  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 gamma-ray band. The spectrum
in the 3 keV to 1 MeV band is well modeled by 4 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 gamma-ray component that dominates between 10-200 keV (with a
minimum detected flux of ~ 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 cut off power law of photon index ~ 0.6 and energy cut off at ~
41 keV; and finally a thermal plasma model of solar abundances and temperature
of 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.

Astrophysics