2009ApJ...695..921W

We propose the timescale-resolved spectroscopy (TRS) as a new method to combine the timing and spectral study. The TRS is based on the time domain power spectrum and reflects the variable amplitudes of spectral components on different timescales. We produce the TRS with the RXTE PCA data for Cyg X-1 and study the spectral parameters (the power-law photon index and the equivalent width of the iron fluorescent line) as a function of timescale. The results of TRS and frequency-resolved spectra have been compared, and similarities have been found for the two methods with the identical motivations. We also discover the correspondences between the evolution of photon index with timescale and the evolution of the equivalent width with timescale. The observations can be divided into three types according to the correspondences and different type is connected with different spectral state.