Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
; Univ Alabama, Dept Phys, Huntsville, AL 35899 USA
; NASA, George C Marshall Space Flight Ctr, Space Sci Lab, Huntsville, AL 35812 USA
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
The luminous electromagnetic emission from distant active galactic nuclei (AGNs) including quasars is believed to be powered by accretion onto supermassive black holes. In the standard unification model for AGNs, a dusty torus covers a significant portion of the viewing angles to the accretion disk and the black hole. The system is classified as a type I AGN if the accretion disk is viewed through the opening part; otherwise, it is called a type II AGN. Therefore, the ratio of type II to type I AGNs serves as a sensitive probe to the unification model. A surprising discovery made from several large sky coverage and/or deep AGN surveys is a significant anticorrelation between the type II fraction and the observed X-ray luminosity between 2 and 10 keV. This suggests two different luminosity functions for the two types of AGNs, thus challenging the AGN unification model. However, this observed anticorrelation is a natural consequence of the AGN unification model with only one intrinsic luminosity function if the inclination angle effects of the X-ray-emitting accretion disk are taken into account. Thus, the AGN unification model survived another critical test.