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A. Valinia (NASA's GSFC), V. Tatischeff (CSNSM), K. Arnaud, K. Ebisawa, R. Ramaty (NASA's GSFC)
We propose a mechanism for the origin of the Galactic ridge hard X-ray background that naturally explains the broad-band continuum, and the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7~keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/gamma-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low energy Cosmic ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Our analysis rules out a significant contribution from low energy Cosmic ray ions, because their nonthrmal X-ray production would be accompanied by a nuclear gamma-ray line diffuse emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT ~7 keV). It is in agreement with the broad band data from ASCA, RXTE and OSSE.
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