The spectral index of the ionizing continuum of quasars
  The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution short-ward of 1200 A. The change of the power-law index from approximately -1 to -2 so far has been interpreted as being intrinsic to quasars. We study an alternative interpretation, in which a tenuous absorption screen is responsible for the change of index. We find that the most successful function of HI with z requires that the ionization fraction be independent of the metagalactic background radiation, that is frozen in. This implies that the putative component should be of sufficiently low density that it could not recombine (while the background is subsiding) and/or cool from a high temperature state. 
L. Binette, M. Rodriguez-Martinez, I. Ballinas