Birthplaces of X-ray emission lines in Cygnus X-3

TL;DR

This study analyzes X-ray emission lines in Cygnus X-3 to understand their formation regions and the surrounding funnel structure, revealing orbital modulation patterns and wind origins through high-resolution Chandra observations.

Contribution

It provides new insights into the spatial origins and dynamics of X-ray emission lines in Cygnus X-3 during a hypersoft state, linking spectral features to the system's geometry.

Findings

Fe XXVI line close to the compact object reflects orbital motion.

Emission lines originate from different regions, including the stellar wind and near the compact star.

Results support the funnel-like structure hypothesis from polarimetry during the hypersoft state.

Abstract

We investigate the formation of X-ray emission lines in the wind of the Wolf-Rayet (WR) companion in Cyg X-3 by analyzing their orbital dynamics using Chandra High Energy Transmission Grating (HEG) observations during a hypersoft state. Our goal is to constrain the X-ray transparency of the recently discovered funnel-like structure surrounding the compact star, as revealed by X-ray polarimetry. All lines exhibit sinusoidal orbital modulation, with the velocity amplitude generally increasing and the orbital phase with the highest blueshift generally decreasing for ions with a higher ionisation potential. The {Fe}{xxvi}-line displays velocity extremes at phase 0.25 (blueshift) and 0.75 (redshift), indicating that the line-emitting region is close to the compact component (disc or corona) and thus reflects the orbital motion. The {Fe}{xxv}-line shows a complex behaviour that cannot be fully resolved with the Chandra/HEG resolution. Other lines display velocity extremes scattered around phase 0.5 (blueshift) and 0.0 (redshift), with velocity amplitudes of 100--300 km/s, suggesting their origin in the WR stellar wind between the two components along 3D \xi-surfaces. Parts of the emission lines of {Ar}{xviii} and {Ca}{xx} originated around the compact star (disc or corona). The recent polarisation funnel-modelling is consistent with the present results during the hypersoft state.