  We analyze the 2.5--10 keV X-ray spectra of the luminous quasar 3C 273 and
simultaneous observations in UV wavelengths from XMM--Newton between 2000 and
2015. The lowest flux level ever was observed in 2015. The continuum emission
from 3C 273 is generally best described by an absorbed power-law but during
extremely low states the addition of fluorescence from the K-shell iron line
improves the fit. We study the spectral evolution of the source during its
extended quiescent state and also examine connections between the X-ray and
ultraviolet emissions, which have been seen in some, but not all, previous
work. We detect a possible anti-correlation between these two bands during the
low state that characterized 3C 273 for most of this period; however, this was
not present during a flaring state. A harder-when-brighter trend for the X-ray
spectrum was observed in these long-term observations of 3C 273 for the first
time. We suggest that the X-ray emission in 3C 273 is the result of inverse
Compton scattering of soft UV seed photons (emitted from the local environment
of the AGN), most likely in a thermal corona. We can explain the significant
temporal variation of the spectral continuum as an outcome of changing optical
depth of the comptonizing medium, along the lines of the wind-shock model
proposed by Courvoisier and Camenzind (1989).
