Evidence for long-period sporadic pulsations in the hard x-ray flux of Cygnus X-1

Power spectral density estimates performed on the data of three balloon observations of Cyg X-1 show a peak at a frequency of 5.75times10-2 Hz. This is interpreted as evidence for the presence of strong correlations in the hard X-ray flux of the source. That peak, however, appears to be present, in the single observation, only for limited periods of time, outside which it tends to disappear or to be displaced. Pulsations of limited duration, about 1 hour, cannot be excluded as a possible cause of the detected phenomenon