Another major change in the radio source associated with Cyg X-1

Abstract

THE sudden appearance1,2 of a faint radio source inside the probable position region for the X-ray source Cyg X-1 was associated with major changes in the X-ray emission observed3 during the period February–May 1971. This coincidence established a probable association with the X-ray source. The accurate position of the radio source led to an identification2,4 with the bright, blue star HDE226868. Subsequent optical investigations have established5,6 this object to be a binary system with a massive, invisible component that may be a black hole.