The Galaxy in Hard X-Rays/soft Gamma-Rays
A recent re-determination of the global Galactic emission in the 20 keV - 8 MeV energy range with the SPI telescope on INTEGRAL shows the transition from a source-dominated to a diffuse-emission dominated sky.
This confirms that individual sources determine the observed emission of the Galaxy between 20 keV and 200 keV, and that the positron-annihilation emission dominates above 300 keV to peak with the bulge-dominated 511 keV line emission.
However, with subtraction of these two components, residual diffuse emission is detected, with a weak surface brightness but a broad spatial extent. This additional, diffuse emission constitutes a significant fraction of the total emission here, and plays the major part from 600 keV up to 2 MeV. Its spatial distribution appears elongated along the plane of the Galaxy. (Bouchet et al., 2008, accepted for publication in ApJ; astro-ph:0801.2086).
Upgraded theoretical models are now able to explain the observed continuum in terms of interstellar processes, thus solving a long-standing mystery of the origin of this emission.