A New Indicator for the Spectral States and State Transitions in Cygnus X-1

Abstract

Based on data from the RXTE All-Sky Monitor, we found that, on time scales of days to weeks, the X-ray intensity of Cyg X-1 is correlated with the spectral hardness of the source in the soft state, but is anti-correlated with the latter in the hard state. The correlation shows interesting evolution during the state transition of 1996 May-August. The entire episode consists of three distinct phases: (1) a ~ 20-d transition phase from the hard state to the soft state, where the correlation changes from negative to positive, (2) a 50-d soft state with a steady positive correlation, and (3) a 20-d transition back to the hard state. The existence and time scales of these three phases are consistent with the timing results from the RXTE/PCA observations. This result confirms the conclusion that the soft X-ray flux alone is not a reliable indicator for a true soft state. We argue that the flux-hardness correlation likely provides a better indicator for defining the spectral states of Cyg X-1.