Other
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...194.3405b&link_type=abstract
American Astronomical Society, 194th AAS Meeting, #34.05; Bulletin of the American Astronomical Society, Vol. 31, p.873
Other
Scientific paper
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_ox <= -2 QSOs from Boroson & Green (1992). SXW QSOs comprise ~ 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 iv absorption in 55 of the 87 QSOs, we find a remarkably strong correlation between alpha_ox and the C 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 iii] luminosities as well as distinctive Hβ -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_Edd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of dot M/dot M_Edd.
Brandt Wiliam N.
Laor Ari
Wills Beverley J.
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