SIGMA DISCOVERY OF A TRANSIENT HARD X-RAY SOURCE IN THE GALACTIC-CENTER REGION

A new X-ray transient source, GRS 1730-312 (= KS 1730-312), was discov ered by the hard X-ray/soft gamma-ray coded mask telescope SIGMA/GRANA T in the Galactic Center region during observations performed in Septe mber 1994. The flare started on September 22 and lasted approximately 3 days, during which the source became the brightest object of the reg ion at energies above 35 keV. The average 35-200 keV spectrum can be d escribed by a power law with photon index of -2.5 or by a thermal brem sstrahlung model with kT(e) approximate to 70 keV. SIGMA data have bee n found consistent with the spectral shape and with the spectral evolu tion observed by the TTM/Mir-Kvant telescope at lower energies. This n ew source belongs to the population of hard X-ray sources already dete cted by SIGMA in the direction of the Galactic Bulge region.