Rossi X-Ray Timing Explorer Observation of Cygnus X-1 in Its High State

We present the results from the Rossi X-Ray Timing Explorer observations of Cygnus X-1 in its high state. In the energy range of 2-200 keV, the observed X-ray spectrum can be described by a model consisting of a soft blackbody component and a broken power law with a high-energy cutoff. The low-energy spectrum (below ~11 keV) varies significantly from observation to observation while the high-energy portion changes little. The X-ray flux varies on all timescales down to milliseconds. The power density spectrum (PDS) can be characterized by excess red noise ("1/f") at low frequencies and a white-noise component that extends to 1-3 Hz before being cut off. At higher frequencies, the PDS becomes power-law again, with a slope of roughly -2 (i.e., "1/f²"). Broad peaks in the range of 3-9 Hz are present and might be due to quasi-periodic oscillations. The PDS shows interesting spectral dependence: the 1/f component becomes more prominent when the low-energy spectrum becomes softer. The difference in the observed spectral and timing properties between the low and high states is qualitatively consistent with a simple "fluctuating corona" model.