On the thermal and double episode emissions in GRB 970828

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 t$_0$+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 E_{tot}^{e^+e^-} = 1.60 \times 10^{53} erg, a baryon load of B = 7 \times 10^{-3} and a bulk Lorentz factor at transparency of Gamma = 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 <n> \approx 10^3 part/cm^3 and dense clouds characterized by typical dimensions of (4 - 8) \times 10^{14} cm and \delta n/n \propto 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.