Afterglow of the gamma-ray burst of July 23, 1992, as observed by the SIGMA/Granat telescope

We present observations of the afterglow of the bright cosmic gamma-ray bur st of July 23, 1992, which was within the field of view of the SIGMA/Granat secondary optics. After the burst of similar to 6-s duration, a decaying s oft gamma-ray (35-300 keV) emission was observed for similar to 1000 s. Abo ut 20% of the burst energy radiated away in this time. The light curve of t he main event smoothly goes into afterglow, in which the flux falls off as a power law with an index of -0.7. At the onset of the flux decline from th e main event, the hardness changes abruptly. The spectral hardness of the a fterglow in the 8 to 200-keV energy band corresponds to a power-law spectru m with a spectral index alpha approximate to 1, while during the burst alph a approximate to 0.0-0.3. The afterglow parameters are discussed in terms o f the relativistic fireball model.