Discrepancy between X-ray and IR line radial velocity amplitudes in Cygnus X-3

Determine the physical explanation for why the X-ray line radial velocities measured in the Fe K band of Cygnus X-3—specifically the photoionized absorption components with a semi-amplitude of approximately 55 km s^-1—are much smaller than the corresponding semi-amplitudes of the near-infrared emission-line velocities attributed to the Wolf-Rayet companion star; specifically, explain why the X-ray lines do not exhibit a speed comparable to the IR lines observed from the primary star.

Background

The authors fit multi-component photoionization models to phase-resolved XRISM/Resolve spectra in the Fe K band for Cygnus X-3 and derived orbital radial velocity curves for emission and absorption features. They find the absorption components have a systemic blueshift around −534 km s^-1 and a semi-amplitude of about 55 km s^-1, while the emission components show a larger semi-amplitude (~194 km s^-1).

Comparing these X-ray-derived velocities to previously reported near-infrared emission-line radial velocities attributed to the Wolf-Rayet companion, the authors note that the X-ray absorption-line semi-amplitudes are much smaller. They explicitly state that it is unclear why the X-ray lines do not show speeds comparable to those inferred from the IR lines, highlighting an unresolved issue about the dynamical origins or locations of the X-ray-absorbing/emitting gas relative to the binary components.