Diffuse Galactic Continuum Gamma Rays: A Model Compatible with EGRET Data and Cosmic‐Ray Measurements

We present a study of the compatibility of some current models of the diffuseGalactic continuum gamma rays with EGRET data. A set of regions sampling thewhole sky is chosen to provide a comprehensive range of tests. The range ofEGRET data used is extended to 100 GeV. The models are computed with ourGALPROP cosmic-ray propagation and gamma-ray production code. We confirm thatthe "conventional model" based on the locally observed electron and nucleonspectra is inadequate, for all sky regions. A conventional model plus hardsources in the inner Galaxy is also inadequate, since this cannot explain theGeV excess away from the Galactic plane. Models with a hard electron injectionspectrum are inconsistent with the local spectrum even considering the expectedfluctuations; they are also inconsistent with the EGRET data above 10 GeV. We present a new model which fits the spectrum in all sky regions adequately.Secondary antiproton data were used to fix the Galactic average protonspectrum, while the electron spectrum is adjusted using the spectrum of diffuseemission itself. The derived electron and proton spectra are compatible withthose measured locally considering fluctuations due to energy losses,propagation, or possibly details of Galactic structure. This model requires amuch less dramatic variation in the electron spectrum than models with a hardelectron injection spectrum, and moreover it fits the gamma-ray spectrum betterand to the highest EGRET energies. It gives a good representation of thelatitude distribution of the gamma-ray emission from the plane to the poles,and of the longitude distribution. We show that secondary positrons andelectrons make an essential contribution to Galactic diffuse gamma-rayemission.Comment: To be published in The Astrophysical Journal v.613, 1 Oct. 2004 issue; 16 pages, 60 ps-figures, 4 tables emulateapj.sty, natbib.sty, aastex.cl