Spectral variability in X-ray sources

Reference is made to Coe and others (Nature 259: 544 (1976)) who reported that Cyg X-1 and the transient source A 0620-00 show anticorrelated spectral variability, in that flux enhancements at low energy tend to be accompanied by a decline at high energy, and vice versa. Both these sources may be considered possible black holes, and it has been suggested that anticorrelated spectral variability, perhaps resulting from inverse Compton scattering in the accretion disk, may be common to x-ray emitting systems that contain black holes. It is here shown that this variability is not confined to sources that are black hole candidates. Spectral data were examined for the foregoing two objects and four other well observed sources - Cen X-3, Her X-1, Cyg X-3 and Cir X-1. The study suggests that anticorrelated spectral variability may occur in x-ray sources other than those favoured as black hole candidates. It would be of interest to ascertain if proposed inverse Compton emission mechanisms that show promise of explaining the observations for black hole candidates can be generated for binary systems involving, for example, neutron stars or white dwarfs. (U.K.)