LOCALIZATION AND SPECTRUM OF THE X-RAY TRANSIENT SOURCE GRS-1915+105

The observation of the recently discovered X-ray transient source GRS 1915+105, carried out by the SIGMA telescope on board GRANAT, has sign ificantly inproved the accuracy of determination of the source positio n. The analysis of the 40-70 keV image provides localization of the so urce at alpha = 19h12m51s, delta = 10-degrees-51.4' (1950 equinox) wit h an error circle with radius of 3.0' (90% confidence). In this observ ation on 1992 September 23-24, the source flux was detected at the lev el of 150 +/- 20 mCrab in the 35-70 keV energy range. The spectrum of GRS. 1915+105, measured by SIGMA in the 35-250 keV energy domain, can be approximated by a power law with a photon index of -2.5 +/- 0.3. No evidence for association of GRS 1915+105 with the Soft Gamma Repeater (SGR) B 1900+14 has been found in the SIGMA data. The position of GRS 1915+105 is approximately 1-degrees-7 apart from the nearest point of SGR's 2 sigma error box, obtained by the VENERA/KONUS experiment (E. Mazets, private communication). No soft gamma-ray bursts were also det ected by SIGMA during 20 hours of observations of the Aquila field. Th e sensitivity of the GRB-cell to the SGR-type bursts is approximately 1/10 of the brightness of the weakest burst, observed by the KONUS exp eriment in 1979, and approximately 2/3 of the weakest burst, detected by BATSE in 1992.