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Our goal is to probe the populations of obscured and unobscured AGNinvestigating their optical-IR and X-ray properties as a function of X-rayflux, luminosity and redshift within a hard X-ray selected sample of 136 X-raysources in the XMM Medium Deep Survey (XMDS) with wide multiwavelengthcoverage. The XMDS area is covered with optical photometry from the VVDS andCFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity andX-ray to optical ratio, 132 sources are likely AGN, of which 122 haveunambiguous optical - IR identification. The observed optical and IR spectralenergy distributions of sources are fitted with AGN/galaxy templates in orderto classify them and compute photometric redshifts. 70% of the AGN are fittedby a type 2 AGN or a star forming galaxy template and are grouped together in asingle class of ``optically obscured'' AGN. They have ``red'' optical colorsand generally show significant X-ray absorption from X-ray spectra or hardnessratios (N$_H > 10^{22}$ cm$^{-2}$). Sources with SEDs typical of type 1 AGNhave ``blue'' optical colors and exhibit X-ray absorption in about 30% ofcases. We performed a stacking analysis for obscured and type 1 AGN. Thestacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN andhas an average spectral slope of Gamma = 1.6. The subsample of objects fittedby a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 -1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lowerluminosities. X-ray absorption is less common than ``optical'' obscuration andits incidence is nearly constant with redshift and luminosity. This impliesthat X-ray absorption is not necessarily related to optical obscuration.Comment: 33 pages, 21 figures, accepted for publication in A&

Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey