Observations of the Soft Gamma-Ray early Afterglow emission from Two Bright Gamma-ray Bursts

We present the results of observations of the soft gamma-ray early afterglows with the energy above 100 keV from two bright Gamma-ray bursts detected by the PHEBUS instrument of the GRANAT orbital observatory. We show that the light curves of GRB 910402 and GRB 920723 events present the afterglow emission with fading fluxes. During our observations (∼ 700 s) for these gamma-ray bursts the afterglow emission was fading as the power law of time with indices equal to −0.70± 0.04 and −0.60 ± 0.05 (at 1σ confidence level). In both cases just after the end of the GRB event we observed the energy spectrum of the afterglow emissions which was softer than the energy spectrum of the main GRB events. The average photon index of the main GRB event (in 100− 800 keV range) is equal to ∼ 1.5. In the beginning of the afterglow emission the energy spectra of both events correspond to the much softer spectra with the photon index of ∼ 2.5. We found that the times of abrupt softening of the burst spectra correspond within statistical errors to the moment when the afterglow emission begins to dominate over GRB emission. We found that during the afterglow emission of GRB910402 the statistically significant hardening of its spectra was observed. This is the first observation of hardening of GRB afterglow emission. Analysis of GRB 910402 and its afterglow showed that this GRB source emits during ∼ 700 s of our observations in soft gamma-rays (100− 500 keV ) only ∼ 1.6% of its total energy released during the main event. For the GRB 920723 we found that in afterglow during ∼ 700 s of our observations ∼ 6% of the GRB total energy was released.