4 Discussion

This is the first time that the behaviour of the GRBs luminosity versus the hardness ratio has been completely analysed. The hardness ratio is a physical parameter which provides information (even if not complete) on the spectral behaviour of GRBs, avalaible for all the events. A deeper analysis obtained fitting a large energy bin number of GRBs spectra with several laws (Band et al. 1993; Tavani 1996; Briggs 1999, etc.) provides more detailed information for the whole energy range but for only a limited number of events, those with a sufficiently large signal-to noise ratio. The spectra of the less luminous GRBs, i.e. those GRBs which appear in the left and right tails of the figures, are excluded. For this reason, at present, it is not possible to perform a complete analysis of the correlations between luminosity and spectral parameters, obtained fitting the spectra with the proposed laws. Analysing the possible meaning of the obtained correlations between luminosity and hardness ratio, we note that the redshift effects alone cannot justify these results. In fact if the event low luminosity is due to redshift effects also the other parameters should be redshifted in the same way. But, for example, the events at the left of the plots, which have lower luminosities, are among the shortest ones (Belli 1995), and it is difficult to think that they are redshifted. So it seems that this luminosity behaviour is principally due to the intrinsic characteristics of the physical process at the origin of the phenomenon, to which possible redshift effects are added. For this reason these results can help us to select among the numerous models proposed for GRBs.