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 discovered by the hard X-ray/soft gamma-ray coded mask telescope SIGMA/GRANAT in the Galactic Center region during observations performed in September 1994. The flare started on September 22 and lasted approximately 3 days, during which the source became the brightest object of the region at energies above 35 keV. The average 35-200 keV spectrum can be described by a power law with photon index of -2.5 or by a thermal bremsstrahlung model with kT(e) approximate to 70 keV. SIGMA data have been found consistent with the spectral shape and with the spectral evolution observed by the TTM/Mir-Kvant telescope at lower energies. This new source belongs to the population of hard X-ray sources already detected by SIGMA in the direction of the Galactic Bulge region.