Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, High Energy Astrophys Lab, Beijing 100039, Peoples R China
Quantities characterizing temporal property, e.g., power density, coherence, and time lag, can be defined and calculated directly in the time domain without using the Fourier transformation. Spectral hardness, variability duration, and correlation between different characteristic quantities on different time scale can be studied in the time domain as well. The temporal analysis technique in the time domain is a powerful tool, particularly in studying rapid variability on short time scales (or in high frequencies). Results of studying variabilities of X-rays from Cyg X-1 with the analysis technique in the time domain and RXTE data reveal valuable clues to understanding production and propagation processes of X-rays and structure of accretion disk in the black hole system.