Chen, L (reprint author), Beijing Normal Univ, Dept Astron, Beijing 100875, Peoples R China.
The power spectral density (PSD) of Cyg X-l is divided into two parts by using the linear state space model (LSSM). One part is the PSD of LSSM, the other is the residual power spectral density (RPSD) defined by the whole PSD minus the model PSD. We focus our attention on analyzing the RPSD. The results show that RPSD has different characteristics in 3 different states-the low/hard, transition and high/soft states. There is a broad peak structure both in the high/soft and low/hard states, with average central frequencies similar to 8Hz and similar to 5 HZ respectively The RPSD in the low/hard state may peak at similar to 0.07 HE. While during the Transition State, the RPSD has a double-peak structure, with the central frequencies in the regions of 0.5-4Hz and 5-11Hz respectively and these are quite close to the QPO central frequencies we have previously obtained. This fact implies that the QPO and the auto-regression that is needed in LSSM belong to different physical processes. Comparing to PSD, the RPSD is more sensitive to variations, such as the QPO, the broken frequencies and so on, The reason for the variable peak in the RPSD is discussed.