A Search for Aperiodic Millisecond Variability in Cygnus X-1

We have conducted a search for aperiodic millisecond variability in the integrated 1-25 keV X-ray region of Cyg X-1. We have examined HEAO A-1 archival data and Rossi X-Ray Timing Explorer (RXTE) guest observer data for evidence of excess power above the Poisson noise floor using the relative integral power analysis and the Fourier transform method. Our results are in disagreement with the results of Meekins and coworkers. We attribute the discrepancy to an instrumental effect for which Meekins and coworkers did not apply a correction. With the correction we see no evidence for excess power above 25 Hz in the HEAO A-1 data. Our analysis of RXTE data is in agreement with previously published results of different data sets and shows no indication of excess power above 30 Hz.