  (abbreviated abstract) We present the long term spectral evolution of the
Galactic black hole candidate Cygnus X-1 in the X-rays and at 15GHz using ~200
pointed observations taken between early 1999 and late 2004 with RXTE and the
Ryle radio telescope. The X-ray spectra are remarkably well described by a
simple broken power law spectrum with an exponential cutoff. Physically
motivated Comptonization models, e.g., compTT or eqpair, can reproduce this
simplicity. Broken power law models reveal a significant linear correlation
between the photon index of the lower energy power law and the hardening of the
power law at approximately 10keV. Comptonization models show that the
bolometric flux of a soft excess (e.g., disk component) is strongly correlated
with the compactness ratio of the Comptonizing medium. We also find that the
fraction of the time spent in low radio emission/soft X-ray spectral states has
increased from ~10% in 1996-2000 to ~34% since early 2000. Radio flares
typically occur during state transitions and failed state transitions. There is
also a strong correlation between the 10-50keV X-ray flux and the radio
luminosity of the source. We demonstrate that rather than there being
distinctly separated states, in contrast to the timing properties the spectrum
of Cyg X-1 shows variations between extremes of properties, with clear cut
examples of spectra at every intermediate point in the observed spectral
correlations.
