3 SPECTRAL STATES OF 1E-1740.7 - 2942 - FROM STANDARD CYGNUS-X-1 TYPE SPECTRUM TO THE EVIDENCE OF ELECTRON-POSITRON ANNIHILATION FEATURE

The source 1E 1740.7-2942 is known to be the brightest hard X-ray source close to the dynamic center of our Galaxy. Three apparently different spectral states of this source were detected by the GRANAT observatory during 1990-1991 observations of the Galactic Center (GC) region. In almost all 1990 observations the source had Cyg X-1-like spectrum with nearly constant flux. The hardest of the states (observed on 1990 October 13-14) exhibits a prominent high-energy bump on the spectrum at 300-600 keV, probably related to the annihilation processes in relatively cold electron-positron plasma. In the most recent measurements on 1991 February-March a long-term low state of 1E 1740.7-2942 was discovered with the source flux on the level of 15%-20% of its 1990 value.