Hard X-ray observations of GRS/KS 1730-312 with GRANAT/SIGMA.

The results of transient X-ray source KS/GRS 1730-312 observations with GRANAT/SIGMA are reported. The source was discovered on September 23, 1994 and at the maximum of the light curve was the brightest one in the Galactic Center region in the 35-200 keV energy domain. Within ≡5 d the hard X-ray flux from GRS/KS 1730-312 declined below the SIGMA sensitivity limit. The average 35-200 keV spectrum can be approximated by the power law spectrum with photon index ≡2.5 or by bremsstrahlung model with temperature ≡70 keV. Some steepening of the 35-200 keV spectrum with time was marginally detected. According to the TTM (Mir-KVANT) data the decline of hard X-ray flux was accompanied by notable increase of the flux in the standard X-ray band (2-10 keV). During 5 d the 2-10 keV band contribution to the 2-300 keV luminosity increased from ≡20-25% to ≡80-90%. Combined TTM and SIGMA data indicate that the broad band (2-300 keV) spectrum evolution may be described in terms of appearance and fast increase of the soft spectral component accompanied with decrease and possible steepening of the hard component.