Measurement of the galactic X-ray/gamma-ray background radiation: Contribution of discrete sources

The Galactic background radiation near the Scutum arm was observed simultan eously with RXTE and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft gamma-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by four components: a high-energ y continuum dominating above 500 keV that can be characterized by a power l aw of photon index similar to 1.6 (an extrapolation from measurements above similar to 1 MeV), a positron annihilation line at 511 keV and positronium continuum, a variable hard X-ray/soft gamma-ray component that dominates b etween 10 and 200 keV (with a minimum detected flux of similar to 7.7 x 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 similar to 0.6 and energy cutoff at similar to 41 keV, and finally a thermal plasma model of solar abundances and temperature sim ilar to 2.6 keV that dominates below 10 keV. We estimate that the contribut ion of bright discrete sources to the minimum flux detected by OSSE was sim ilar to 46% at 60 keV and similar to 20% at 100 keV. The remaining unresolv ed emission may be interpreted either as truly diffuse emission with a hard spectrum (such as that from inverse Compton scattering) or the superpositi on of discrete sources that have very hard spectra.