Sigma discovery of variable e(+)-e(-) annihilation radiation from the near Galactic center variable compact source 1E 1740.7 - 2942

Abstract

A series of observations of the Galactic center have been performed by the hard X-ray and low-energy gamma-ray coded imaging telescope SIGMA, aboard the GRANAT space observatory, in March, April, September, and October 1990, in the energy range 35 keV-1.3 MeV. The emission of the close nucleus center region above 35 keV was found to be dominated up to 300 keV by the previously observed X-ray source 1E 1740.7 - 2942, situated about 48 arcmin away from the radio source Sgr A. During the October 13, 1990, observation, it was discovered that the source was exhibiting a remarkable feature in its emission spectrum (E is greater than 300 keV), which appears as a bump, reaching a maximum intensity around 500 keV, followed by an approximately 700-keV cutoff. In an observation performed the day after, this feature was absent, showing that the source had recovered its nominal state in a very short time.