1996A&A...313..457W

Guided by the HEXAGONE gamma-ray observations of the Galactic Center region we performed a series of Monte Carlo calculations to describe the gamma-ray spectrum between 100 and 500keV by a simple model with as few parameters as possible. This energy range is governed by the continuum spectrum of orthopositronium decay, by Compton scattered annihilation radiation (511keV line + orthopositronium continuum), and by the 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 the interpretations as a "3γ" continuum or as Compton scattered annihilation radiation cannot be achieved for a wide range of the parameters f_ps and τ_st. But the observation of a 170keV backscattering 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∼0.2, an optical thin (back)scattering medium (τ_tot<1), and the gamma-ray source lying in front of the scattering medium, i.e. τ_st∼0. The small positronium fraction is contradictory to the generally assumed value of f_ps∼0.9 for positron annihilation in the interstellar medium. In order to explain the redshift of the 170keV feature we propose a plausible alternative scenario for the Compton scattering region near the Galactic Center containing a jet which has an inhomogeneous structure. A single clump or blob which is moving away from us serves as the backscattering medium while the annihilation region is confinded to a relatively small, not directly observable area, nearly at rest, at the bottom of the jet acceleration region.