Evidence for a Soft X-Ray Excess in the Spectrum of GRS 1758-258

We report the results of the first simultaneous observation of GRS 1758-258 at soft and hard X-ray energies. The data have been obtained with the ROSAT Position Sensitive Proportional Counter (PSPC) (0.1-2.4 keV) and with SIGMA/Granat coded mask telescope at energies greater than 40 keV. The ROSAT observation shows the presence of a soft spectral component above the low-energy extrapolation of the flat power law measured at high energies with SIGMA. This soft spectral component was weaker in 1990, when the hard X-ray flux from GRS 1758-58 was in its highest observed state. These characteristics, similar to those observed in black hole candidates such as Cyg X-1 and the ultrasoft X-ray transients, support the interpretation of GRS 1758-258 as an accreting black hole in a binary system.