DIFFUSE GAMMA-RAY EMISSION: GALACTIC AND EXTRAGALACTIC

ABSTRACT. Here is reviewed our current understanding of Galactic and extragalactic diffuse -ray emission. The spectrum of the extragalactic -ray background above 30 MeV can be well described by a power law with photon index = 2.1. In the 2-10 MeV range a preliminary analysis of COMPTEL data indicates a lower intensity than previously found, with no evidence for an MeV bump. Most of the models of a truly diffuse background seem to be in conflict with the observed spectrum. Though AGN are the most likely input from discrete sources, two independent attempts to model the high energy background as the superposition of unresolved AGN indicate that AGN underproduce the observed intensity. Therefore the origin of the extragalactic -ray background is still unknown.

The Galactic diffuse -ray continuum is more intense than expected both at very low energies ( 100 keV) and at high energies ( 1 GeV). The published models for these excesses all involve cosmic ray electron interactions. While the low energy excess may have something to do with in-situ acceleration of electrons, the excess at high energies may be understood if the sources of cosmic ray electrons are discrete. The measured energy spectrum of the diffuse Galactic -ray continuum radiation thus may provide new insights into the acceleration of cosmic rays.

Table of Contents

The Extragalactic GAMMA-RAY Background

COMPTEL Results (1-30 MeV)

EGRET Results (30 MeV to 100 GeV)

Models of the Extragalactic -ray Background

THE GALACTIC DIFFUSE GAMMA-RAY EMISSION

-ray Line Emission

Galactic Continuum Emission

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