Cygnus X-1: A Case for a Magnetic Accretion Disk?
  With the advent of RXTE, which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce -- or at the very least not fundamentally disagree with - all of Cygnus X-1's most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other currently discussed models. 
Michael A. Nowak (JILA), Brian A. Vaughan (Caltech), James Dove   (JILA), Joern Wilms (IAA-Tuebingen)