NEW CONSTRAINTS ON THE POSITRON-ANNIHILATION MEDIA FROM THE DIRECTION OF THE GALACTIC-CENTER BASED ON THE 511 KEV LINE-PROFILES

We present here an analysis of the profiles and intensities of the 511 keV annihilation line observed in the direction of the Galactic center by high energy resolution detectors. We first investigate the HEXAGONE 1989 May data, where the point source was in a rather low state. We find that a warm medium (temperature of 8000 K) can describe the annihilation of the positrons from the diffuse component of the line. We compare these results with the Bell/Sandia 1977, and GRIS 1988 October flights during high states of the central source, and we then show that the discrepancy in the line shape and width between these two sets of data is explained if the time-variable component of the line coming from the annihilation of the positrons emitted by the central source annhihilates in a cold medium (temperature around 80 K). This constraint on the annihilation medium temperature supports the association with the molecular cloud G0.86-0.08 in the direction of 1E 1740.7-2942.