X-ray flux in the SED modelling: An application of X-CIGALE in the
  XMM-XXL field
  X-CIGALE, built upon the spectral energy distribution (SED) code of CIGALE,
implements important new features: The code accounts for obscuring material in
the polars of the AGN and has the ability to fit X-ray fluxes. In this work, we
use ~2500 spectroscopic, X-ray AGN from the XMM-XXL-North field and examine the
improvements the new features bring in the SED modelling analysis. Based on our
results, X-CIGALE successfully connects the X-ray with the UV luminosity in the
whole range spanned by our sample (log Lx(2-10 keV) = (42 - 46) erg/s). The
addition of the new features globally improves the efficiency of X-CIGALE in
the estimation and characterization of the AGN component. The classification
into type 1 and type 2 based on their inclination angle is improved, especially
at redshifts lower than 1. The statistical significance of the AGN fraction,
fracAGN, measurements is increased, in particular for luminous X-ray sources
(log LX > 45 erg/s). These conclusions hold under the condition that (mid-) IR
photometry is available in the SED fitting process. The addition of polar dust
increases the AGN fraction and the efficiency of the SED decomposition to
detect AGN among X-ray selected sources. X-CIGALE estimates a strong AGN
(fracAGN > 0.3) in more than 90% of the infrared selected AGN and 75% of X-ray
detected AGN not selected by IR colour criteria. The latter drops to ~50% when
polar dust is not included. The ability of X-CIGALE to include X-ray
information in the SED fitting process can be instrumental in the optimal
exploitation of the wealth of data that current (eROSITA) and future (ATHENA)
missions will provide us.
