets. A goal of processing of WASP-18 and the other data sets is consecutive analysis of exoplanetary atmospheres of the observed systems.
Keywords:
observational methods; photometric techniques; planetary systems
Available at various institutes of the ASCR
Automation of processing and photometric data analysis for transiting exoplanets observed with ESO NIR instrument HAWK-I
Nowadays, when amount of data still increases, it is necessary to automatise their processing. State-of-the-art instruments are capable to produce even tens of thousands of images during a single ...
Z CVn - Still mysterious
Skarka, Marek; Liška, J.; Dřevěný, R.; Sódor, A.; Kolenberg, B.
2018 - English
We comment on short- and long-term pulsation period variations of Z CVn, a classical RR Lyrae star with the Blazhko effect. Z CVn shows cyclic-like O-C diagram that can be interpreted as a consequence of binarity throught the light travel time effect. We show that this hypothesis is false and that the observed long-term period variations must be caused by some effect that is intrinsic to the star. We also show that the Blazhko period is not simply anti-correlated with the long-term period variations as was suggested by previous authors.
Keywords:
long-term pulsation; short-term pulsation; Blazhko effect
Available at various institutes of the ASCR
Z CVn - Still mysterious
We comment on short- and long-term pulsation period variations of Z CVn, a classical RR Lyrae star with the Blazhko effect. Z CVn shows cyclic-like O-C diagram that can be interpreted as a consequence ...
Analytical solution for charged fluid pressure profiles - circulation in combined electromagnetic field
Kovář, J.; Slaný, P.; Stuchlík, Z.; Karas, Vladimír; Trova, A.
2017 - English
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.
Keywords:
charged perfect fluid; toroidal structures; compact object
Fulltext is available at external website.
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 - English
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.
Keywords:
black hole; microquasars; accretion flows
Fulltext is available at external website.
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 ...
Flows along arch filaments observed in the GRIS `very fast spectroscopic mode'
Gonzalez Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor Yabar, A.; Collados Vera, M.; Verma, M.; Balthasar, H.; Diercke, A.; Fischer, C.E.; Gömöry, P.; Bello González, N.; Schlichenmaier, R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmid, W.; Sigwarth, M.; Sobotka, Michal; Solanki, S.K.; Soltau, D.; Staude, J.; Strassmeier, K.G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.A.
2017 - English
A new generation of solar instruments provides improved spectral, spatial, and temporal resolution, thus facilitating a better understanding of dynamic processes on the Sun. High-resolution observations often reveal multiple-component spectral line profiles, which provides information about the chromospheric velocity and magnetic fine structure. We observed an emerging flux region, including two small pores and an arch filament system, on 2015 April 17 with the "very fast spectroscopic mode" of the GREGOR Infrared Spectrograph (GRIS) situated at the 1.5-meter GREGOR solar telescope at Observatorio del Teide, Tenerife, Spain. We discuss this method of obtaining fast (one per minute) spectral scans of the solar surface and its potential to follow dynamic processes on the Sun. We demonstrate the performance of the "very fast spectroscopic mode" by tracking chromospheric high-velocity features in the arch filament system.
Keywords:
Sun; chromosphere; photosphere
Available at various institutes of the ASCR
Flows along arch filaments observed in the GRIS `very fast spectroscopic mode'
A new generation of solar instruments provides improved spectral, spatial, and temporal resolution, thus facilitating a better understanding of dynamic processes on the Sun. High-resolution ...
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