The soft X-ray properties of a complete sample of optically selected quasars .2. Final results

We present the final 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 less than or equal to 0.400 and N-HI(Gal) < 1.9 x 10(20) cm(-2). Pointed ROSAT PSPC observations were made for all 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 similar to 30%, with a single power-law model at rest-frame 0.2-2 keV. There is no evidence for significant soft excess emission with respect to the best-fit power law. We place a limit (95% confidence) of similar to 5 x 10(19) cm(-2) on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are similar to 1 x 10(19) cm(-2) in the two highest S/N spectra. 2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350 Angstrom continuum recently determined by Zheng et al. (1996) for z > 0.33 quasars. This suggests that there is no steep soft component below 0.2 keV. 3. Significant X-ray absorption (tau > 0.3) by partially ionized gas (''warm absorber'') in quasars is rather rare, occurring for less than or similar to 5% of the population, which is in sharp contrast to lower luminosity active galactic nuclei (AGNs), where significant absorption probably occurs for similar to 50% of the population. 4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emission-line parameters indicates that the strongest correlation is between the spectral slope alpha(x) and the H beta FWHM. A possible explanation for this remarkably strong correlation is a dependence of alpha(x) on L/L(Edd), as seen in Galactic black hole candidates. 5. The strong correlations between alpha(x) and L([O III]), Fe II/H beta, and the peak [O III] to H beta flux ratio are verified. The physical origin of these correlations is still not understood. 6. There appears to be a distinct class of ''X-ray-weak'' quasars, which form similar to 10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10-30, than expected based on their luminosity at other bands and on their H beta 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 0.2-2 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 diffuse H II gas component of the interstellar medium is mostly ionized to He II or He III. We finally note the intriguing possibility that although [alpha(x)] in radio-loud quasars (-1.15 +/- 0.14) is significantly flatter than in radio-quiet quasars (-1.72 +/- 0.09) the X-ray emission may not be related to the presence of radio emission. The difference in [alpha(x)] may result from the strong alpha(x) versus H beta FWHM correlation and the tendency of radio-loud quasars to have broader H beta.