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

Recent studies of Quasi-Stellar Objects (QSOs) with ROSAT 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 Broad Absorption Line QSOs (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 αox ≤ −2 QSOs from the Boroson & Green (1992) sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈ 11% of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C iv absorption in the 55 BG92 QSOs with available C iv data, we find a remarkably strong correlation between αox and the C iv absorption equivalent width. This correlation suggests 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. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective ( > ∼ 80% successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption. 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 and equivalent