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Weekly Seminars for October 2007 Print E-mail

Monday 29th, 16.00
Aula Majorana (Department of Physics, Ancient Building)

Speaker: Prof. Sandip Kumar Chakrabarti from S.N. Bose National Centre for Basic Sciences, Kolkata (INDIA) and Centre for Space Physics, Kolkata (INDIA)

Title: Physics and Astrophysics of the "Boundary Layer" of a Black Hole: the Shocking story

Abstract: Accretion processes on black holes being necessarily transonic, the flow cannot be Keplerian close to the black hole. The deviation of the main Keplerian component, together with supply of winds and sub-Keplerian flows forces us to study the physics of the sub-Keplerian matter around a black hole very closely. The centrifugal barrier close to the black hole fights with gravity and creates a standing or oscillating shock at a few Schwarzschild radii of the black hole. The post-shock region, i.e., the region between the horizon and the shock acts as the "boundary layer" where most of the energy is dissipated.
We claim that most if not all of the astrophysical signatures of a black hole are in reality signatures of this region. We demostrate this in terms of the spectral state transition, variability class transition, jet/outflow formation, quasi-periodic oscillations etc. of black hole candidates.

Wednesday 10th, 16.00
Aula Majorana (Department of Physics, Ancient Building)

Speaker: Prof. David Arnett from Steward Observatory, University of Arizona, Tucson AZ, USA

Title: Symmetry Breaking and Convection in Stars

Abstract: Fully four-dimensional simulations of turbulent convection in stars have been performed to study the difficult problem of transport and mixing in stellar evolution; the specific case of thermonuclear oxygen burning in a presupernova is illustrated. The time and angle averaged Reynolds decompositons of the full fluid dynamics equations are closed by simple physical modeling based on the simulations. Strengths and weaknesses of previous phenomenological models will be discussed. The new results can be viewed in a broad way as the consequence of space-time symmetry breaking in a high-energy density plasma, mediated by the four basic interactions. The approach (Convection Algorithms Based on Simulations, or CABS) has great promise for reducing the largest remaining uncertainties in the theory of the evolution of stars, from birth to death as supernovae or planetary nebulae.

Wednesday 24th, 17.00
Sala Persico (Department of Physics, Ancient Building, 1th floor)

Speaker: Dr. Sabrina Casanova

Title: Searching for very high energy Gamma-Ray Burst with Milagro

Abstract: Gamma-Ray Bursts (GRBs) have been detected at GeV energies by EGRET and many models predict emission at > 100 GeV energies. Milagro is a wide field (2 sr) high duty cycle (> 90%) ground based water Cherenkov detector that records extensive air showers in the energy range 100GeV to 100TeV. We have searched for very high emission from a sample of about one hundred gamma-ray burst (GRB) detected since the beginning of the 2000 by BATSE, BeppoSax, HETE-2, INTEGRAL, Swift or the IPN. No evidence for emission from any of the bursts has been found and we present upper limits from these bursts. 


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