Evidence for a 39-d period in Cyg X – 1

Abstract

THE detection of a 39-d period in Cyg X – 1 (HDE226868), as seen in the ultraviolet (ultraviolet-filter) polarisation¹, would have at least one interesting interpretation, that of a third-body orbital period (M. Milgrom and J. Shaham, personal communication). We report here the result of long-term monitoring of the optical polarisation of Cyg X – 1², which has been in progress since 1974 at Pine Mountain Observatory, Oregon. Up to early August 1977, over 225 nights of ultraviolet-filter data were obtained. Power spectra of the ultraviolet-filter polarisation computed in March 1977 suggested a peak at the approximate period 39 d, common to the separate power spectra of our two instrumental Stokes parameters² Q and U. The amplitude of the feature corresponds to a peak-to-peak polarisation variation of about 0.25%. The Q and U variations proved to be coherent with each other and to be of a rectilinear type, describable as a simple variation along a certain rotated Stokes parameter Q₀, defined approximately by Q₀=p cos[2(θ − 117°)]. The normalised conjugate parameter U₀=p sin[2(θ − 117°)] shows no significant variation at the 39-d period.