Gamma Ray Bursts (GRBs) are the most violent phenomenons in the Universe. They are associated with the birth of stellar mass black holes either from the collapse of hypermassive stars or the merger of compact objects. The Fireball model is the most popular scenario to explain GRB. In this theoretical framework, GRB central engines release collimated, bipolar and highly relativistic
jets mainly composed of electrons, positrons, photons, and a small amount of baryons. During a first phase lasting from a fraction of second to few tens of seconds -- and which is the only phase discussed in this talk -- charged particles are accelerated and release non-thermal radiation. The Fireball model also predicts a thermal like component in the same energy range coming from the jet photosphere.
This first phase would be responsible for the GRB prompt emission observed by gamma ray telescopes such as Fermi/GBM in keV-MeV energy range. Until now, GRB prompt emission spectra were considered as adequately fitted with the empirical Band function, which is a smoothly broken power law. However, although the Band function is purely empirical, its parameters are very often incompatible with the Fireball model predictions for both the thermal and non-thermal components.
We will see that observation with the Fermi Gamma Ray Space Telescope break the paradigm of the Band function and that deviations from this function exists in many GRBs. Those deviations are adequately fitted with an additional thermal-like component and/or an additional power law to the Band function, and we can follow the evolution of the various components with time. Importantly, with the three components together, theory becomes much more compatible with observations. In
addition, we will also see how this work on the prompt emission may have an impact well beyond the physics of GRBs itself. Indeed, this work may confirm a relation between the hardness of the GRB prompt emission and its luminosity which may be used to scale GRBs as standard-like candles for use in cosmology.