2000A&A...363.1013R

We analyzed a large number of RXTE/PCA observations of Cyg X-1 in the hard and soft spectral states with total exposure time of ≃190 and ≃10ksec respectively and time resolution better than ≃250µs in order to investigate its variability down to a few milliseconds time scales. The model of modifications of the power density spectra due to the dead time effect was tested using RXTE observations of an extremely bright source - Sco X-1 . The results of these tests demonstrated, that although some problems still remain, for sources like Cyg X-1 (which is an order of magnitude less bright than Sco X-1 ) present knowledge of the instrument is sufficient to reliably analyze the power spectra in the kHz frequency range. In both spectral states of Cyg X-1 we detected statistically significant variability up to ∼150-300Hz with a fractional rms amplitude above 100Hz at the level of ∼2-3%. The power spectrum in the hard state shows a steepening at frequency of ∼40-80Hz with the power law slope changing from ∼1.7 to ∼2.3-2.4. In the soft state the slope of power spectrum changes from ∼1 to ∼2 at the frequency of ∼15-20Hz without any evidence of further steepening up to ∼ 100-150Hz. These break frequencies represent the highest characteristic frequencies detected in the power density spectrum of Cyg X-1 so far.