On the origin of the 170keV feature from the Galactic Center region.

Details On the origin of the 170keV feature from the Galactic Center region. On the origin of the 170keV feature from the Galactic Center region.

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...313..457w&link_type=abstract

Astronomy and Astrophysics, v.313, p.457-464

Astronomy and Astrophysics

Astrophysics

1

Galaxy: Center, Gamma-Rays: Theory, Radiation Mechanisms: Non-Thermal, Scattering

Scientific paper

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.

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