SIGMA soft gamma-ray observations of 1E 1740.7-2942 in the spring of 1992 : discovery of a sub-luminous state of emission and precise gamma-ray position measurement.

We present here approximately 240 hours of 35 keV to 1.3 MeV live time observations of the remarkable hard source 1E 1740.7-2942 obtained between 1992 February 18 and 1992 April 9 by the SIGMA coded aperture telescope on board the GRANAT spacecraft. In contrast with all the 1991 SIGMA observations, during which 1E 1740.7-2942 was often below the SIGMA detection limit, the source is again found to dominate the GC region emission in the soft γ-ray band. However, the state of emission we have observed clearly differs from the standard state previously identified by SIGMA in 1990. With respect to the standard state, the source flux was down by about a factor of 2.4 in the 40-150 keV band, while its spectral shape was found to be quite similar. The unprecedented statistics of the imaging observations of 1E 1740.7 - 2942 performed by SIGMA in the spring of 1992, supplemented with the 1990 SIGMA observations during which 1E 1740.7-2942 was within the full sensitivity SIGMA field of view, has made possible the determination of a more precise position of 1E 1740.7-2942 at soft γ-ray energies.