On the cosmic evolution of AGN obscuration and the X-ray luminosity function: XMM-Newton and Chandra spectral analysis of the 31.3 deg² Stripe 82X

We present the X-ray spectral analysis of XMM and Chandra observations in the 31.3 deg² Stripe-82X (S82X) field. Of the 6181 unique X-ray sources in this field, we analyze a sample of 2937 candidate active galactic nuclei (AGN) with solid redshifts (0 < z < 4) and sufficient counts determined by simulations. Correcting for observational biases, we derive the intrinsic, model-independent, fraction of AGN that are obscured (22 < log NH/cm² < 24), finding a significant increase in the obscured AGN fraction with redshift and a decline with increasing luminosity. This work constrains the AGN obscuration and spectral shape of the still uncertain high-luminosity and high-redshift regimes (log Lx/erg/s/cm² > 45.5, z > 3), where the obscured AGN fraction reaches 64±12%. The total, unobscured, and obscured X-ray luminosity functions (XLFs) are determined up to z=4. We report a luminosity and density evolution of the total XLF, with obscured AGN dominating at all luminosities at z > 2 and unobscured sources prevailing at log Lx/erg/s/cm² > 45 at lower redshifts. Our results agree with the evolutionary models in which the bulk of AGN activity is triggered by gas-rich environments and in a downsizing scenario. Also, the black hole accretion density (BHAD) is found to evolve similarly to the star formation rate density, confirming the co-evolution between AGN and host-galaxy, but suggesting different time scales in their growing history. Intriguingly, the derived BHAD evolution shows that Compton-thick (log NH/cm² > 24) AGN contribute to the accretion history of AGN as much as all other AGN populations combined, significantly exceeding previous estimates (e.g., Ueda et al. 2014, Aird et al. 2015).