The EGRET observations and the EGRET diffuse emission model

In the years between 1991 and 2000, the EGRET instrument performed observations in the gamma band, in the energy range from 30 MeV to 10 GeV. EGRET observations covered the whole sky, providing data on Galaxy emission, both in the plane and at high latitudes.
These observations have demonstrated the correlation between the galactic structures (arm - interarm regions) and the gamma emission, thus confirming the model of gamma emission based on the interaction between cosmic rays and interstellar medium. A further relevant result of EGRET observations on diffuse emission is the demonstration of the galactic origin of cosmic rays (at least for energy smaller than $10^{15} eV$). The flux of gamma-rays from the Large Magellanic Cloud is consistent with a cosmic ray density similar to that of Milky Way, while the upper limit for the Small Magellanic Cloud implies a cosmic ray density several times less [Sreekumar, 1993].
The EGRET galactic diffuse emission model is a 3D grid of gamma-ray emissivity of the Galaxy, centered on the Sun with coordinates given by galactic longitude, galactic latitude and distance along the line of sight [Bertsch et al., 1993], [Hunter et al., 1997]. The bin size of this matrix is $0.5^{\circ}$ for longitude and latitude and 0.5 kpc for radial distance. The cosmic ray model used to derive the gamma emissivity assumes that cosmic rays have the same energy spectrum as the local one [Skibo, 1993]. Also the ratio between protons and electrons fluxes is assumed to be equal to the local value. The normalization of the cosmic-ray energy spectrum is instead dependent on position: it is assumed that the cosmic-ray intensity is proportional to the matter surface density in the Galaxy.
The atomic hydrogen distribution is derived mainly from the 21 cm galactic plane survey of Weaver and Williams (1973) and the Maryland-Parkes southern survey [Kerr et al., 1986], while the molecular clouds distribution is obtained from the ``superbeam'' survey [Dame et al., 1987]. These radio data were the best ones available when the EGRET model was made.
The good agreement of this model with the results of EGRET observations demonstrates that the computation of gamma-ray emissivity based on proton-proton and Bremsstrahlung interaction of cosmic ray on interstellar matter and on inverse Compton interaction on interstellar radiation field can reproduce quite well the observed diffuse-emission features (spectrum and spatial distribution), also at the level of single molecular clouds systems[Hunter et al., 1994], [Digel et al., 1996], [Digel et al., 1999] and [Digel et al., 2001].

Figure 2.1: Spectrum of the inner Galaxy ( $\vert l\vert<60^{\circ}$, $\vert b\vert<10^{\circ}$ ) with calculated components from bremsstrahlung (EB), inverse Compton (IC), $\pi ^0$ decay (NN), and extragalactic isotropic emission (ID) [Hunter et al., 1997].