Long-Term Variability of the Hard X-ray Source GRS 1758-258

Abstract

The results of hard x-ray observations of GRS 1758-258 in 1990–1997 byGRANAT/SIGMA are presented. The long-term light curve of the source showsvariations in the 40–200 keV luminosity by a factor of more than ~ 7 — from less than 1.5 × 1036 erg s−1 up to 10 × 1036 erg s−1. The average 40–200 keV luminosity is ~ 6 × 1036 erg s−1 and the averaged spectrum has best-fit power-law photon index α = 2.00 ± 0.06 with a steepening above ~ 100 keV. The source long-term light curve and the spectrum are similar to that of 1E1740.7-2942 andCyg X-1.