Observational study of the prompt emission in gamma-ray bursts with the Fermi Telescope: 
The role of the photosphere of the relativistic jet.
Contact person: Prof. Felix Ryde (felix@particle.kth.se)

Gamma-ray bursts are the largest known explosions in the universe. Due to their huge brightness we are able to detect them from very large distances, thereby viewing the very early universe. Most of the observable energy in a GRB is released in the gamma-rays. In spite of this fact we do not yet know how it arises. This problem is therefore one of the most fundamental in high-energy astrophysics today, and much attention has been devoted to it both observationally and theoretically. The aim of the proposed PhD project is to attack this problem to study and determine the radiation and particle acceleration processes involved in producing the GRBs. In particular, this includes detailed studies of the spectral and temporal data available from  the Fermi Gamma-ray Space Telescope, combined with in-depth theoretical considerations in  order to understand these observations. The expected result will be better and deeper understanding  of the radiation processes of GRBs, in particular of the broad-band spectra, up to 300 GeV and  beyond will be understood. There are strong indications that the jet photosphere is important in  shaping the spectrum, which is why significant effort will be devoted to understanding its behaviour.
It is of great importance to solve this problem since
(i) it is vital in giving us more clues to what  the central engine is and how it behaves
(ii) it will let us understand the shock physics and the  ultra-relativistic environments in which the gamma-rays are created
(iii) it will enable us to use  bursts for cosmological studies.