Two distinct modes in the low (hard) state of Cygnus X-1 and 1E 1740.7-2942.

The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and 1E 1740.7-2942 in 1990-1994 was analyzed in order to search for correlations between primary observational characteristics of the hard X-ray (40-400 keV) emission - hard X-ray luminosity L_X, hardness of the spectrum (quantified in terms of the best-fit thermal bremsstrahlung temperature kT) and the rms of short-term flux variations. Two distinct modes of the kT vs. L_X dependence were found for both sources. At low luminosity - below the level corresponding approximately to the γ₁ state of Cyg X-1 (Ling et al. 1987) - the kT increases as the L_X increases. Quantitatively it corresponds to increase of the temperature from 70 keV at ≈0.5 L_γ1 to 150 keV at ≈1.2 L_γ1. Above the luminosity level of ≈1.2 L_γ1 the spectrum hardness is nearly constant (T ≈ 150 keV) and does not depend on the luminosity. In the case of Cyg X-1 (1E 1740.7-2942 is not bright enough and is located in the crowded Galactic Center region) the correlation of similar kind was found between the spectrum hardness and rms of the short-term flux variations. The increase of the kT, corresponding to the increasing branch on the kT vs. L_X diagram, is accompanied with increase of the rms from ⪉ few percent level to ≈10-15%. Further increase of the rms is not accompanied with change of the kT and does not correlate with changes in the luminosity.