INTEGRAL SPI Observation of the Galactic Central Radian: Contribution of Discrete Sources and Implication for the Diffuse Emission

The INTEGRAL observatory has been performing a deep survey of the Galactic central radian since 2003, with the goal of both extracting a catalog of sources and gaining insight into the Galactic diffuse emission. This paper concentrates on the estimation of the total point-source emission contribution. It is now clear that unresolved point sources contribute to the observed diffuse emission; the increasing sensitivity of instruments with time has led to a steady decrease in estimates of this "diffuse emission." We have analyzed the first-year data obtained with the spectrometer and imager SPI on board INTEGRAL. First, a catalog of 63 hard X-ray sources detected, time averaged, during our 2003 Galactic plane survey is derived. Second, after extracting the spectra of the sources detected by SPI, their combined contribution is compared to the total (resolved and unresolved) emission from the Galactic ridge. The data analysis is complex: it requires us to split the total emission into several components, as discrete sources and diffuse emission are superimposed in SPI data. The main result is that point-source emission dominates in the hard X-ray/soft γ-ray domain and contributes around 90% of the total emission around 100 keV, while above 250 keV diffuse electron-positron annihilation, through its three-photon positronium continuum with a positronium fraction ~0.97 and the 511 keV electron-positron line, dominates over the sources.