Seminars
Our seminars take place in the lecture room of the building at
Praha–Sporilov.
Next seminar:
14.03.2012 13:00
Jiri Svoboda
European Space Astronomy Centre, Madrid, Spain
Relativistic iron lines as the probe of black hole innermost accretion discs
Abstract
Jiri Svoboda
Relativistic iron lines as the probe of black hole innermost accretion discs
X-ray spectroscopy of active galaxies and black hole binaries provides an opportunity to explore the innermost regions of black hole accretion discs. I will make a brief introduction to relativistic distortion of the iron fluorescent lines and its application for measurement of black hole angular momentum. Very steep radial decrease of the disc reflection emissivity has been detected in several X-ray sources suggesting the disc-irradiating corona to be compact and very centrally localised. I will discuss whether the special conditions on the corona properties are indeed required, and/or whether the steep radial emissivity could be an artifact of model assumptions. I will present two different effects which might account for the steep radial emissivities, the angular directionality of the reflected radiation properly calculated in the fully relativistic regime and the radial dependence of the accretion disc ionisation. I will show that these effects may also influence the measurements of the black hole angular momentum.
Previous seminar:
20.02.2012 13:00
Richard Wunsch
Astronomical Institute, Academy of Sciences, Prague
Tree-based gravity solver for grid-based hydrodynamic codes
Abstract
Richard Wunsch
Tree-based gravity solver for grid-based hydrodynamic codes
I will present our MPI parallel tree gravity solver for the hydrodynamic code Flash. Finite difference hydrodynamic codes (like Flash) traditionally use either spectral methods (FFT, multipole expansion) or multi-grid methods to solve for the gravitational potential. On the other hand, particle based codes (N-body, SPH) typically compute the gravitational potential either by direct integration or by tree based algorithms.
We develop an octal tree-based gravity solver for grid based hydrodynamic codes that is efficient on massively parallel architectures. There are two main reasons for this efficiency: (i) the simplicity of the algorithm allows us to make the communication between processes - often the main bottleneck of parallel algorithms - minimalistic; (ii) the regular structure of the grid enables very efficient implementation of interaction lists that make the tree-walk faster. Another advantage of the tree-code is that it can be relatively easily implemented on graphics card based architectures. The accuracy and scaling tests show that the algorithm is competitive with multi-grid methods and scales very well at least up to 512 processors.
If you would like to give a seminar in our group, please contact Vladimir Karas or Jaroslav Hamersky.
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