Complex sentence: This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV.
Split into simpler sentences: This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines. The best-known of which is the iron feature around 6.4 keV.