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

Abstract

We present a study of the compatibility of some current models of the diffuse\nGalactic continuum gamma rays with EGRET data. A set of regions sampling the\nwhole sky is chosen to provide a comprehensive range of tests. The range of\nEGRET data used is extended to 100 GeV. The models are computed with our\nGALPROP cosmic-ray propagation and gamma-ray production code. We confirm that\nthe "conventional model" based on the locally observed electron and nucleon\nspectra is inadequate, for all sky regions. A conventional model plus hard\nsources in the inner Galaxy is also inadequate, since this cannot explain the\nGeV excess away from the Galactic plane. Models with a hard electron injection\nspectrum are inconsistent with the local spectrum even considering the expected\nfluctuations; they are also inconsistent with the EGRET data above 10 GeV.\n We present a new model which fits the spectrum in all sky regions adequately.\nSecondary antiproton data were used to fix the Galactic average proton\nspectrum, while the electron spectrum is adjusted using the spectrum of diffuse\nemission itself. The derived electron and proton spectra are compatible with\nthose measured locally considering fluctuations due to energy losses,\npropagation, or possibly details of Galactic structure. This model requires a\nmuch less dramatic variation in the electron spectrum than models with a hard\nelectron injection spectrum, and moreover it fits the gamma-ray spectrum better\nand to the highest EGRET energies. It gives a good representation of the\nlatitude distribution of the gamma-ray emission from the plane to the poles,\nand of the longitude distribution. We show that secondary positrons and\nelectrons make an essential contribution to Galactic diffuse gamma-ray\nemission.\n