ASCA observations of the galactic bulge hard X-ray source GRS 1758-258

GRS 1758-258 is one of the few persistent hard X-ray emitters (E > 100 keV) in the Galaxy. Using the ASCA satellite, we have obtained the first detailed data on GRS 1758-258 in the 1-10 keV range, where previous observations were affected by confusion problems caused by the nearby strong source GX 5-1. The spectrum is well described by a power law with photon index 1.7 without strong Fe emission lines. A prominent soft excess, as observed with ROSAT when the hard X-ray flux was in a lower intensity state, was not detected. However, the presence of a soft spectral component, accounting for at most 5% of the 0.1-300 keV flux, cannot be excluded. The accurate measurement of interstellar absorption [N-H = (1.5 +/- 0.1) x 10(22) cm(-2)] corresponds to an optical extinction that definitely excludes the presence of a massive companion.