Diffuse continuum gamma rays from the Galaxy
Abstract
We have carried out a new study of the diffuse Galactic gamma-ray continuum radiation using a cosmic-ray propagation model which includes nucleons, antiprotons, electrons, positrons, and synchrotron radiation. We show that models based on locally measured electron and nucleon spectra and synchrotron constraints are consistent with gamma-ray measurements in the 30 MeV - 500 MeV range, but outside this range excesses are apparent. A harder nucleon spectrum is considered but fitting to gamma-rays causes it to violate limits from positrons and antiprotons. A harder interstellar electron spectrum allows the gamma-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 gamma-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 favoured by both analyses. The halo contribution of Galactic emission to the high-latitude gamma-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 <30 MeV gamma-ray emission 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 gamma-rays, which can then be seen as an extension of the hard X-ray continuum measured by OSSE, GINGA and RXTE.Keywords
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