What is the period of low X-ray emission called ?
The observed X-ray background is thought to result from , at the " soft " end ( below 0.3 keV ) , Galactic X-ray emission ( the " galactic " X-ray background ) , and , at the " hard " end ( above 0.3keV ) , from a combination of many unresolved X-ray sources outside of the Milky Way ( the " cosmic " X-ray background ) .