2007A&A...474L..13B

The discovery by Swift and HETE-2 of an afterglow emission associated possibly with short GRBs opened the new problematic of their nature and classification. This issue has been further enhanced by the observation of GRB 060614 and by a new analysis of the BATSE catalog which led to the identification of a new class of GRBs with ``an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission''. We plan a twofold task: a) to fit this new class of ``hybrid'' sources within our ``canonical GRB'' scenario, where all GRBs are generated by a ``common engine'' (i.e. the gravitational collapse to a black hole); b) to propose GRB 970228 as the prototype of the above mentioned class, since it shares the same morphology and observational features. We analyze BeppoSAX data on GRB 970228 within the ``fireshell'' model and we determine the parameters describing the source and the CircumBurst Medium (CBM) needed to reproduce its light curves in the 40-700 keV and 2-26 keV energy bands. We find that GRB 970228 is a ``canonical GRB'', like e.g. GRB 050315, with the main peculiarity of a particularly low average density of the CBM <n_cbm≳10^–3particles/cm³. We also simulate the light curve corresponding to a rescaled CBM density profile with <n_cbm≥1particle/cm³. From such a comparison it follows that the total time-integrated luminosity is a faithful indicator of the nature of GRBs, contrary to the peak luminosity which is merely a function of the CBM density. We call attention on discriminating the short GRBs between the ``genuine'' and the ``fake'' ones. The ``genuine'' ones are intrinsically short, with baryon loading B≲10<sup>–5</sup>, as stated in our original classification. The ``fake'' ones, characterized by an initial spikelike emission followed by an extended emission lasting tenths of seconds, have a baryon loading 10^–4≲B≤10^–2. They are observed as such only due to an underdense CBM consistent with a galactic halo environment which deflates the afterglow intensity.