On the origin of the iron K line in the spectrum of the galactic X-ray background

We propose a mechanism for the origin of the Galactic ridge X-ray backgroun d that naturally explains the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7 keV and the apparent broad ness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/gamma -ray background, we consider a mod el that is a mixture of thermal plasma components of perhaps supernova orig in and nonthermal emission from the interaction of low-energy cosmic-ray el ectrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomi c collisions of fast electrons produce characteristic nonthermal, narrow X- ray emission lines that can explain the complex Galactic background spectru m. Using the ASCA GIS archival data from the Scutum arm region, we show tha t a two-temperature thermal plasma model with kT similar to 0.6 and similar to 2.8 keV, plus a LECRe component models the data satisfactorily. Our ana lysis rules out a purely nonthermal origin for the emission. It also rules out a significant contribution from low-energy cosmic-ray ions, because the ir nonthermal X-ray production would be accompanied by a nuclear gamma -ray line diffuse emission exceeding the upper limits obtained using OSSE, as w ell as by an excessive Galaxy-wide Be production rate. The proposed model n aturally explains the observed complex line features and removes the diffic ulties associated with previous interpretations of the data which evoked a very hot thermal component (kT similar to 7 keV).