Observation of an absorption dip in the x-ray intensity of Cygnus X-1

On 1973 January 2 the MIT X-ray detectors on the OSO-7 satellite recorded a transient change in the X-ray spectrum of Cygnus X-I indicative of an increase in absorption which lasted at least 37 minutes and occurred at a time near phase zero of the secondary of the likely optical counterpart, the spectroscopic binary system HD 226868. The effect is interpreted as a partial occultation by a stream of matter passing in front of the X-ray source and having a column density along the line of sight equivalent to 2 X 1022 H atoms cm2 of cold matter. Subject headings: binaries - X-ray sources