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

¹ Dept of Physics, PO Box 84, 00014 University of Helsinki, Finland
² Institutt for Fysikk, Norwegian University of Science and Technology, Högskoleringen 5, Trondheim 7491, Norway

^★ Corresponding authors: osmi.vilhu@gmail.com; karri.koljonen@ntnu.no

Received: 2 April 2025
Accepted: 12 June 2025

Abstract

Aims. 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.

Methods. We analysed Chandra/HEG observations and measured radial velocities and emission line intensities as a function of orbital phase for six emission lines: Si XIV 6.184 Å, S XVI 4.734 Å, Ar XVIII 3.731 Å, Ca XX 3.018 Å, Fe XXV 1.859 Å, and Fe XXVI 1.778 Å. We constructed radial velocity and emission intensity light curves for these lines using ten phase bins, which allowed us to investigate their orbital variability using a simple spherical WR-wind model.

Results. 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), with a velocity amplitude of ~500 km/s, 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.

Conclusions. The Fe XXVI-line originates in the disc or corona of the compact object and can be used to constrain the system masses (Appendix A). The origin of the Fe XXV-line remains uncertain due to the limitations of Chandra/HEG resolution. The other lines formed in the WR-wind between the component stars, likely above the orbital plane along ionisation ξ-surfaces. Parts of the emission lines of Ar XVIII and Ca XX originated around the compact star. The wind-related component of the Ca XX line formed closest to the ionizing source (the compact star) due to its highest ionisation potential. The recent polarisation funnel-modelling is consistent with the present results during the hypersoft state.