The OPTX Project III: X-ray versus Optical Spectral Type for AGNs
  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 > 3.5x10^-15 erg/cm2/s), highly
spectroscopically complete (>80% for f_2-8 keV > 10^-14 erg/cm2/s and >60%
below) 2-8 keV X-ray sample observed in three Chandra fields (CLANS, CLASXS,
and the CDF-N) that cover ~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, Geff,
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% +/- 3% (33% +/- 4%) have Geff<1.2 (Geff > 1.2). Thus, one cannot 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 Geff.
However, this dependence breaks down at higher redshifts due to the highly
redshift-dependent nature of Geff. We therefore also caution that any
classification scheme which depends on Geff is likely to suffer from serious
redshift bias.
