1999A&A...344L..53B

We present a GRANAT/SIGMA observation of the soft gamma-ray afterglow immediately after GRB 920723. The main burst is very bright. After ∼6s, 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 1000s; beyond this time, the afterglow emission is lost in the background fluctuations. At least ∼20% of main burst energy is emitted in the afterglow. At approximately ∼6s after the trigger, we also observe an abrupt change in the burst spectrum. At t<6s, the ratio of 8-20 and 75-200keV fluxes corresponds to the power law spectral index α=0.0-0.3. At t=6s, the value of α increases to α≃1 and stays at this level afterwards. The observed afterglow characteristics are discussed in connection with the relativistic fireball model of gamma-ray bursts.