We compare the optical spectral types with the X-ray spectral properties for a uniformly selected ( sources with fluxes greater than the 3 ~ { } \sigma level and above a flux limit of f _ { 2 - 8 ~ { } keV } > 3.5 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } ) , highly spectroscopically complete ( > 80 % for f _ { 2 - 8 ~ { } keV } > 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } and > 60 % below ) 2 - 8 keV X-ray sample observed in three Chandra fields ( CLANS , CLASXS , and the CDF-N ) that cover \sim 1.2 deg ^ { 2 } .
For our sample of 645 spectroscopically observed sources , we confirm that there is significant overlap of the X-ray spectral properties , as determined by the effective photon indices , \Gamma _ { eff } , obtained from the ratios of the 0.5 - 2 keV to 2 - 8 keV counts , for the different optical spectral types .
For example , of the broad-line AGNs ( non-broad-line AGNs ) , 20 % \pm 3 % ( 33 % \pm 4 % ) have \Gamma _ { eff } < 1.2 ( \Gamma _ { eff } \geq 1.2 ) .
Thus , one can not use the X-ray spectral classifications and the optical spectral classifications equivalently .
Since it is not understood how X-ray and optical classifications relate to the obscuration of the central engine , we strongly advise against a mixed classification scheme , as it can only complicate the interpretation of X-ray AGN samples .
We confirm the dependence of optical spectral type on X-ray luminosity , and for z < 1 , we find a similar luminosity dependence of \Gamma _ { eff } .
However , this dependence breaks down at higher redshifts due to the highly redshift-dependent nature of \Gamma _ { eff } .
We therefore also caution that any classification scheme which depends on \Gamma _ { eff } is likely to suffer from serious redshift bias .