  We present improved synthesis models of the evolving spectrum of the UV/X-ray
diffuse background, updating and extending our previous results. A "minimal
cosmic reionization model" is presented in which the galaxy UV emissivity
traces recent determinations of the cosmic history of star formation, the
luminosity-weighted escape fraction of hydrogen-ionizing radiation increases
rapidly with redshift, from 1% at z=3 to 50% at z=8.5, the clumping factor of
the intergalactic medium is C=3, and Population III stars make a negligible
contribution to the metagalactic flux. The model provides a good fit to the
hydrogen-ionization rates inferred from flux decrement measurements, and yields
an optical depth to Thomson scattering, tau_e=0.085, that is in agreement with
WMAP results.
