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Stepan J.,Czech Republic Astronomical Institute | Bueno J.T.,Institute of Astrophysics of Canarias | Bueno J.T.,University of La Laguna
Astronomy and Astrophysics | Year: 2013

The interpretation of the intensity and polarization of the spectral line radiation produced in the atmosphere of the Sun and of other stars requires solving a radiative transfer problem that can be very complex, especially when the main interest lies in modeling the spectral line polarization produced by scattering processes and the Hanle and Zeeman effects. One of the difficulties is that the plasma of a stellar atmosphere can be highly inhomogeneous and dynamic, which implies the need to solve the non-equilibrium problem of the generation and transfer of polarized radiation in realistic three-dimensional (3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel radiative transfer code we have developed for the simulation of the spectral line polarization caused by scattering processes and the Hanle and Zeeman effects in 3D models of stellar atmospheres. The numerical method of solution is based on the non-linear multigrid iterative method and on a novel short-characteristics formal solver of the Stokes-vector transfer equation which uses monotonic Bézier interpolation. Therefore, with PORTA the computing time needed to obtain at each spatial grid point the self-consistent values of the atomic density matrix (which quantifies the excitation state of the atomic system) scales linearly with the total number of grid points. Another crucial feature of PORTA is its parallelization strategy, which allows us to speed up the numerical solution of complicated 3D problems by several orders of magnitude with respect to sequential radiative transfer approaches, given its excellent linear scaling with the number of available processors. The PORTA code can also be conveniently applied to solve the simpler 3D radiative transfer problem of unpolarized radiation in multilevel systems. © ESO, 2013.

Horak J.,Czech Republic Astronomical Institute | Lai D.,Cornell University
Monthly Notices of the Royal Astronomical Society | Year: 2013

We study the dynamics of spiral waves and oscillation modes in relativistic rotating discs around black holes. Generalizing the Newtonian theory, we show that wave absorption can take place at the corotation resonance, where the pattern frequency of the wave matches the background disc rotation rate. We derive the general relativistic expression for the disc vortensity (vorticity divided by surface density), which governs the behaviour of density perturbation near corotation. Depending on the gradient of the generalized disc vortensity, corotational wave absorption can lead to the amplification or damping of the spiral wave. We apply our general theory of relativistic wave dynamics to calculate the non-axisymmetric inertial-acoustic modes (also called p modes) trapped in the innermost region of a black hole accretion disc. Because general relativity changes the profiles of the radial epicyclic frequency and disc vortensity near the inner disc edge close to the black hole, these p modes can become overstable under appropriate conditions. We present the numerical results of the frequencies and growth rates of p modes for various black hole spin and model disc parameters (the surface density profile and sound speed), and discuss their implications for understanding the enigmatic high-frequency quasi-periodic oscillations observed in black hole X-ray binaries. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Durech J.,Charles University | Sidorin V.,Czech Republic Astronomical Institute | Kaasalainen M.,Tampere University of Technology
Astronomy and Astrophysics | Year: 2010

Context. Apart from a few targets that were directly imaged by spacecraft, remote sensing techniques are the main source of information about the basic physical properties of asteroids, such as the size, the spin state, or the spectral type. The most widely used observing technique-time-resolved photometry-provides us with data that can be used for deriving asteroid shapes and spin states. In the past decade, inversion of asteroid lightcurves has led to more than a hundred asteroid models. In the next decade, when data from all-sky surveys are available, the number of asteroid models will increase. Combining photometry with, e.g., adaptive optics data produces more detailed models. Aims. We created the Database of Asteroid Models from Inversion Techniques (DAMIT) with the aim of providing the astronomical community access to reliable and up-to-date physical models of asteroids-i.e., their shapes, rotation periods, and spin axis directions. Models from DAMIT can be used for further detailed studies of individual objects, as well as for statistical studies of the whole set. Methods. Most DAMIT models were derived from photometric data by the lightcurve inversion method. Some of them have been further refined or scaled using adaptive optics images, infrared observations, or occultation data. A substantial number of the models were derived also using sparse photometric data from astrometric databases. Results. At present, the database contains models of more than one hundred asteroids. For each asteroid, DAMIT provides the polyhedral shape model, the sidereal rotation period, the spin axis direction, and the photometric data used for the inversion. The database is updated when new models are available or when already published models are updated or refined. We have also released the C source code for the lightcurve inversion and for the direct problem (updates and extensions will follow). © ESO, 2010.

Marin F.,Czech Republic Astronomical Institute
Monthly Notices of the Royal Astronomical Society | Year: 2014

The anisotropic nature of active galactic nuclei (AGN) is thought to be responsible for the observational differences between type-1 (pole-on) and type-2 (edge-on) nearby Seyfert-like galaxies. In this picture, the detection of emission and/or absorption features is directly correlated to the inclination of the system. The AGN structure can be further probed by using the geometry-sensitive technique of polarimetry, yet the pairing between observed polarization and Seyfert type remains poorly examined. Based on archival data, I report here the first compilation of 53 estimated AGN inclinations matched with ultraviolet/optical continuum polarization measurements. Corrections, based on the polarization of broad emission lines, are applied to the sample of Seyfert-2 AGN to remove dilution by starburst light and derive information about the scattered continuum alone. The resulting compendium agrees with past empirical results, i.e. type-1 AGN show low polarization degrees (P ≤ 1 per cent) predominantly associated with a polarization position angle parallel to the projected radio axis of the system, while type-2 objects show stronger polarization percentages (P > 7 per cent) with perpendicular polarization angles. The transition between type-1 and type-2 inclination occurs between 45° and 60° without noticeable impact on P. The compendium is further used as a test to investigate the relevance of four AGN models. While an AGN model with fragmented regions matches observations better than uniform models, a structure with a failed dusty wind along the equator and disc-born, ionized, polar outflows is by far closer to observations. However, although the models correctly reproduce the observed dichotomy between parallel and perpendicular polarization, as well as correct polarization percentages at type-2 inclinations, further work is needed to account for some highly polarized type-1 AGN. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kopacek O.,Czech Republic Astronomical Institute | Karas V.,Czech Republic Astronomical Institute
Astrophysical Journal | Year: 2014

While the motion of particles near a rotating, electrically neutral (Kerr), and charged (Kerr-Newman) black hole is always strictly regular, a perturbation in the gravitational or the electromagnetic field generally leads to chaos. The transition from regular to chaotic dynamics is relatively gradual if the system preserves axial symmetry, whereas non-axisymmetry induces chaos more efficiently. Here we study the development of chaos in an oblique (electro-vacuum) magnetosphere of a magnetized black hole. Besides the strong gravity of the massive source represented by the Kerr metric, we consider the presence of a weak, ordered, large-scale magnetic field. An axially symmetric model consisting of a rotating black hole embedded in an aligned magnetic field is generalized by allowing an oblique direction of the field having a general inclination with respect to the rotation axis of the system. The inclination of the field acts as an additional perturbation to the motion of charged particles as it breaks the axial symmetry of the system and cancels the related integral of motion. The axial component of angular momentum is no longer conserved and the resulting system thus has three degrees of freedom. Our primary concern within this contribution is to find out how sensitive the system of bound particles is to the inclination of the field. We employ the method of the maximal Lyapunov exponent to distinguish between regular and chaotic orbits and to quantify their chaoticity. We find that even a small misalignment induces chaotic motion. © 2014. The American Astronomical Society. All rights reserved.

Gunar S.,University of St. Andrews | Gunar S.,Czech Republic Astronomical Institute | Mackay D.H.,University of St. Andrews
Astrophysical Journal | Year: 2015

We present the first 3D whole-prominence fine structure model. The model combines a 3D magnetic field configuration of an entire prominence obtained from nonlinear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along multiple fine structures within the 3D magnetic model. Through the use of a novel radiative transfer visualization technique for the Hα line such plasma-loaded magnetic field model produces synthetic images of the modeled prominence comparable with high-resolution observations. This allows us for the first time to use a single technique to consistently study, in both emission on the limb and absorption against the solar disk, the fine structures of prominences/filaments produced by a magnetic field model. © 2015. The American Astronomical Society. All rights reserved.

Hellinger P.,Czech Republic Astronomical Institute
Astrophysical Journal | Year: 2016

Proton and alpha particle collisional transport coefficients (isotropization, relative deceleration frequencies, and heating rates) at 1 au are quantified using the Wind/Solar Wind Experiment data. In agreement with previous studies, the ion-ion Coulomb collisions are generally important for slow solar wind streams and tend to reduce the temperature anisotropies, the differential streaming, and the differences between the proton and alpha particle temperatures. In slow solar wind streams the Coulomb collisions between protons and alpha particles are important for the overall proton energetics, as well as the relative deceleration between the two species. It is also shown that ion temperature anisotropies and differential streaming need to be taken into account for evaluation of the collisional transport coefficients. © 2016. The American Astronomical Society. All rights reserved.

Ehlerova S.,Czech Republic Astronomical Institute | Palous J.,Czech Republic Astronomical Institute
Astronomy and Astrophysics | Year: 2013

We analyse the all-sky Leiden/Argentina/Bonn HI survey, where we identify shells belonging to the Milky Way. Methods. We used an identification method based on the search of continuous regions of a low brightness temperature that are compatible with given properties of HI shells. Results. We found 333 shells in the whole Galaxy. The size distribution of shells in the outer Galaxy is fitted by a power law with the coefficient of 2.6 corresponding to the index 1.8 in the distribution of energy sources. Their surface density decreases exponentially with a scale length of 2.8 kpc. The surface density of shells with radii ≥100 pc in the solar neighbourhood is ∼4 kpc-2 and the 2D porosity is ∼0.7. © 2013 ESO.

Slany P.,Silesian University in Opava | Kovar J.,Silesian University in Opava | Stuchlik Z.,Silesian University in Opava | Karas V.,Czech Republic Astronomical Institute
Astrophysical Journal, Supplement Series | Year: 2013

A Newtonian model of non-conductive, charged, perfect fluid tori orbiting in combined spherical gravitational and dipolar magnetic fields is presented and stationary, axisymmetric toroidal structures are analyzed. Matter in such tori exhibits a purely circulatory motion and the resulting convection carries charges into permanent rotation around the symmetry axis. As a main result, we demonstrate the possible existence of off-equatorial charged tori and equatorial tori with cusps that also enable outflows of matter from the torus in the Newtonian regime. These phenomena qualitatively represent a new consequence of the interplay between gravity and electromagnetism. From an astrophysical point of view, our investigation can provide insight into processes that determine the vertical structure of dusty tori surrounding accretion disks. © 2013. The American Astronomical Society. All rights reserved.

Jurcak J.,Czech Republic Astronomical Institute
Astronomy and Astrophysics | Year: 2011

Aims. I try to determine the properties of the magnetic field on the inner and outer penumbral boundaries and find out if either magnetic field strength or inclination are constant there and if these plasma parameters depend on the sunspot area. Methods. The spectropolarimetric data obtained with the Hinode satellite were analysed. Active regions located mostly around the disc centre were selected to compare sunspots of different sizes. The magnetic field strength and inclination were estimated using the inversions of observed Stokes profiles. Results. Both the magnetic field strength and inclination do not vary along individual outer penumbral boundaries, and the magnetic field probably becomes weaker and more vertical with decreasing sunspot area. The magnetic field strength and inclination are changing along the inner penumbral boundaries and also depend on the umbral area. Weaker magnetic fields are more vertical on the inner penumbral boundaries, which leads to a constant vertical component of the magnetic field on these boundaries. The vertical component of the magnetic field is possibly independent of the umbral area. Conclusions. The inner penumbral boundaries are defined by the critical value of the vertical component of the magnetic field. This implies that the penumbral filaments have a convective origin. © 2011 ESO.

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