Rossi X-ray timing explorer observation of Cygnus X-1. II. Timing analysis

We present timing analysis for a Rossi X-Ray Timing Explorer (RXTE) observa tion of Cygnus X-l in its hard/low state. This was the first RXTE observati on of Cyg X-l taken after it transited back to this state from its soft/hig h state. RXTE's large effective area, superior timing capabilities, and abi lity to obtain long, uninterrupted observations have allowed us to obtain m easurements of the power spectral density (PSD), coherence function, and Fo urier time lags to a decade lower in frequency and 0.5 decade higher in fre quency than typically was achieved with previous instruments. Notable aspec ts of our observations include a weak 0.005 Hz feature in the PSD coinciden t with a coherence recovery; a "hardening" of the high-frequency PSD with i ncreasing energy; a broad frequency range measurement of the coherence func tion, revealing rollovers from unity coherence at both low and high frequen cy; and an accurate determination of the Fourier time lags over 2.5 decades in frequency. As has been noted in previous similar observations, the time delay is approximately proportional to f(-0.7), and at a fixed Fourier fre quency the time delay of the hard X-rays(-)compared to the softest energy c hannel tends to increase logarithmically with energy. Curiously, the 0.01-0 .2 Hz coherence between the highest and the lowest energy bands is actually slightly greater than the coherence between the Second highest and the low est energy bands. We carefully describe all of the analysis techniques used in this paper, and we make comparisons of the data to general theoretical expectations. In a companion paper, we make specific comparisons to a Compt on corona model that we have successfully used to describe the energy spect ral data from this observation.