Yes (31)
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31
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A one-parametric formula relating the frequencies of twin-peak quasi-periodic oscillations
Török, G.; Goluchová, K.; Šrámková, J.; Horák, Jiří; Bakala, P.; Urbanec, M.
2017 - English
Timing analysis of X-ray flux in more than a dozen low-mass X-ray binary systems containing a neutron star reveals remarkable correlations between frequencies of two characteristic peaks present in the power-density spectra. We find a simple analytic relation that well reproduces all these individual correlations. We link this relation to a physical model which involves accretion rate modulation caused by an oscillating torus.
Keywords:
X-rays binaries; accretion Disks; neutron stars
Fulltext is available at external website.
A one-parametric formula relating the frequencies of twin-peak quasi-periodic oscillations
Timing analysis of X-ray flux in more than a dozen low-mass X-ray binary systems containing a neutron star reveals remarkable correlations between frequencies of two characteristic peaks present in ...
Chaotic motion in the Johannsen-Psaltis spacetime
Zelenka, Ondřej; Lukes-Gerakopoulos, Georgios
2017 - English
The Johannsen-Psaltis spacetime is a perturbation of the Kerr spacetime designed to avoid pathologies like naked singularities and closed timelike curves. This spacetime depends not only on the mass and the spin of the compact object, but also on extra parameters, making the spacetime deviate from Kerr. In this work we consider only the lowest order physically meaningful extra parameter. We use numerical examples to show that geodesic motion in this spacetime can exhibit chaotic behavior. We study the corresponding phase space by using Poincaré sections and rotation numbers to show chaotic behavior, and we use Lyapunov exponents to directly estimate the sensitivity to initial conditions for chaotic orbits.
Keywords:
chaos; geodesic motion; black holes
Fulltext is available at external website.
Chaotic motion in the Johannsen-Psaltis spacetime
The Johannsen-Psaltis spacetime is a perturbation of the Kerr spacetime designed to avoid pathologies like naked singularities and closed timelike curves. This spacetime depends not only on the mass ...
Identification of features in solar ALMA images and comparison with solar atmospheric models
Skokić, Ivica; Brajša, R.; Sudar, D.; Kuhar, M.; Benz, A. O.
2017 - English
Various solar features were identified in the full-disc single-dish ALMA images taken at wavelength of 1.2 and 2.8 mm and compared with H.alpha., Ca II K and EUV images and magnetograms. Assuming thermal bremsstrahlung as the dominant radiation mechanism, several models were constructed to calculate the brightness temperature of various solar features (quiet and active solar regions, filaments and coronal holes) in a wavelength range from 0.3 to 10 mm. It is found that model predictions are in a qualitative agreement with ALMA images where active regions appear brighter, while filaments and coronal holes are not discernible from the quiet Sun level. It is also found that in ALMA images regions around magnetic neutral lines appear darker than the quiet Sun. Models predict a critical frequency below which filaments should appear in absorption and above it in emission.
Keywords:
ALMA; images regions; magnetic neutral lines
Fulltext is available at external website.
Identification of features in solar ALMA images and comparison with solar atmospheric models
Various solar features were identified in the full-disc single-dish ALMA images taken at wavelength of 1.2 and 2.8 mm and compared with H.alpha., Ca II K and EUV images and magnetograms. Assuming ...
Numerical implementation of equations for photon motion in Kerr spacetime
Bursa, Michal
2017 - English
Raytracing is one of the essential tools for accurate modeling of spectra and variability of various astrophysical objects. It has a major importance in relativisticenvironments, where light endures to a number of relativistic effects. Because the trajectories of light rays in curved spacetimes, and in Kerr spacetime in particular, are highly non-trivial, we summarize the equations governing the motion of photon\n(or any other zero rest mass particle) and give analytic solution of the equations thatcan be further used in practical computer implementations.
Keywords:
Kerr spacetime; raytracing; photon motion
Fulltext is available at external website.
Numerical implementation of equations for photon motion in Kerr spacetime
Raytracing is one of the essential tools for accurate modeling of spectra and variability of various astrophysical objects. It has a major importance in relativisticenvironments, where light ...
Polarization properties of bow shock sources close to the Galactic centre
Zajaček, M.; Karas, Vladimír; Hosseini, E.; Eckart, A.; Shahzamanian, B.; Valencia-S, M.; Peissker, F.; Busch, G.; Britzen, S.; Zensus, J. A.
2017 - English
Several bow shock sources were detected and resolved in the innermost parsec from the supermassive black hole in the Galactic centre. They show several distinct characteristics, including an excess towards mid-infrared wavelengths and a significant linear polarization as well as a characteristic prolonged bow-shock shape. These features give hints about the presence of a non-spherical dusty envelope generated\nby the bow shock. The Dusty S-cluster Object (also denoted as G2) shows similar characteristics and it is a candidate for the closest bow shock with a detected proper motion in the vicinity of Sgr A*, with the pericentre distance of only approx. 2000 Schwarzschild radii. However, in the continuum emission it is a point-like source and hence we use Monte Carlo radiative transfer modeling to reveal its possible three-dimensional structure. Alongside the spectral energy distribution, the detection of polarized continuum emission in the near-infrared Ks-band (2.2 micrometers) puts additional constraints on the geometry of the source.
Keywords:
black hole physics – Galaxy: centre –; galaxy centra; radiative transfer
Fulltext is available at external website.
Polarization properties of bow shock sources close to the Galactic centre
Several bow shock sources were detected and resolved in the innermost parsec from the supermassive black hole in the Galactic centre. They show several distinct characteristics, including an excess ...
ALMA Development Plan Study: Solar Research with ALMA. Final report
Brajša, R.; Skokić, Ivica; Bárta, Miroslav; Bastian, T.S.; Shimojo, M.; White, S. M.; Iwai, K.
2017 - English
The final report of the research/development project Solar Research with ALMA: Development study. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. The main goal of the project is development of the specific Solar Observing Mode for ALMA observatory. The report on demand of ESO summarizes in detail results of the WP3: Optimum strategies for project preparation, execution and post-processing. This summary of the second term of the project is precedded by a brief annoation of results described in the prevoius two mid-term reports. Main result: Based on the test (Science Verification) solar observations, which we took part in Chile in December 2014 and 2015, it has been shown that suggested procedures for solar observations with ALMA, including subsequent calibration and imaging, do work and can be used for regular ALMA science program. The report has been reviewed by international expert panel nominated by ESO.\n\n\n\n
Keywords:
ALMA; solar research; observing modes
Fulltext is available at external website.
ALMA Development Plan Study: Solar Research with ALMA. Final report
The final report of the research/development project Solar Research with ALMA: Development study. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. The main ...
Modelling the bow–shock evolution along the DSO/G2 orbit in the Galactic centre
Štofanová, L.; Zajaček, M.; Kunneriath, D.; Eckart, A.; Karas, Vladimír
2017 - English
A radially directed flow of gaseous environment from a supermassive black hole affects the evolution of a bow–shock that develops along the orbit of an object passing through the pericentre. The bow–shock exhibits asymmetry between the approaching and receding phases, as can be seen in calculations of the bow-shock size, the velocity profile along the shocked layer, and the surface density of the bow–shock,\nand by emission-measure maps. We discuss these effects in the context of the recent pericentre transit of DSO/G2 near Sagittarius A*.
Keywords:
galactic center; Sagittarius A*; astrophysical processes
Fulltext is available at external website.
Modelling the bow–shock evolution along the DSO/G2 orbit in the Galactic centre
A radially directed flow of gaseous environment from a supermassive black hole affects the evolution of a bow–shock that develops along the orbit of an object passing through the pericentre. The ...
ALMA Development Plan Study: Solar Research with ALMA. Science cases
Brajša, R.; Bárta, Miroslav; Skokić, Ivica; Wedemeyer, S.; Bastian, T.S.; Shimojo, M.; White, S. M.
2016 - English
A mid-term progress report of the research/development project “Solar Research with ALMA: Development study”. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. The main goal of the project is development of the specific Solar Observing Mode for ALMA observatory. The report on demand of ESO summarizes results of the WP1: Solar science cases. Possible solar science cases have been identified and optimal observation, calibration and imaging strategies for individual cases suggested. The report has been reviewed by international expert panel nominated by ESO.\n\n\n
Keywords:
ALMA; solar research; observing modes
Fulltext is available at external website.
ALMA Development Plan Study: Solar Research with ALMA. Science cases
A mid-term progress report of the research/development project “Solar Research with ALMA: Development study”. The project has been demanded by ESO to the Czech node of the European ALMA Regional ...
ALMA Development Plan Study: Solar Research with ALMA. Progress Report
Brajša, R.; Bárta, Miroslav; Skokić, Ivica; Bastian, T.S.; Shimojo, M.; White, S. M.; Iwai, K.
2016 - English
A mid-term progress report of the research/development project Solar Research with ALMA: Development study. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. The main goal of the project is development of the specific Solar Observing Mode for ALMA observatory. The report on demand of ESO summarizes results of the WP2: Observing modes and ALMA software requirements. Technical ALMA requirements of solar observing mode are studied and summarised. The ALMA user software was reviewed and its changes complying with the Solar Observing Mode has been proposed. The report has been reviewed by international expert panel nominated by ESO.\n\n\n
Keywords:
ALMA; solar research; observing modes
Fulltext is available at external website.
ALMA Development Plan Study: Solar Research with ALMA. Progress Report
A mid-term progress report of the research/development project Solar Research with ALMA: Development study. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. ...
Infrared-excess Source DSO/G2 Near the Galactic Center: Theory vs. Observations
Zajaček, M.; Eckart, A.; Peissker, F.; Karssen, G.; Karas, Vladimír
2015 - English
Based on the monitoring of the Dusty S-cluster Object (DSO/G2) during its closest approach to the Galactic Center supermassive black hole in 2014 and 2015 with ESO VLT/SINFONI, we further explore the model of a young, accreting star to explain observed spectral and morphological features. The stellar scenario is supported by our ndings, i.e., ionized-hydrogen emission from the DSO that remains spatially compact before and after the peribothron passage. The detection of DSO/G2 object as a compact single-peak emission-line source is not consistent with the original hypothesis of a core-less cloud that is necessarily tidally stretched, hence producing a double-peak emission line prole around the pericentre passage. This strengthens the evidence that the DSO/G2 source is a dust-enshrouded young star that appears to be in an accretion phase. The infall of material from the circumstellar disc onto the stellar surface can contribute signicantly to the emission of Br line as well as the observed large line width of the order of 10 angstrom.
Keywords:
galactic centre
Fulltext is available at external website.
Infrared-excess Source DSO/G2 Near the Galactic Center: Theory vs. Observations
Based on the monitoring of the Dusty S-cluster Object (DSO/G2) during its closest approach to the Galactic Center supermassive black hole in 2014 and 2015 with ESO VLT/SINFONI, we further explore the ...
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