Lecturer: Dr. Omer Bromberg Affiliation: Princeton University Abstract: A the major questions in the study of GRBs is the composition of their jets. The answer for this question has important implications on the energy dissipation processes that lead to the emission of the high energy photons. It can also provide clues for the type of the compact object that powers the jet. In this talk I will discuss the two main option for the jet composition: hydrodynamic or Poynting flux dominated jets in the context of long GRBs. I will discuss the propagation of these jets inside the progenitor star, and the differences in the time it takes them to break out of the stellar envelope. The breakout time leaves a distinguish mark on the duration distribution of GRB, and allows us to place constraints on the jet composition. In addition this provides us to better way to separate long - collapsar GRBs from the short - non collapsar GRBs and by that allows us to obtain much more homogeneous samples of these objects. This is mostly important for the study of short - non collapsar GRBs, as the observed number of these object is still small and thus even a minor contamination by wrongly classified collapsar GRBs can affect the conclusions we infer on their properties. Additional details of the upcoming Astrophysics' seminars can be found on the following link.