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

Abstract

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 that includes all αox ~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 widths as well as distinctive Hβ line profiles. They tend to lie toward the weak [O III] end of Boroson & Green eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed Seyfert galaxies and QSOs with similar values of eigenvector 1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate relative to the Eddington rate (M/MEdd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of M/MEdd. Finally, we present and discuss correlations between αox and other QSO observables after removal of the SXW QSOs.