Model of Cygnus X-2

Abstract

OBSERVATIONS on the optical object associated with Cyg X-2 (ref. 1) have shown that the system is a spectroscopic binary (for example, ref. 2), with one component a G star³, but the nature of the other component is not obvious. Kraft and Demoulin³ identified the G component from weak but definitely present G-type features, including the λ4226 line of Ca I, which they measured for radial velocities. By assuming an absolute visual magnitude of + 5.5 for the G star, they estimated the distance of Cyg X-2 to be 500 to 700 pc. This in turn, together with the X-ray flux observations, means that the X-ray luminosity is about 25 L_⊙. As well as the G spectrum, broad Balmer absorption lines, of equivalent width appropriate for about type A8, and relatively sharp absorption lines of Ca II (H and K) are prominent. The profiles of the Balmer lines are unique⁴ in that they are very broad, yet show no wings of any kind. The widths are far too great to result from thermal Doppler motions, for hydrogen could not be neutral at the indicated temperature. Sofia⁵ has shown that all broadening mechanisms except mass motions (of the order of 10³ km s⁻¹) can be eliminated. A further unusual feature is the ratio of the strengths of the Balmer lines, with the high number lines relatively strong, H_β quite weak, and H_α entirely absent or perhaps present weakly in emission². Beyond the fact that they show binary motion, the most noteworthy feature of the radial velocity measures is their very large scatter and lack of repeatability. Although the radial velocities were determined in difficult conditions, so that the errors of measurement are much greater than normally expected, the scatter in the Balmer line velocities is far too large to be explained in this way. Within these uncertainties, however, we can say that there seem to be two velocity systems represented: that of Ca I and He II, and that of Ca II and the Balmer lines.