Properties of the hard X - ray radiation from Cygnus X - 1 and 1 E 1740 . 7 - 2942

The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and 1E1740.7-2942 in 1990–1994 was analyzed in order to search for correlations between primary observational characteristics of the hard X-ray (40–200 keV) emission – the hard X-ray luminosity L X , the hardness of the spectrum (quantified in terms of the best-fit thermal bremsstrahlung temperature kT) and the rms of short-term flux variations. Although no strict point-to-point correlations were detected certain general tendencies are evident. It was found that for Cyg X–1 the spectral hardness is in general positively correlated with the relative amplitude of short-term variability. A correlation of a similar kind was found for the X-ray transient GRO J0422+32 (X-ray Nova Persei 1992). For both sources an approximate correlation between kT and L X was found. At low hard X-ray luminosity – below ∼ 10 37 erg/sec – kT increases with L X. At higher luminosity the spectral hardness depends weaker or does not depend at all on the hard X-ray luminosity. The low luminosity end of these approximate correlations (low kT and low rms) corresponds to extended episodes of very low hard X-ray flux which occurred during SIGMA observations.