Diffuse Continuum Gamma Rays from the Galaxy

Abstract

A new study of the diffuse Galactic γ-ray continuum radiation is presented, using a cosmic-ray propagation model which includes nucleons, antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of the inverse Compton scattering includes the effect of anisotropic scattering in the Galactic interstellar radiation field (ISRF) and a new evaluation of the ISRF itself. Models based on locally measured electron and nucleon spectra and synchrotron constraints are consistent with γ-ray measurements in the 30-500 MeV range, but outside this range excesses are apparent. A harder nucleon spectrum is considered but fitting to γ-rays causes it to violate limits from positrons and antiprotons. A harder interstellar electron spectrum allows the γ-ray spectrum to be fitted above 1 GeV as well, and this can be further improved when combined with a modified nucleon spectrum which still respects the limits imposed by antiprotons and positrons. A large electron/inverse Compton halo is proposed which reproduces well the high-latitude variation of γ-ray emission; this is taken as support for the halo size for nucleons deduced from studies of cosmic-ray composition. Halo sizes in the range 4-10 kpc are favored by both analyses. The halo contribution of Galactic emission to the high-latitude γ-ray intensity is large, with implications for the study of the diffuse extragalactic component and signatures of dark matter. The constraints provided by the radio synchrotron spectral index do not allow all of the γ-ray emission at less than 30 MeV to be explained in terms of a steep electron spectrum unless this takes the form of a sharp upturn below 200 MeV. This leads us to prefer a source population as the origin of the excess low-energy γ-rays, which can then be seen as a continuation of the hard X-ray continuum measured by OSSE, Ginga, and RXTE.