AGN Populations in Large Volume X-ray Surveys: Photometric Redshifts and
  Population Types found in the Stripe 82X Survey
  Multi-wavelength surveys covering large sky volumes are necessary to obtain
an accurate census of rare objects such as high luminosity and/or high redshift
active galactic nuclei (AGN). Stripe 82X is a 31.3 deg$^2$ X-ray survey with
$Chandra$ and $XMM$-Newton observations overlapping the legacy Sloan Digital
Sky Survey (SDSS) Stripe 82 field, which has a rich investment of
multi-wavelength coverage from the ultraviolet to the radio. The wide-area
nature of this survey presents new challenges for photometric redshifts for AGN
compared to previous work on narrow-deep fields because it probes different
populations of objects that need to be identified and represented in the
library of templates. Here we present an updated X-ray plus multi-wavelength
matched catalog, including $Spitzer$ counterparts, and estimated photometric
redshifts for 5961 (96% of a total of 6181) X-ray sources, which have a
normalized median absolute deviation, $\sigma_{\rm nmad}$ = 0.06 and an outlier
fraction, $\eta$ = 13.7%. The populations found in this survey, and the
template libraries used for photometric redshifts, provide important guiding
principles for upcoming large-area surveys such as $eROSITA$ and 3$XMM$ (in
X-ray) and the Large Synoptic Survey Telescope (LSST; optical).
