LOW-FLUX HARD STATE OF 1E 1740.7-2942

GRANAT observations of IE 1740.7-2942 in 1991 October revealed the source at low 40-150 keV flux level, approximately 20-30 mCrab (approximately 1/5 of the typical 1990 value). The source spectrum in the 150-600 keV energy domain exhibits notable excess above a power-law extrapolation of the lower energy part (4-150 keV) of the spectrum. Comparison of the 1991 October spectrum with the ''standard'' state spectrum observed in 1990 suggests some analogy with gamma2-gamma1 states transition, observed for the well-known black hole candidate Cygnus X-1 by HEAO 3 (Ling et al.). Previously a transient high-energy feature, possibly associated with electron-positron annihilation, has been detected in the spectrum of IE 1740.7-2942 by GRANAT on 1990 October 13-14, when 40-150 keV flux was approximately 5 times higher than during 1991 fall observations. This indicates that the hardness of the 1E 1740.7-2942 spectrum is not connected with some specific value of X-ray luminosity and mass accretion rate. During GRANAT observations of the source in 1991 August-September the source was below ART-P and SIGMA detection limits, i.e., in the lowest state whenever observed by GRANAT in 1990-1991, indicating that the hard X-ray flux is variable by a factor of more than 10 on the time scale of approximately 1 yr. On this basis constraints on the parameters of the molecular cloud possibly associated with IE 1740.7-2942 were derived.