Weekly Seminars for September 2010

Friday 3rd September 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Conversi (Physics Dept., Old Building - 1st Floor)

Speaker:  Prof. Apostolos Mastichiadis (Section of Astrophysics, Astronomy and Mechanics, Physics Department - University of Athens Panepistimiopolis)

Title: The Supercritical Pile for Gamma-Ray Bursts

Abstract: I will present the spectral and temporal radiative signatures expected within the "Supercritical Pile" model of Gamma Ray Bursts (GRBs). This model is motivated by the need for a process that provides the dissipation necessary in GRBs and presents a well defined scheme for converting the energy stored in the relativistic protons of the Relativistic Blast Waves (RBW) associated with GRB into radiation; at the same time it leads to spectra which exhibit a peak in the burst vFv distribution at an energy Ep ~ 1 MeV in the observer's frame, in agreement with observation and largely independent of the Lorentz factor Γ of the associated relativistic outflow. Futhermore, this scheme does not require acceleration of particles at the shock other than that provided by the isotropization of the flow bulk kinetic energy on the RBW frame.

Friday 3rd September 2010 - 5:30 P.M.

Università "La Sapienza" Roma - Aula Conversi (Physics Dept., Old Building - 1st Floor)

Speaker:  Dr. Maria Petropoulou (Physics Department - University of Athens Panepistimiopolis)

Title: Effects of γmax on Multiwavelength Spectra and Light Curves of GRB afterglows

Abstract: Gamma-Ray Bursts (GRBs) are attributed to a release of a very large amount of energy (~1051-1052 ergs) into a small region of space (< 100 km) over a short period of time (~10-100 s for long GRBs and <2 s for short GRBs). These energetic events have two characteristic radiative signatures : (i) the prompt γ-ray and (ii) the afterglow emission. While many issues concerning the prompt emission are still open, the afterglow, i.e. the lower energy long lasting emission, is believed to arise from the interaction of the relativistic ejecta with the ambient matter and can be adequately described by the so-called "standard" model. A significant amount of work has been performed by many researchers in calculating the multiwavelength (MW ) spectra and light curves (LC) of GRB afterglows either based directly on the "standard" model or using different variations of it. In my presentation I will focus on the effects that a low γmax will bring on the MW spectra and LC of the afterglow emission. This has not been treated so far as it is implicitly assumed that γmax is very large and its radiative signature does not contribute to any observable band. However, if it has a low value, then a break might might appear, say in the X-ray band, as the synchrotron component gives gradually its position to the SSC one. This can produce LC which do not exhibit a pure power law decay with time, but more complicated behavior..