Properties of the hard X-ray radiation from Cygnus X-1 and 1E1740.7-2942

The entire data set of the GRANAT/SIGMA observations of Cyg X-1 and 1E1740.7 -2942 in 1990-1994 was analysed 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-fitting thermal bremsstrahlung temperature kT) and the rms of short-term flux variations. Although no strict point-to-point correlations were detected, certain general tendencies were 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 (Xray Nova Persei 1992). For both sources, an approximate correlation between kT and L-X was found. At low hard X-ray luminosity -below similar to 10(37) erg s(-1) -kT increases with L-X. At higher luminosity the spectral hardness depends more weakly, 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 the SIGMA observations.