2000ApJ...531..963L

Using the data from the Rossi X-Ray Timing Explorer (RXTE), we report the different energy dependence of the variability of the four hard X-ray sources in the hard/low 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 anticorrelates 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.