THE OPTX PROJECT. III. X-RAY VERSUS OPTICAL SPECTRAL TYPE FOR ACTIVE GALACTIC NUCLEI

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 x 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 similar to 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, broad-line active galactic nuclei (AGNs) are expected to be unobscured and hence X-ray soft (Gamma(eff) >= 1.2), yet we find 20% +/- 3% have Gamma(eff) >= 1.2. Non-broad-line AGNs are expected to be obscured and hence X-ray hard (Gamma(eff) < 1.2), yet we find 33% +/- 4% have Gamma(eff) >= 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 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.