The hard, power-law tail in the x-ray spectrum of Cyg X-1 and other black-hole candidates is commonly explained as inverse Compton scattering of photons produced in an accretion disk in a hot electron corona surrounding the disk. We are using XTE's timing and spectral capabilities in concert to undertake a thorough observational test of the Comptonization models of Cygnus X-1. We model Comptonization spectra, as well as mean photon scattering times and their variances, via a fully self-consistent Non-Linear Monte Carlo code. Guided by these models, we use XTE power and cross power spectra to look for: time lag shelfs between hard and soft energies, energy dependence of hard X-ray lags, and loss of coherence between variability at hard and soft energies. These observations are helping to determine cloud temperature, mean free scattering path, and whether the cloud fluctuates. In addition, our timing studies allow us to investigate the soft input spectrum as well.
Observations by previous instruments suggested that a loss of cross spectral
coherence occurs between photons of low and high energies at high Fourier
frequencies. We have investigated this in the papers listed below.