Recent observations performed with the Fermi gamma-ray space telescope show that the empirical Band function alone, which challenged the theoretical scenario for so long, is usually not sufficient to describe the prompt emission spectra of bright GRBs either in time-integrated or in time-resolved spectroscopy. In some bursts, the nuFnu spectra are better described with two humps rather than the single one of the Band function alone. While the high energy one is usually still adequately fit with the Band function corresponding to non-thermal radiations, the low energy bump is likely of thermal origin and can be interpreted as the jet photospheric emission.

In this presentation we will study in details a very intriguing and important burst and try to answer the following question: is this burst a peculiar soft and intense short GRB or a regular short GRB with a intense thermal component? We will compare this GRB to other cases with possible thermal emission we reported with Fermi. Then we will see how including a thermal component in the time-integrated and time-resolved fits strongly affects the resulting shape of the prompt emission spectra and makes the Band function perfectly compatible with the synchrotron fast cooling scenario. Finally, we will discuss the strong and unexpected impact of the thermal component on the often discussed Epeak-Luminosity relation and see how it can help to reveal the possible physical nature of this relation.