On the thermal and double episode emissions in GRB 970828

Abstract

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.

7 pages, 10 figures, submitted to ApJ