Orbital Dynamics of Cygnus X‐3

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Abstract
Orbital-phase-resolved infrared spectra of Cygnus X-3 in outburst and quiescence, including tomographic analysis, are presented. We confirm the phasing of broad He II and N V lines in quiescence, such that maximum blueshift corresponds to the X-ray minimum at Φ = 0.00 ± 0.04. In outburst, double-peaked He I structures show a similar phasing with two significant differences: (1) although varying in relative strength, there is continuous line emission in blue and red peaks around the orbit; and (2) an absorption component, ~ of an orbit out of phase with the emission features, is discerned. Doppler tomograms of the double-peaked profiles are consistent with a disk-wind geometry, rotating at velocities of 1000 km s-1. Regrettably, the tomography algorithm will produce a similar ring structure from alternative line sources if contaminated by overlying P Cygni profiles. This is certainly the case in the strong 2.0587 μm He I line, leading to an ambiguous solution for the nature of double-peaked emission. The absorption feature, detected of an orbit out of phase with the emission features, is consistent with an origin in the He star wind and yields for the first time a plausible radial velocity curve for the system. We directly derive the mass function of the system, 0.027 M☉. If we assume a neutron star accretor and adopt a high orbital inclination, i > 60°, we obtain a mass range for the He star of 5 M☉ MWR 11 M☉. Alternatively, if the compact object is a black hole, we estimate MBH 10 M☉. We discuss the implications of these masses for the nature and size of the binary system.
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