High frequencies in the power spectrum of Cyg X-1 in the hard and soft spectral states

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 approximate to 190 and approximate to 10 ksec respectively and time resolution better than approx imate to 250 mus in order to investigate its variability down to a few mill iseconds time scales. The model of modifications of the power density spect ra due to the dead time effect was tested using RXTE observations of an ext remely bright source - Sco X-1. The results of these tests demonstrated, th at 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 in strument is sufficient to reliably analyze the power spectra in the kHz fre quency range. In both spectral states of Cyg X-1 we detected statistically significant va riability up to similar to 150-300 Hz with a fractional rms amplitude above 100 Hz at the level of similar to 2-3%. The power spectrum in the hard sta te shows a steepening at frequency of similar to 40-80 HZ With the power la w slope changing from similar to 1.7 to similar to 2.3-2.4. In the soft sta te the slope of power spectrum changes from similar to 1 to similar to 2 at the frequency of similar to 15-20 Hz without any evidence of further steep ening up to similar to 100-150 Hz. These break frequencies represent the hi ghest characteristic frequencies detected in the power density spectrum of Cyg X-1 so far.