SIGMA-DETECTION OF HARD X-RAY-EMISSION FROM THE SOFT TRANSIENT TYPE-I X-RAY BURSTER KS-1731-260

During the observations of the Galactic center region performed by the French hard X-ray/soft gamma-ray telescope SIGMA, several transient sources were detected. One of them was the soft transient type I X-ray burster KS 1731 - 260 recently discovered by the TTM experiment aboard the Soviet MIR-KVANT observatory. The SIGMA observations of KS 1731 - 260 reveal the existence of a hard state for which the spectrum extends up to 150 keV, with a power-law shape having a photon spectral index of almost-equal-to 2.9 (+/-0.8). Consequently KS 1731 - 260 can be added to the list of hard X-ray sources recently discovered by SIGMA in the vicinity of the Galactic center. Furthermore, this result indicates that the high-energy tail commonly observed among the ultra-soft transient sources, which are thought to be black holes, may also be an attribute of transient systems where the compact object is a weakly magnetized neutron star.