  Using the data from the Rossi X-ray Timing Explorer (RXTE), we report the
different energy dependence of the variability of the four persistent hard
X-ray sources in the low-hard state: Cygnus X-1, GX 339-4, GRS 1758-258 and 1E
1740.7-2942. Cygnus X-1 is found to have a flatter power density spectrum (PDS)
shape at higher energies. The other three sources have energy independent PDS
shapes. The energy dependence of the overall variability (the integrated rms
amplitude) varies from source to source and from observation to observation.
1E~1740.7-2942, for example, has a variability generally increasing with energy
while GX 339-4 has a decreasing variability. A general trend is found in the
four sources that the integrated rms amplitude anti-correlates with the X-ray
flux. We compare these distinct energy dependent behaviors with several
emission models. None of the models can fully explain all the features that we
have found.
