Astronomy and Astrophysics, volume 474, L13-16 (2007/10-4)
GRB 970228 and a class of GRBs with an initial spikelike emission.
BERNARDINI M.G., BIANCO C.L., CAITO L., DAINOTTI M.G., GUIDA R. and RUFFINI R.
Abstract (from CDS):
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 <ncbm≳10–3particles/cm3. We also simulate the light curve corresponding to a rescaled CBM density profile with <ncbm≥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≲10–5, 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.
gamma rays: bursts - black hole physics - stars: binaries: general - galaxies: halos