Using COMPTEL to Study the
Cosmic Diffuse Gamma Ray Background

Georg Weidenspointner, Martin Varendorff and the COMPTEL collaboration

The Cosmic Diffuse Gamma Ray Background (CDG) is a powerful probe for cosmology because of the transparency of the universe to gamma rays. The study of the CDG is one of the principal scientific goals of the Compton telescope COMPTEL. To understand the origin of the CDG radiation, accurate descriptions of its energy spectrum and spatial homogeneity are required.

A large number of possible origins for the CDG have been proposed, attributing the CDG to either a truly diffuse origin or to the emission of unresolved point sources or both. Over the last years significant progress has been made in the determination of the energy spectrum of the CDG. A multiwavelength spectrum of the CDG from X-rays through g-rays is depicted in Fig. 1. Recent analyses indicate that the CDG is most likely composed of a number of different components, each dominating a different energy range [1997a].

The COMPTEL energy range from 0.8-30 MeV is of particular interest. At lower and higher energies the CDG seems to be dominated by emission from unresolved Seyfert galaxies and Blazars, respectively [1997a], but the situation is still unclear in the transition region covered by COMPTEL. A preliminary spectrum of the CDG was measured with COMPTEL by Kappadath et al. [1996]. Recently, this measurement was refined above 9 MeV [1997b].

P. Sreekumar et al., The Extragalactic Diffuse Gamma-Ray Emission, 1997a
S.C. Kappadath et al., The preliminary cosmic diffuse g-ray spectrum from 800 keV to 30 MeV measured with COMPTEL, 1996
S.C. Kappadath et al., The Total Cosmic Diffuse Gamma-Ray Spectrum from 9 to 30 MeV Measured with COMPTEL, 1997b

Figure1: Multiwavelength spectrum of the CDG from X-rays through gamma-rays (figure taken from [1997a]).


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