Intensity transitions in Cyg XR-1 observed at high energies from OSO 8

THE high energy X-ray source, Cygnus XR-1, exhibits a variable intensity between states which differ in flux at lower energies (typically 3 to 6 keV) by a factor of three (refs 1-5). Although the transitions between these `high' and `low' flux states occur at irregular intervals, the transition itself usually takes no more than one day in either direction. We report here an observation of such a transition at energies above 20 keV which shows that the spectrum of Cyg XR-1 exhibits the pivoting effect during intensity transitions expected from two-temperature accretion disk models of the X-ray emitting region^6,7.