The evolving X-ray spectrum of active galactic nuclei: evidence for an
  increasing reflection fraction with redshift
  The cosmic X-ray background (XRB) spectrum and active galaxy number counts
encode essential information about the spectral evolution of active galactic
nuclei (AGNs) and have been successfully modeled by XRB synthesis models for
many years. Recent measurements of the $8$--$24$ keV AGN number counts by
NuSTAR and Swift-BAT are unable to be simultaneously described by existing XRB
synthesis models, indicating a fundamental breakdown in our understanding of
AGN evolution. Here we show that the $8$--$24$ keV AGN number counts can be
successfully modeled with an XRB synthesis model in which the reflection
strength ($R$) in the spectral model increases with redshift. We show that an
increase in reflection strength with redshift is a natural outcome of (1)
connecting $R$ to the incidence of high column density gas around AGNs, and (2)
the growth in the AGN obscured fraction to higher redshifts. In addition to the
redshift evolution of $R$, we also find tentative evidence that an increasing
Compton-thick fraction with $z$ is necessary to describe the $8$--$24$ keV AGN
number counts determined by NuSTAR. These results show that, in contrast to the
traditional orientation-based AGN unification model, the nature and covering
factor of the absorbing gas and dust around AGNs evolve over time and must be
connected to physical processes in the AGN environment. Future XRB synthesis
models must allow for the redshift evolution of AGN spectral parameters. This
approach may reveal fundamental connections between AGNs and their host
galaxies.
