The High resolution Fe K Spectrum of Cygnus X-3

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze the Fe K spectrum of Cygnus X-3, revealing detailed ionization features and providing insights into the system's compact object and stellar wind interactions.

Contribution

It presents the first detailed analysis of the Fe K spectrum of Cygnus X-3 using third-order Chandra data, resolving complex emission lines and discussing their implications for the system's properties.

Findings

Resolved Fe XXV and Fe XXVI lines with high energy resolution.

Detected inner-shell emission and absorption features.

Estimated the compact object mass to be comparable to the Wolf-Rayet star.

Abstract

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 XXV He{\alpha} complex and the Fe XXVI Ly{\alpha} lines. 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 inner-shell 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 XXV n = 1-2 forbidden and intercombination lines suggest that there is a substantial contribution from resonantly scattered inner-shell emission from the Li- and Be-like ionization states. The Fe XXV 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, and specifically of the radial velocity curve of the Fe XXV forbidden line.