GRANAT/SIGMA observation of the early afterglow from GRB 920723 in soft gamma-rays

We present a GRANAT/SIGMA observation of the soft gamma-ray afterglow immed iately after GRB 920723. The main burst is very bright. After similar to 6 s, the burst light curve makes a smooth transition into an afterglow where flux decays as t(-0.7). The power-law decay lasts for at least 1000 s; beyo nd this time, the afterglow emission is lost in the background fluctuations . At least similar to 20% of main burst energy is emitted in the afterglow. At approximately similar to 6 s after the trigger, we also observe an abru pt change in the burst spectrum. At t < 6 s, the ratio of 8-20 and 75-200 k eV fluxes corresponds to the power law spectral index alpha = 0.0-0.3. At t = 6 s, the value of alpha increases to alpha approximate to 1 and stays at this level afterwards. The observed afterglow characteristics are discusse d in connection with the relativistic fireball model of gamma-ray bursts.