Spectral variability of Cygnus X-1 in the soft state

Cygnus X-1 made a transition to the relatively rare soft state in 1996 May, and stayed in this state until 1996 August. A number of Target of Opportunity (TOO) observations of the source were made during this time with the Rossi X-ray Timing Explorer (RXTE). The hardness-intensity diagram showed similarity to that of GRS 1915-105 in recent observations. Time-dependent model fits to the spectrum show that the photon index of the hard tail varies significantly on minute timescales.