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 \langle n \rangle \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 .