On the Nature of Soft X‐Ray Weak Quasi‐stellar Objects

Recent studies of QSOs with ROSAT suggest the existence of a significantpopulation of Soft X-ray Weak QSOs (SXW QSOs) where the soft X-ray flux is ~10-30 times smaller than in typical QSOs. As a first step in a systematic studyof these objects, we establish a well-defined sample of SXW QSOs which includesall alpha_ox<=-2 QSOs from the Boroson & Green (1992) sample of 87 BQS QSOs.SXW QSOs comprise about 11% of this optically selected QSO sample. From ananalysis of CIV absorption in the 55 BG92 QSOs with available CIV data, we finda remarkably strong correlation between alpha_ox and the CIV absorptionequivalent width. This correlation suggests that absorption is the primarycause of soft X-ray weakness in QSOs, and it reveals a continuum of absorptionproperties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs andBAL QSOs. From a practical point of view, our correlation demonstrates thatselection by soft X-ray weakness is an effective (>=80% successful) andobservationally inexpensive way to find low-redshift QSOs with strong andinteresting ultraviolet absorption. We have also identified several notabledifferences between the optical emission-line properties of SXW QSOs and thoseof the other BG92 QSOs. SXW QSOs show systematically low [O III] luminositiesas well as distinctive H-beta profiles. They tend to lie toward the weak-[OIII] end of BG92 eigenvector 1, as do many low-ionization BAL QSOs. UnabsorbedSeyferts and QSOs with similar values of eigenvector 1 have been suggested tohave extreme values of a primary physical parameter, perhaps mass accretionrate relative to the Eddington rate (M-dot/M-dot_{Edd}). If these suggestionsare correct, it is likely that SXW QSOs also tend to have generally high valuesof (M-dot/M-dot_{Edd}). (Abridged)