The inverse compton and extragalactic components of diffuse gamma-ray emission

We present spectra of the inverse Compton and extragalactic components of the diffuse high-energy gamma radiation based on an analysis of EGRET data in eight high-latitude (29°.5 < |b| < 74°) regions. The spectrum of gamma radiation that is correlated with atomic hydrogen (H I) column density indicates that this component originates in cosmic-ray/matter interactions. The gamma-ray emission uncorrelated with H I varies as a function of Galactic longitude and is correlated with radio continuum emission (408 MHz), which is dominated by synchrotron radiation from cosmic-ray electrons. We interpret the longitude-dependent variation in the intensity of gamma-ray emission uncorrelated with H I as a model-independent, direct measure of the spatially varying part of the inverse Compton (IC) emission from the Galactic halo. We find an intensity change (center versus anticenter) of (3.3 ± 0.7) × 10-6 photons cm-2 s-1 sr-1 for E > 100 MeV, a statistically significant (>4 σ) result. This value is a lower limit to the intensity of inverse Compton gamma-ray emission toward the Galactic center at high latitudes. We correlate the gamma-ray intensity with a model consisting of an isotropic component, a component proportional to the 408 MHz synchrotron radiation, and an H I component with different emissivities in eight Galactic octants. Using this fit, we measure the spectrum and absolute intensity of the IC emission. We obtain a spectral index of -1.88 ± 0.14, in agreement with measurements of the near-Earth electron cosmic-ray spectrum, and an average intensity of (5.0 ± 0.8) × 10-6 photons cm-2 s-1 sr-1 for E > 100 MeV. Extrapolating the IC emission to zero galactic radio continuum intensity, we find that the extragalactic gamma radiation has a spectral index of -2.15 ± 0.06 and an intensity of (1.24 ± 0.06) × 10-5 photons cm-2 s-1 sr-1 for E > 100 MeV. The spectrum is consistent with those measured for gamma-ray-loud active galactic nuclei (AGNs) and lends support to interpretation of the extragalactic radiation as due to unresolved gamma-ray-emitting AGNs. © 1996. The American Astronomical Society. All rights reserved.