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Similar Shot Noise in Cyg X-1, GRO J0422+32 and 1E 1724-3045

We study the rapid variability in Cyg X-1 observed with the Rossi Timing Explorer (RXTE) on Jan 17 and 20 of 1997. The power spectra below 200 Hz can be characterized by two ``shot noise'' components and a peaked-noise centered at 0.2 Hz. This is similar to those ``shot noise'' observed in another black hole candidate GRO J0422+32 and a neutron star X-ray binary 1E 1724-3045. The similarity suggests that the generation mechanism of ``shot noise'' and peaked-noise is probably similar in both black hole systems and neutron star systems, and in both high mass X-ray binaries (HMXBS) and low mass X-ray binaries (LMXBs). We have also analyzed ~ 500 brightest shots selected from the Cyg X-1 light curves. The time scale of the spectral variation around the shot can be as long as ~ a few seconds, consistent with Ginga results. The difference between the superposed shot profile in different energy bands can be attributed to the time lag and the different shot width between different energy bands. The shot width defined from the auto-correlation function of shot profile does not show a bimodal distribution.