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, 20-30 mCrab. The source spectrum in the 150-600 keV energy domain exhibits notable excess above a power-law extrapolation of the lower energy part 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 HEA0 3. 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 about five times higher than during 1991 fall observations. This indicates that the hardness of the IE 1740.7-2942 spectrum is not connected with some specific value of X-ray luminosity and mass accretion rate. GRANAT observations of the source in 1991 August-September indicate that the hard X-ray flux is variable by a factor of more than 10 on the time scale of about 1 yr. On this basis constraints on the parameters of the molecular cloud possibly associated with IE 1740.7-2942 were derived.