3 YEARS OF MONITORING GRS 1758-258 - AN EXTREMELY HARD X-RAY SOURCE NEAR GX 5-1

One of the hardest X-ray sources in the vicinity of the Galactic center, GRS 1758-258, has been monitored by the GRANAT/SIGMA above 35 keV since 1990 spring. The hardness of the observed spectrum (clearly detected up to approximately 200 keV) suggests considering GRS 1758-258 to be a black hole candidate. The range of flux variations on a time scale of about a year, observed by SIGMA, corresponds to factor of more than 10. During all observations in 1990 it was one of the two brightest sources within a few degrees of the Galactic center (IE 1740.7-2942 was another one) with 40-150 keV flux at the level of approximately 90 mCrab, while in 1991 fall its flux had declined below the SIGMA detection limit (<9 mCrab, 3 sigma upper limit). The source reappeared in 1992 fall with 40-150 keV flux at the level of approximately 30 mCrab. The long-term light curve of GRS 1758-258 and its 35-300 keV spectral properties, observed by SIGMA during 3 years, are quite similar to the light curve and spectra of IE 1740.7-2942, suggesting a similar nature of these two brightest sources in the 6-degrees neighborhood of the Galactic center.