Seyfert galaxies were classified on the basis of their optical spectroscopic properties into types 1 and 2, (Weedman 1970, Weedman 1973 and Khachikian & Weedman 1974). This enduring classification highlighted an important physical distinction and ultimately led to Unified Schemes for AGN, which have formed the basis for most observational and theoretical progress in AGN for the last 15 years. The Xray band is another region of the electromagnetic spectrum rich in atomic transitions. With the advent of the high resolution X-ray spectroscopy from Chandra and XMM-Newton, resonant absorption lines and photoelectric edges from neutral (mostly in type 2s) as well as highly ionized (mostly in type 1s) ions of C, O, N, Ne, Mg, Si, S and Fe, have been clearly detected in the X-ray spectra of most of the nearby bright AGNs. We are analyzing several type-1 optically classified AGNs observed at high spectral resolution with the Chandra gratings.
As a first approach, we have compared the lines: OVII resonance (r), intercombination (i), forbidden (f); and OVIII LyΒ«. The lines have been fitted in Sherpa as gaussians added to a power law that fits the local continuum. There are uncertainties as to determine the underlying continuum (Morales, Nicastro and Elvis, 2002). Figure shows for each source the best fit to the lines mentioned above (e.g. the OVII(f) line may be strong or entirely absent).
The most striking result of this comparison is the variety of behavior in the X-ray lines of objects optically classified as Seyfert 1s. The differences are both dynamical (presence of blueshifts and/or redshifts) and physical (presence of absorption and/or emission); see table below. We propose to take a fresh look at AGN classification based purely on their X-ray spectroscopic properties. It is reasonable that X-ray spectral classification will prove as valuable for the warm high ionization medium in AGN as optical classification proved to be for cooler, lower ionization gas.