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 afterglo ws 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 th e light curves of GRB 910402 and GRB 920723 events present the afterglow em ission with fading fluxes. During our observations (similar to 700 S) for t hese 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 1 sigma conf idence level). In both cases just after the end of the GRB event we observe d 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 mai n GRB event (in 100-800 keV range) is equal to similar to 1.5. In the begin ning of the afterglow emission the energy spectra of both events correspond to the much softer spectra with the photon index of similar to 2.5. We fou nd that the times of abrupt softening of the burst spectra correspond withi n statistical errors to the moment when the afterglow emission begins to do minate over GRB emission. We found that during the afterglow emission of GR B 910402 the statistically significant hardening of its spectra was observe d. This is the first observation of hardening of GRB afterglow emission. An alysis of GRB 910402 and its afterglow showed that this GRB source emits du ring similar to 700 s of our observations in soft gamma-rays (100-500 keV) only similar to 1.6% of its total energy released during the main event. Fo r the GRB 920723 we found that in afterglow during similar to 700 s of our observations similar to 6% of the GRB total energy was released.