The Correlated Intensity and Spectral Evolution of Cygnus X-1 During State Transitions

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Abstract
Using data from the All-Sky Monitor (ASM) aboard the Rossi X-Ray Timing Explorer (RXTE), we found that the 1.5-12 keV X-ray count rate of Cygnus X-1 is, on timescales from 90 s to at least 10 days, strongly correlated with the spectral hardness of the source in the soft state but is weakly anticorrelated with the latter in the hard state. The correlation shows an interesting evolution during the 1996 spectral state transition. The entire episode can be roughly divided into three distinct phases: (1) a 20 day transition phase from the hard state to the soft state, during which the correlation changes from being negative to positive, (2) a 50 day soft state with a steady positive correlation, and (3) a 20 day transition back to the hard state. The pointed RXTE observations confirmed the ASM results but revealed new behaviors of the source at energies beyond the ASM passband. We discuss the implications of our findings.