Induced gravitational collapse in the BATSE era: The case of GRB 970828

Following the recently established “Binary-driven HyperNova” (BdHN) model, we here interpret GRB 970828 in terms of the four episodes typical of such a model. The “Episode 1,” up to 40 s after the trigger time t⁰, with a time varying thermal emission and a total energy of E^iso,1st = 2.60 × 10⁵³ erg, is interpreted as due to the onset of an hyper-critical accretion process onto a companion neutron star, triggered by the companion star, an FeCO core undergoing a SN explosion. The “Episode 2,” observed up t⁰ + 90 s, is interpreted as a canonical gamma ray burst, with an energy of $$E_{tot}^{e^ + e^ - } = 1.60 \times 10^{53}$$ erg, a baryon load of B = 7× 10^-3 and a bulk Lorentz factor at transparency of G = 142.5. From this Episode 2, we infer that the GRB exploded in an environment with a large average particle density 〈n〉 ≈103 particles/cm³ and dense clouds characterized by typical dimensions of (4-8) × 10¹⁴ cm and δn/n ~ 10. The “Episode 3” is identified from t⁰ +90 s all the way up to 10^5-6 s: despite the paucity of the early X-ray data, typical in the BATSE, pre-Swift era, we find extremely significant data points in the late X-ray afterglow emission of GRB 970828, which corresponds to the ones observed in all BdHNe sources. The “Episode 4,” related to the Supernova emission, does not appear to be observable in this source, due to the presence of darkening from the large density of the GRB environment, also inferred from the analysis of the Episode 2.