AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 34. Imaging, Spectra and Variability of X-ray Emission in AGN
Oral, Monday, May 31, 1999, 2:00-3:30pm, Waldorf

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[34.05] On the Nature of Soft X-ray Weak Quasi-Stellar Objects

W.N. Brandt (Penn State), A. Laor (Technion), B.J. Wills (UT-Austin)

Recent X-ray studies of QSOs suggest the existence of a significant population of Soft X-ray Weak QSOs (SXW~QSOs) where the soft X-ray flux is ~~10--30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to BAL~QSOs and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW~QSOs which includes all \alpha\rm ox\leq -2 QSOs from Boroson & Green (1992). SXW~QSOs comprise \approx 11% of the optically selected QSO population, and soft X-ray weakness is found in both radio-quiet and radio-loud QSOs. We look for clues to the origin of soft X-ray weakness by comparing the multiwavelength properties of our SXW~QSOs to those of the other Boroson & Green QSOs. From an analysis of C~{\sc iv} absorption in 55 of the 87 QSOs, we find a remarkably strong correlation between \alpha\rm ox and the C~{\sc iv} absorption equivalent width. This correlation provides highly suggestive evidence that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW~QSOs and BAL~QSOs.

We have also identified several notable differences between the optical emission-line properties of SXW~QSOs and those of the other Boroson & Green QSOs. SXW~QSOs show systematically low [O~{\sc iii}] luminosities as well as distinctive H\beta-line profiles. They tend to lie toward the negative end of Boroson & Green eigenvector~1, as do many low-ionization BAL~QSOs. Unabsorbed Seyferts/QSOs with large negative values of eigenvector~1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate compared to the Eddington rate (\dot M/\dot M\rm Edd). If these suggestions are correct, it is likely that SXW~QSOs also tend to have generally high values of \dot M/\dot M\rm Edd.

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