Dissecting Photometric redshift for Active Galactic Nuclei using XMM-
  and Chandra-COSMOS samples
  With this paper, we release accurate photometric redshifts for 1692
counterparts to Chandra sources in the central square degree of the COSMOS
field. The availability of a large training set of spectroscopic redshifts that
extends to faint magnitudes enabled photometric redshifts comparable to the
highest quality results presently available for normal galaxies. We demonstrate
that morphologically extended, faint X-ray sources without optical variability
are more accurately described by a library of normal galaxies (corrected for
emission lines) than by AGN-dominated templates, even if these sources have
AGN-like X-ray luminosities. Preselecting the library on the bases of the
source properties allowed us to reach an accuracy sigma_(Delta z/(1+z_spec))
\sim0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS
sample. In addition, we release revised photometric redshifts for the 1735
optical counterparts of the XMM-detected sources over the entire 2 sq. deg.of
COSMOS. For 248 sources, our updated photometric redshift differs from the
previous release by Delta z>0.2. These changes are predominantly due to the
inclusion of newly available deep H-band photometry H_AB=24 mag. We illustrate
once again the importance of a spectroscopic training sample and how an
assumption about the nature of a source together with the number and the depth
of the available bands influence the accuracy of the photometric redshifts
determined for AGN. These considerations should be kept in mind when defining
the observational strategies of upcoming large surveys targeting AGN, such as
eROSITA at X-ray energies and ASKAP/EMU in the radio band.
