  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.
