We analized 190 ksec of observations of Cyg X-1 in the low/hard state and reanalyzed 10 ksec of high/soft state observations. We detected statistically significant variability up to frequencies of 300-400 Hz with the fractional rms amplitude in the 100-400 Hz frequency range 3% in the hard state and 2% in the soft state. The upper limits on the fractional rms amplitude of a QPO feature with Lorentzian profile in the 500-2000 Hz frequency range are 2% for and 0.9% for in the hard state and 2.5% () and 1.5% () in the soft state. Assuming that the model for the instrumental noise level is exact, the upper limits on the continuum variability in the 400-2000 Hz frequency range are 1% and % for the hard and soft state respectively.
We detected statistically significant steepening of the power spectrum in the hard state at frequencies Hz. The power law approximation to the spectrum at frequencies higher than 80-100 Hz gives the slope of 2.3-2.4. In the soft state the slope of power spectrum changes from to at the frequency of Hz without any evidence of further steepening up to the frequency of Hz.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000