The $\it{XXL}$ survey: XL. Obscuration properties of red AGN in
  $\it{XXL-N}$
  The combination of optical and mid-infrared (MIR) photometry has been
extensively used to select red active galactic nuclei (AGNs). Our aim is to
explore the obscuration properties of these red AGNs with both X-ray
spectroscopy and spectral energy distributions (SEDs). In this study, we
re-visit the relation between optical/MIR extinction and X-ray absorption. We
use IR selection criteria, specifically the $W1$ and $W2$ WISE bands, to
identify 4798 AGNs in the $\it{XMM-XXL}$ area ($\sim 25$deg$^2$). Application
of optical/MIR colours ($r- W2 > 6$) reveals 561 red AGNs (14$\%$). Of these,
47 have available X-ray spectra with at least 50 net (background-subtracted)
counts per detector. For these sources, we construct SEDs from the optical to
the MIR using the CIGALE code. The SED fitting shows that 44 of these latter 47
sources present clear signs of obscuration based on the AGN emission and the
estimated inclination angle. Fitting the SED also reveals ten systems
($\sim20\%$) which are dominated by the galaxy. In these cases, the red colours
are attributed to the host galaxy rather than AGN absorption. Excluding these
ten systems from our sample and applying X-ray spectral fitting analysis shows
that up to $76\%$ (28/37) of the IR red AGNs present signs of X-ray absorption.
Thus, there are nine sources ($\sim20\%$ of the sample) that although optically
red, are not substantially X-ray absorbed. Approximately $50\%$ of these
sources present broad emission lines in their optical spectra. We suggest that
the reason for this apparent discrepancy is that the r-W2 criterion is
sensitive to smaller amounts of obscuration relative to the X-ray spectroscopy.
In conclusion, it appears that the majority of red AGNs present considerable
obscuration levels as shown by their SEDs. Their X-ray absorption is moderate
with a mean of $\rm N_H \sim 10^{22}\, \rm{cm^{-2}}$.
