SIGMA HARD X-RAY OBSERVATIONS OF GX 339-4 DURING THE HARD STATE AND A TRANSITION TO THE SOFT STATE

The X-ray system GX 339-4 has been observed (more than 135 hr), between 1990 March and 1991 September, with the SIGMA hard X-ray/soft gamma-ray (35 keV-1.3 MeV) imaging telescope on board the GRANAT observatory. In particular, the 1991 outburst has been well covered by SIGMA. The source stayed in a hard state during more than 3 weeks with an intensity greater than 380 mCrab up to 350 keV. Then, the last two observations performed on 1991 September 28 and October 5 revealed that the source had begun a transition from the hard to the soft state, as the intensity in the 35-230 keV band decreased below 100 mCrab. In the hard state, the spectra we observe are usually compatible with a power law of index approximately 2.2. During the transition phase, this index rises to approximately 3. Furthermore, some deviations from a simple power law appear in the high-intensity hard state. We suggest the possible existence of a spectral component above 130 keV, which seems to vary on a time scale of hours.