X-ray flux in SED modelling: An application of X-CIGALE in the XMM-XXL field

X-CIGALE is built on the spectral energy distribution (SED) code of CIGALE and implements important new features: the code accounts for obscuring material in the poles of AGNs and has the ability to fit X-ray fluxes. In this work, we used ∼2500 spectroscopic X-ray AGNs from the XMM-XXL-north field and examined the improvements the new features bring to 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 L_X(2-10 keV) = (42-46) erg s^-1). The addition of the new features generally improves the efficiency of X-CIGALE in the estimation and characterisation of the AGN component. Classification as type 1 or type 2 based on their inclination angle is improved, especially at redshifts lower than 1. The statistical significance of AGN fraction, frac_AGN, measurements is increased, in particular for luminous X-ray sources (L_X > 10⁴⁵ erg s^-1). 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 AGNs among X-ray selected sources. X-CIGALE estimates a strong AGN (frac_AGN > 0.3) in more than 90% of the IR-selected AGNs and 75% of X-ray-detected AGNs 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.