Two Distinct Modes In The Low (Hard) State Of Cyg X-1 And 1E1740-294

Abstract

The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and 1E1740.7-294 in 1990-1994 was analyzed in order to search for correlations between primary observational characteristics of the hard X-ray 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 gamma_1 state of Cyg X-1 - the kT increases as the L_X increases. Above the luminosity level of ~1.2L_gamma_1 the spectrum hardness is nearly constant (kT ~150 keV) and does not depend on the luminosity. In the case of Cyg X-1 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.