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We present the Ðnal results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low-z quasars. This sample includes all 23 quasars from the Bright Quasar Survey with z ¹ 0.400 and cm~2. Pointed ROSAT PSPC observations were made for all N H I Gal \ 1.9 ] 1020 quasars, yielding high signal-to-noise (S/N) spectra for most objects, which allowed an accurate determination of the spectral shape. The following main results were obtained :1. The spectra of 22 of the 23 quasars are consistent, to within D30%, with a single power-law model at rest-frame 0.2È2 keV. There is no evidence for signiÐcant soft excess emission with respect to the bestÐt power law. We place a limit (95% conÐdence) of D5 ] 1019 cm~2 on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are D1 ] 1019 cm~2 in the two highest S/N spectra.2. The mean 0.2È2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050È350 continuum recently determined by et al. for z [ 0.33 quasars. This sug-A Zheng (1996) gests that there is no steep soft component below 0.2 keV.3. SigniÐcant X-ray absorption (q [ 0.3) by partially ionized gas ("" warm absorber ÏÏ) in quasars is rather rare, occurring for of the population, which is in sharp contrast to lower luminosity active [5% galactic nuclei (AGNs), where signiÐcant absorption probably occurs for D50% of the population.4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emissionline parameters indicates that the strongest correlation is between the spectral slope and the Hb a x FWHM. A possible explanation for this remarkably strong correlation is a dependence of on a x L /L Edd , as seen in Galactic black hole candidates.5. The strong correlations between and Fe II/Hb, and the peak [O III] to Hb Ñux ratio are a x L *O III+ , veriÐed. The physical origin of these correlations is still not understood.6. There appears to be a distinct class of "" X-rayÈweak ÏÏ quasars, which form D10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10È30, than expected based on their luminosity at other bands and on their Hb luminosity. These may be quasars in which the direct X-ray source is obscured and only scattered X-rays are observed. 7. Thin accretion disk models cannot reproduce the observed 0.2È2 keV spectral shape, and they also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be maintaining a strong correlation between the optical and soft X-ray emission.8. The H I/He I ratio in the high Galactic latitude ISM must be within 20%, and possibly within 5%, of the total H/He ratio of 10, which indicates that He in the di †use H II gas component of the interstellar medium is mostly ionized to He II or He III.We Ðnally note the intriguing possibility that although in radio-loud quasars ([1.15^0.14) is Sax T signiÐcantly Ñatter than in radio-quiet quasars ([1.72^0.09) the X...

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).

The radiative efficiency of AGN is commonly estimated based on the total mass accreted and the total AGN light emitted per unit volume in the universe integrated over time (the Soltan argument). In individual AGN, thin accretion disk model spectral fits can be used to deduce the absolute accretion rateṀ , if the black hole mass M is known. The radiative efficiency η is then set by the ratio of the bolometric luminosity L bol toṀ c 2 . We apply this method to determine η in a sample of 80 PG quasars with well determined L bol , whereṀ is set by thin accretion disk model fits to the optical luminosity density, and the M determination based on the bulge stellar velocity dispersion (13 objects) or the broad line region (BLR). For the BLR-based masses, we derive a mean log η = −1.05 ± 0.52 consistent with the Soltan argument based estimates. We find a strong correlation of η with M , rising from η ∼ 0.03 at M = 10 7 M ⊙ and L/L Edd ∼ 1 to η ∼ 0.4 at M = 10 9 M ⊙ and L/L Edd ∼ 0.3. This trend is related to the overall uniformity of L opt /L bol in our sample, particularly the lack of the expected increase in L opt /L bol with increasing M (and decreasing L/L Edd ), which is a generic property of thermal disk emission at fixed η. The significant uncertainty in the M determination is not large enough to remove the correlation. The rising η with M may imply a rise in the black hole spin with M , as proposed based on other indirect arguments.