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

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Abstract
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 Lx, 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 (X-ray Nova Persei 1992). For both sources, an approximate correlation between kT and Lx was found. At low hard X-ray luminosity — below ∼ 1037 erg s−1 — kT increases with Lx. 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.
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