Cygnus X-1: X-ray Emission Mechanism and Geometry

On 1996 May 10, the All-Sky Monitor aboard RXTE revealed that Cyg X–1 started a transition from its hard state to soft state (Cui 1996). Throughout this interesting episode, snapshots were taken with more sensitive detectors on ASCA, RXTE, and CGRO to monitor the temporal and spectral variability of the source over a broad energy range. The results (both spectral and temporal) from these observations seem to converge toward a self-consistent picture, which not only specifies the X-ray emission geometry but also qualitatively describes how the geometry evolves during the transition.