THE PHYSICAL INTERPRETATION OF X-RAY PHASE LAGS AND COHERENCE - RXTE OBSERVATIONS OF CYGNUS X-1 AS A CASE-STUDY

There have been a number of recent spectral models that have been succ essful in reproducing the observed X-ray spectra of galactic black hol e candidates (GBHC). However, there still exists controversy over such issues as: what are the sources of hard radiation, what is the system 's geometry, is the accretion efficient or inefficient, etc. A potenti ally powerful tool for distinguishing among these possibilities: made possible by the Rossi X-ray Taming Explorer (RXTE), is the variability data, especially the observed phase lags and variability coherence. T hese data, in conjunction with spectral modeling, have the potential o f determining physical sizes of the system, as well as placing strong constraints on both Compton corona and advection models. As an example , we present RXTE variability data of Cygnus X-1.