On the cosmic evolution of AGN obscuration and the X-ray luminosity function: XMM-Newton and Chandra spectral analysis of the 31.3 deg${}^2$ Stripe 82X

We present X-ray spectral analysis of XMM and Chandra observations in the 31.3 deg${}^2$ Stripe-82X (S82X) field. Of the 6181 X-ray sources in this field, we analyze a sample of 2937 active galactic nuclei (AGN) with solid redshifts and sufficient counts determined by simulations. Our results show a population with median values of spectral index $\mathrm{\Gamma }=1.94_{-0.39}^{+0.31}$, column density log$N_{\mathrm{H}}\mathrm{/}{\mathrm{c}\mathrm{m}}^{-2}=20.7_{-0.5}^{+1.2}$ and intrinsic, de-absorbed, 2-10 keV luminosity log$L_{\mathrm{X}}\mathrm{/}{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}}^{-1}=44.0_{-1.0}^{+0.7}$, in the redshift range 0-4. We derive the intrinsic fraction of AGN that are obscured ($22\le \mathrm{l}\mathrm{o}\mathrm{g}{N}_{\mathrm{H}}\mathrm{/}{\mathrm{c}\mathrm{m}}^{-2}<24$), finding a significant increase in the obscured AGN fraction with redshift and a decline with increasing luminosity. The average obscured AGN fraction is $57\pm 4\mathrm{\%}$ for log$L_{\mathrm{X}}\mathrm{/}{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}}^{-1}>43$. This work constrains the AGN obscuration and spectral shape of the still uncertain high-luminosity and high-redshift regimes (log$L_{\mathrm{X}}\mathrm{/}{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}}^{-1}>45.5$, $z>3$), where the obscured AGN fraction rises to $64\pm 12\mathrm{\%}$. We report a luminosity and density evolution of the X-ray luminosity function, with obscured AGN dominating at all luminosities at $z>2$ and unobscured sources prevailing at log$L_{\mathrm{X}}\mathrm{/}{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}}^{-1}>45$ at lower redshifts. Our results agree with 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. The derived BHAD evolution shows that Compton-thick AGN contribute to the accretion history of AGN as much as all other AGN populations combined.