Power spectral density analysis of hard X-ray emission of Cyg X-1

We report on results of analysis sperformed on data of three balloon observations of Cyg X-l in the range 20-200 keV. Evidence of periodic, even if not stable, pulsations were previously obtained at about 0.058 Hz. The continuous power spectrum, still showing a broad peak around the above frequency, can be explained on the basis of a model, similar to the shot noise one, where the single shot is replaced by a short sequence of equispaced pulses with a decaying amplitude. Such a model could also explain the bursting activity observed at lower energies.Now at Istituto Plasma Spazio/CNR, Via G.Galilei, 00044 Frascati, ITALY.