Photoionized plasma in Cygnus X-3

ASCA observations of Cygnus X-3 in its X-ray high-intensity state have for the first time resolved the K alpha lines from He-like and H-like ions of S, Ar, Ca, and Fe. In addition to these emission lines, we detected an emission-line-like feature at 3.5 keV. We identified this feature as being the free-bound continuum emitted in recombination into the 1(2)S state of H-like S. This is direct evidence of a photoionized plasma in the Cyg X-3 system, which has a dense stellar wind from a Wolf-Rayet companion star irradiated by strong X-rays from the compact star. The electron temperature of this plasma was estimated to be 16(-6)(+5) measuring the shape of the free-bound continuum.