GRANAT/SIGMA Observation of the Early Afterglow from GRB 920723 in Soft Gamma-Rays

R. A. Burenin, A. A. Vikhlinin, M. R. Gilfanov, O. V. Terekhov, A. Yu. Tkachenko, S. Yu. Sazonov, E. M. Churazov, R. A. Sunyaev, P. Goldoni, A. Claret, A. Goldwurm, J. Paul, J. P. Roques, E. Jourdain, F. Pelaez, G. Vedrenne

Published 1999-02-01, updated 1999-02-14Version 2

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