Towards a New Model for GRB Prompt Emission and a New Hardness-Luminosity Relation for Cosmology

GRB prompt emission spectra are usually considered as adequately fitted with the so-called Band function. Although purely empirical, the Band function has usually a non-thermal shape, but incompatible with the most popular synchrotron emission scenarios. Despite decades of use, the Band function clearly shows its limitations for interpreting the initial phase of the GRB phenomenon. Recent observations of bright GRBs with Fermi reveal significant spectral deviations to the Band function: a thermal-like component adequately approximated with a black body (BB) spectral shape and interpreted as the jet photospheric emission, and/or an additional power law (PL).In this presentation, I show a sample of famous bright Fermi GRBs exhibiting the presence of the three components simultaneously. This new model completely changes the view that we had on those GRBs previously. While the fit of a Band function alone to the data results in dramatic changes of its parameter values on very short time scales - difficult to interpret - the three components of our new models vary much smoother. More importantly, in our new model, the Band function shape becomes (more) compatible with synchrotron model predictions. In addition, despite the presence of three distinct components, we succeed in reducing the complexity of this new model in making it statistically very competitive with a single Band function with only one additional degree of freedom. Through the presentation, we will see how the various components evolve with time and how this new model leads to an hardness-intensity relation which could eventually be used as a tool for cosmology. Finally, we will see that the validity of this new model is supported by observations with other instruments such as CGRO/BATSE, Swift and WIND/Konus.