Periodic Light Variations in HDE 226868 (Cyg X-1)

Abstract

The positional coincidence of the ninth magnitude B01b supergiant HDE 226868 with the radio counterpart of Cyg X-1 (refs. 1, 2) and the radial velocity variations in the single-lined spectrum of this star have led to the speculation that a collapsed companion may be the source of the observed X-ray emission^3,4. We have photometrically monitored the Hα index and the intermediate-band b magnitude of HDE 226868 for several months using the 24-inch telescope of the Pine Mountain Observatory. No significant variations were detected in the photometric Hα index. We have, however, observed changes of 0.07 in the b magnitude for which the minima coincide with the spectroscopic phases of conjunction as given by Bolton's radial measurements⁴. An independent observation of similar optical variations has been reported by Lyutiy⁵. The reported absence of optical variation⁶ is explained by the coincidence of those measurements with the two minima of approximately equal depth.