Seminars
Our seminars take place usually in the library of the Main Building (Praha–Sporilov, see
Contacts).
Scheduled:
20.07.2009 14:00
Tapas K. Das
Harish Chandra Research Institute, Allahabad, India
On the multi-critical properties of axisymmetric matter flow in the Kerr metric
Abstract
Tapas K. Das
On the multi-critical properties of axisymmetric matter flow in the Kerr metric
I will demonstrate that the accretion of non self-gravitating classical fluid onto astrophysical black holes may have multiple critical points by identifying the equations describing the space gradient of the dynamical flow velocity of the accreting matter to be equivalent to first order autonomous dynamical systems. Fixed point analysis ensures that such a flow must be multi-transonic for certain astrophysically relevant initial boundary conditions, and such multi-transonic flow allows the formation of Rankine-Hugoniot shock in the accretion disc. I also plan to demonstrate how such multi-transonic shocked flow model can be applied to explain various astrophysical phenomena, like generation of the AGN outflow and the X-ray flares from our Galactic centre, or the origin of the quasi periodic oscillations of black hole candidates.
Previous:
08.06.2009 14:00
Jan Ebr
Institute of Physics, Academy of Sciences, Prague
Between us and particle astrophysics: high-energy hadronic interactions
Abstract
Jan Ebr
Between us and particle astrophysics: high-energy hadronic interactions
With the advent of large ground-based cosmic-ray observatories, from which the Pierre Auger Observatory is currently the largest, a new window into the universe has opened. Particles with laboratory energies as large as 1020 eV are observed and the statistic of the data in the 1019 eV range increases rapidly. Nevertheless, the interpretation of the data is not easy: not only do we observe only the extensive air showers of secondary particles in the atmosphere instead of "seeing" the incoming particle directly - the crucial difficulty lies in the fact that the sensitivty range of ground-based detectors does not overlap with the energies currently accessible in accelerator experiments. Thus some way of extrapolation is necessary - a genuinely tricky task considering the lack of fundamental understading even for the existing data in the accelerator energy range.
This talk aims to clarify these difficulties to a non-particle physicist. A brief introduction to strong interaction phenomenology is given. The Quantum Chromodynamics (QCD) is introduced, showing its strengths and weaknesses alike. A small digression into the field-theoretical aspects is made, explaining the important differences between different coupling regimes of QCD, gently touching the crucial idea of a running coupling constant. Then the somehwat mystical pomeron is introduced and the general ideas of current highest-energy hadronic Monte Carlo event generators are presented. At the end, the listener shall understand why, even with the amazing amount of data and theories at hand, the utility of ground-based cosmic ray observatories as "particle telescopes" for astrophysics is still sohewhat limited.
We try hard to meet regularly to discuss ongoing research projects and recent astro-ph papers.
If you would like to give a seminar in our group, please contact Vladimir Karas.
Everybody is welcome.
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