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 α ox ≤ −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 α ox 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 Soft X-ray Weak QSO SelectionWe have selected our SXW QSOs from the Boroson & Green (1992; hereafter BG92) sample. This sample is composed of all 87 QSOs from the Bright Quasar Survey (BQS; Schmidt & Green 1983) with z < 0.5, and our intention is to perform a thorough analysis of all the SXW QSOs in this sample. The BQS consists of the broad-line active galaxies with dominant star-like nuclei to the survey limits: B < ∼ 16.2 and U − B < ∼ − 0.44 (for Galactic latitudes greater than 30 • and declinations above −10 • ). To these limits it is thought to be > ∼ 70% complete at z < 0.5 (see Véron et al. 1999), and a large amount of high-quality and uniformly analyzed data are available for the BG92 QSOs. Since the BQS QSOs were optically selected, the BQS should not be directly biased with respect to the inclusion or exclusion of SXW QSOs. Possible indirect biases are, unfortunately, inescapable when using current large-area QSO surveys (see Goodrich 1997 andKrolik &Voit 1998 for examples of potentially relevant indirect biases).
We have produced the next generation of quasar spectral energy distributions (SEDs), essentially updating the work of Elvis et al. (1994) by using high-quality data obtained with several space and ground-based telescopes, including NASA's Great Observatories. We present an atlas of SEDs of 85 optically bright, non-blazar quasars over the electromagnetic spectrum from radio to X-rays. The heterogeneous sample includes 27 radio-quiet and 58 radio-loud quasars. Most objects have quasi-simultaneous ultraviolet-optical spectroscopic data, supplemented with some far-ultraviolet spectra, and more than half also have Spitzer mid-infrared IRS spectra. The X-ray spectral parameters are collected from the literature where available. The radio, far-infrared, and near-infrared photometric data are also obtained from either the literature or new observations. We construct composite spectral energy distributions for radio-loud and radio-quiet objects and compare these to those of Elvis et al., finding that ours have similar overall shapes, but our improved spectral resolution reveals more detailed features, especially in the mid and near-infrared.
We have observed the narrow-line regions (NLRs) of the seven brightest radio-quiet PG (or BQS) quasars (z < 0.5) with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). Linear-ramp filters were used to image the [OIII] lambda 5007 line emission with 0.0455-0.1 arcsec pixel resolution. We find that the NLRs are very compact with typical extents of 2-4 arcsec. Two quasars show compact filamentary structures similar to Seyfert NLRs. They may be related to radio outflows. Most interestingly, when including a sample of Seyfert galaxies observed with HST, we tentatively find that the size of the NLR is proportional to the square root of the [OIII] luminosity. This is comparable to the scaling found for the size of the broad-line region with continuum luminosity, which has been interpreted in terms of a constant photoionization parameter. The relation determined here connects the NLR of radio-quiet quasars and Seyferts over three orders of magnitude in [OIII] luminosity.Comment: 6 pages, 4 figures, accepted for publication in ApJ Letter
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