Rossi X-Ray Timing Explorer observation of Cygnus X-1. III. Implications for Compton corona and advection-dominated accretion flow models

We have recently shown that a "sphere+disk" geometry Compton corona model p rovides a good description of Rossi X-Ray Timing Explorer (RXTE) observatio ns of the hard/low state of Cygnus X-1. Separately, we have analyzed the te mporal data provided by RXTE. In this paper we consider the implications of this timing analysis for our best-fit "sphere+disk" Comptonization models. We focus our attention on the observed Fourier frequency-dependent time de lays between hard and soft photons. We consider whether the observed time d elays are created in the disk but are merely reprocessed by the corona, cre ated by differences between the hard and soft photon diffusion times in cor onae with extremely large radii, or are due to "propagation" of disturbance s through the corona. We find that the time delays are most likely created directly within the corona; however, it is currently uncertain which specif ic model is the most likely explanation. Models that posit a large coronal radius (or equivalently, a large advection-dominated accretion flow region) do not fully address all the details of the observed spectrum. The Compton corona models that do address the full spectrum do not contain dynamical i nformation. We show, however, that simple phenomenological propagation mode ls for the observed time delays for these latter models imply extremely slo w characteristic propagation speeds within the coronal region.