Hard X-ray/softγ-ray observations of the Galactic Background Radiation: unresolved discrete sources?

Abstract

The Galactic background radiation near the Scutum Arm was observed simultaneously with the Rossi X-Ray Timing Explorer (RXTE) and the Oriented Scintillation Spectrometer Experiment (OSSE) in order to constrain 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 4 components: a high energy continuum dominating above 500 keV that can be characterized by a power law of photon index ∼1 .6 below 1 MeV; a positron annihilation line at 511 keV and positronium continuum; a variable hard X-ray/soft γ-ray component that dominates between 10-400 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 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 Raymond-Smith thermal plasma model of solar abundances and temperature ∼2.6 keV that dominates below 10 keV. We estimate that at 60 and 100 keV, the contribution of bright discrete sources to the minimum flux detected by OSSE was ∼49% and ∼20%, respectively. This implies that the spectrum of the residual, unresolved emission is harder than that measured with the above power law. This 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.