Following the recent theoretical interpretation of GRB 090618 and GRB 101023,
we here interpret GRB 970828 in terms of a double episode emission: the first
episode, observed in the first 40 s of the emission, is interpreted as the
proto-black-hole emission; the second episode, observed after t0+50 s, as a
canonical gamma ray burst. The transition between the two episodes marks the
black hole formation. The characteristics of the real GRB, in the second
episode, are an energy of Etote+e−=1.60×1053 erg, a
baryon load of B=7×10−3 and a bulk Lorentz factor at transparency
of Γ=142.5. The clear analogy with GRB 090618 would require also in
GRB 970828 the presence of a possible supernova. We also infer that the GRB
exploded in an environment with a large average particle density ≈103 part/cm3 and dense clouds characterized by typical dimensions
of (4−8)×1014 cm and δn/n∝10. Such an environment
is in line with the observed large column density absorption, which might have
darkened both the supernova emission and the GRB optical afterglow.Comment: 7 pages, 10 figures, submitted to Ap