MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, cilt.265, sa.2, ss.347-350, 1993 (SCI-Expanded)
We calculate the power density spectrum of fluctuations in the X-ray turn-on times of Her X-1, including new data which extend the X-ray and optical flux observations to a span of over 20 yr with 221 distinct 35-d high-low flux cycles. If we assume that turn-on times define the period of the 35-d clock, this statistical interpretation of turn-on behaviour is consistent with a white-noise process in the first derivative of the 35-d phase fluctuations (or a random walk in clock phase). We discuss the implications of considering the 35-d clock mechanism as a noise process.