Population properties, dissipation and radiative processes in GRBs.

Gamma Ray Bursts (GRBs) are short and intense flashes of γ–rays with typical energies between keV and a few MeV. They reach luminosities (assuming isotropy) of 1054 erg/s. The γ–ray emission, called “prompt”, is highly variable (with timescales as short as few milliseconds) and can last a fraction of a second (short GRBs, T90 < 2 s) or few tens of seconds (long GRBs, T90 > 2 s). The prompt is followed by the “afterglow” emission, at lower frequencies (in the X–ray, Optical and Radio band) which has been detected also up to several months after the trigger and is typically smooth and decaying as a function of time. GRBs are cosmological sources having average redshift <z> ∼ 2.5. The progenitors of long GRBs are thought to be very massive stars that collapse at the end of their life, while the progenitors of short GRBs are thought to be the merging of two neutron stars. Two of the key properties characterizing the population of GRBs are their cosmic formation rate ψ(z) (GRBFR) and their luminosity function φ(L) (LF). Recovering ψ (z) and φ(L) of GRBs allows us to test the nature of their progenitor (e.g. through the comparison with the cosmic star formation rate), to study the possible presence of sub–classes of GRBs and to infer intrinsic properties such as the structure of their jetted outflows. The knowledge of the intrinsic population properties is becoming even more compelling with the recent association of short GRBs with gravitational wave signals produced by the merger of two neutron stars. I also concentrated my work on the prompt emission dissipation and radiation mechanism operating in GRBs.