Letter to the Editor

GRB 970228 and a class of GRBs with an initial spikelike emission

¹ ICRANet and ICRA, Piazzale della Repubblica 10, 65122 Pescara, Italy
² Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy e-mail: [maria.bernardini;bianco;letizia.caito;dainotti;roberto.guida;ruffini]@icra.it
³ ICRANet, Université de Nice Sophia Antipolis, Grand Château, BP 2135, 28 avenue de Valrose, 06103 Nice Cedex 2, France

Received: 17 July 2007
Accepted: 5 September 2007

Abstract

Context.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”.

Aims.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.

Methods.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.

Results.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  10^-3 particles/cm³. We also simulate the light curve corresponding to a rescaled CBM density profile with  = 1 particle/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.

Conclusions.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.