ON THE ORIGIN OF THE 170 KEV FEATURE FROM THE GALACTIC-CENTER REGION

Guided by the HEXAGONE gamma-ray observations of the Galactic Center r egion we performed a series of Monte Carlo calculations to describe th e gamma-ray spectrum between 100 and 500 keV by a simple model with as few parameters as possible. This energy range is governed by the cont inuum spectrum of orthopositronium decay, by Compton scattered annihil ation radiation (511 keV line + orthopositronium continuum), and by th e high energy part of an inverse power law X-ray spectrum. The excess flux above the extrapolated power law spectrum is often ascribed to a orthopositronium continuum. We find that a clear distinction between t he interpretations as a ''3 gamma'' continuum or as Compton scattered annihilation radiation cannot be achieved for a wide range of the para meters f(ps) and tau(st). But the observation of a 170 keV backscatter ing feature strongly requires the presence of Compton scattering. From our models we conclude that the observed spectrum can be best modeled with a small positronium fraction f(ps) similar to 0.2, an optical th in (back)scattering medium (tau(tot) < 1), and the gamma-ray source ly ing in front of the scattering medium, i.e. tau(st) similar to 0. The small positronium fraction is contradictory to the generally assumed v alue of f(ps) similar to 0.9 for positron annihilation in the interste llar medium. In order to explain the redshift of the 170 keV feature w e propose a plausible alternative scenario for the Compton scattering region near the Galactic Center containing a jet which has an inhomoge neous structure. A single clump or blob which is moving away from us s erves as the backscattering medium while the annihilation region is co nfined to a relatively small, not directly observable area, nearly at rest, at the bottom of the jet acceleration region.