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تسجيل مستخدم جديدX-ray constraints on the fraction of obscured AGN at high accretion luminosities
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The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured AGN at high accretion luminosities, $L_X (rm 2-10 , keV) > 10^{44} , erg ,s ^{-1}$, and out to redshift $zapprox1.5$. The sample covers an area of about $rm14,deg^2$ and provides constraints on the space density of powerful AGN over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, $rm N_Happrox10^{24},cm^{-2}$. The fraction of obscured Compton-thin ($rm N_H=10^{22}-10^{24},cm^{-2}$) AGN is estimated to be $approx0.35$ for luminosities $L_X(rm 2-10,keV)>10^{44},erg,s^{-1}$ independent of redshift. For less luminous sources the fraction of obscured Compton-thin AGN increases from $0.45pm0.10$ at $z=0.25$ to $0.75pm0.05$ at $z=1.25$. Studies that select AGN in the infrared via template fits to the observed Spectral Energy Distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGN (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGN as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGN is primarily a function of redshift. This fraction increases from about 20-30% at z=0.25 to about 50-70% at $z=1.5$.
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