The High resolution Fe K Spectrum of Cygnus X-3

We analyze features of the Fe K spectrum of the high mass X-ray binary Cygnus X-3. The spectrum was obtained with the Chandra High Energy Transmission Grating Spectrometer in the third diffraction order. The increased energy resolution of the third order enables us to fully resolve the Fe XV He$\alpha$ complex, the Fe XVI Ly$\alpha$ lines and features of the radiative recombination continua. The emission line spectrum shows the expected features of photoionization equilibrium, excited in the dense stellar wind of the companion star. We detect discrete emission from innershell transitions, in addition to absorption likely due to multiple unresolved transitions in lower ionization states. The emission line intensity ratios observed in the range of the spectrum occupied by the Fe XV $n=1-2$ forbidden and intercombination lines suggest that there is a substantial contribution from resonantly scattered innershell emission from the Li- and Be-like ionization states. The Fe XV forbidden and intercombination lines arise in the ionization zone closest to the compact object, and since they are not subject to radiative transfer effects, we can use them in principle to constrain the radial velocity amplitude of the compact object. We infer that the results indicate a compact object mass of the order of the mass of the Wolf-Rayet companion star, but we note that the presence of resonantly scattered radiation from Li-like ions may complicate the interpretation of the He-like emission spectrum.