Extremely hard GRB spectra prune down the forest of emission models

We consider the evidence for very hard low energy spectra during the prompt phase of Gamma Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness by applying different tests. These are among the bursts with the hardest low energy spectrum, sufficiently bright to allow time resolved spectral studies on time intervals of the order of tenths of a second. We discuss the hard spectra of these bursts and their evolution in the context of several non–thermal emission models, which all appear inadequate to account for these cases. The extremely hard spectra at the beginning of their prompt emission are also compared with a black body spectral model: the resulting fits are remarkably good, except for an excess at high energies (in several cases) which could be simply accounted for by the presence of a supra–thermal component. The findings on the possible thermal character of the evolving spectrum and the implications on the GRB physical scenario are considered in the frameworks of photospheric models for a fireball which is becoming optically thin, and of Compton drag models, in which the fireball boosts “ambient” seed photons by its own bulk motion. Both models appear to be qualitatively and quantitatively consistent with the found spectral characteristics.