The 150(D) 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 A ll Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE) were a nalysed 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 pe aks at 1608 and 406 days. The similar to 300 day modulation reported previo usly (e.g. Priedhorsky et al. 1983) is not confirmed by the present ASM dat a. Truncation of the high counts above 30 cnts/s partly eliminates the dist ortion and reveals the true period of 150(d) in the power spectra. It is in teresting that the high/soft state (HS) and the failed state transitions (F ST) always occur on the maximum phase of the 150(d) cycle.