The 150d Modulation of the X-ray Emission of CYG X-1

The X-ray observations of Cyg X-1, covering a period of 3.8 years, by the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE) were analysed to search for long periodicities. The periodicities in the original data were seen to be distorted by the high/soft state and the failed state counts of the system, and thus the power spectra showed two unrealistic peaks at 1608and 406 days. The ∼ 300 day modulation reported previously(e.g. Priedhorsky et al. 1983) is not confirmed by the present ASM data. Truncation of the high counts above 30 cnts/s partly eliminates the distortion and reveals the true period of 150d in the power spectra. It is interesting that the high/soft state (HS) and the failed state transitions (FST) always occur on the maximum phase of the150d cycle.