Long-term variability of the hard X-ray source GRS 1758-258

The results of hard X-ray observations of GRS 1758-258 in 1990-1997 by GRAN AT/SIGMA are presented. The long-term light curve of the source shows varia tions in the 40-200 keV luminosity by a factor of more than similar to 7 - from less than 1.5 x 10(36) erg s(-1) up to similar to 10 x 10(36) erg s(-1 ). The average 40-200 keV luminosity is similar to 6 x 10(16) erg s(-1) and the averaged spectrum has best-fit power-law photon index alpha = 2.00 +/- 0.06 with a steepening above similar to 100 keV. The source long-term ligh t curve and the spectrum are similar to that of 1E1740.7-2942 and Cyg X-1.