The soft gamma-ray source 1E 1740.7-2942 revisited : SIGMA observation of a new transient activity beyond 200 keV.

We present results obtained between 1992 September 14 and 1992 September 23 by the SIGMA coded aperture telescope on board the GRANAT spacecraft, which further demonstrate that the remarkable soft γ-ray source 1E 1740.7-2942 is indeed subject to repeated outbursts of hard emission. During the single 1992 September 19-20 observation session, the source was found anew to exceed markedly the SIGMA detection limit beyond 200 keV. Such a high energy excess was not apparent during the four previous observation sessions devoted to 1E 1740.7-2942, nor during the next one. During the whole September 1992 observation program, the spectral shape of the 40-200 keV source emission was found to be quite similar to that observed when the source emission was in the so called standard state previously identified by SIGMA. The spectral shape of the 1992 September 19-20 high energy excess does not differ significantly from that of the high energy outburst observed by SIGMA on 1990 October 13-14, which was suggestive of positron annihilation radiation. Together with the SIGMA observations of a high energy excess in October 1991, this new SIGMA observation confirms that such a hard state is not an exceptional situation, but appears as a strongly variable phenomenon (by more than a factor 4-5).