Measurement of the Galactic X‐Ray/Gamma‐Ray Background Radiation: Contribution of Discrete Sources

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Abstract
The Galactic background radiation near the Scutum arm was observed simultaneously with RXTE and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft γ-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by four components: a high-energy continuum dominating above 500 keV that can be characterized by a power law of photon index ~1.6 (an extrapolation from measurements above ~1 MeV), a positron annihilation line at 511 keV and positronium continuum, a variable hard X-ray/soft γ-ray component that dominates between 10 and 200 keV (with a minimum detected flux of ~7.7 × 10-7 photons cm-2 s-1 keV-1 deg-2 at 100 keV averaged over the field of view of OSSE) and that is well modeled by an exponentially cutoff power law of photon index ~0.6 and energy cutoff at ~41 keV, and finally a thermal plasma model of solar abundances and temperature ~2.6 keV that dominates below 10 keV. We estimate that the contribution of bright discrete sources to the minimum flux detected by OSSE was ~46% at 60 keV and ~20% at 100 keV. The remaining unresolved emission may be interpreted either as truly diffuse emission with a hard spectrum (such as that from inverse Compton scattering) or the superposition of discrete sources that have very hard spectra.
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