ar X iv : a st ro - p h / 04 07 23 3 v 1 1 2 Ju l 2 00 4 GRB 970228 Within the EMBH Model

We consider the gamma-ray burst of 1997 February 28 (GRB 970228) within the ElectroMagnetic Black Hole (EMBH) model. We first determine the value of the two free parameters that characterize energetically the GRB phenomenon in the EMBH model, that is to say the dyadosphere energy, E dya = 5.1 × 10 52 ergs, and the baryonic remnant mass M B in units of E dya , B = M B c 2 /E dya = 3.0 × 10 −3. Having in this way estimated the energy emitted during the beam-target phase, we evaluate the role of the InterStellar Medium (ISM) number density (n ISM) and of the ratio R between the effective emitting area and the total surface area of the GRB source, in reproducing the observed profiles of the GRB 970228 prompt emission and X-ray (2-10 keV energy band) afterglow. The importance of the ISM distribution three-dimensional treatment around the central black hole is also stressed in this analysis. The GRB 970228 [1] had an important role in solving the origin of GRBs through the first detection of counterparts at other wavelengths: the afterglow phenomenon, long-lived multi-wavelength emission, was discovered following GRB 970228 at X-ray ([2], Costa et al. [3]) and optical ([4], van Paradijs et al. [5]) wavelengths. We consider of great interest to compare the predictions of the ElectroMagnetic Black Hole (EMBH) theory (see Ruffini et al. [6] and references therein) with the first afterglow observed by the Beppo-SAX satellite. We are also interested in testing the efficiency of the model in reproducing the GRB 970228 prompt emission: in the 40-700 keV energy band the burst was characterized by an initial 5 s strong pulse followed, after about 30 s, by three additional pulses of decreasing intensity (Frontera et al. [7]). The InterStellar Medium (ISM) number density (n ISM) inhomogeneities have an important role in interpreting this profile within the EMBH model. Our analysis starts establishing the value of the two free parameters that determine energetically the GRB phenomenon in the EMBH model: the total energy deposited in the dyadosphere E dya (Ruffini et al. [8]) and the amount of the baryonic matter left over in the collapse process of the EMBH progenitor star (Ruffini et al. [8]), that can be parametrized by the dimensionless parameter B = M B c 2 /E dya. With the choice of E dya = 5.1 × 10 52 ergs and …